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FLOW VISUALIZATION AND FLUID-STRUCTURE INTERACTION OF TORNADO-LIKE VORTICES by JOHN LYLE FOUTS, B.S.M.E. A THESIS IN MECHANICAL ENGINEERING Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE IN MECHANICAL ENGINEERING Approved December, 2003
Transcript of FLOW VISUALIZATION AND FLUID-STRUCTURE INTERACTION ... …
FLOW VISUALIZATION AND FLUID-STRUCTURE
INTERACTION OF TORNADO-LIKE VORTICES
by
JOHN LYLE FOUTS, B.S.M.E.
A THESIS
IN
MECHANICAL ENGINEERING
Submitted to the Graduate Faculty of Texas Tech University in
Partial Fulfillment of the Requirements for
the Degree of
MASTER OF SCIENCE
IN
MECHANICAL ENGINEERING
Approved
December, 2003
ACKNOWLEDGEMENTS
I would like to thank my advisors, Dr. Darryl L. James and Dr. Chris
Letchford, for their continued guidance and support throughout this project. I
would also like to thank the State of Texas and the Wind Science and
Engineering Research Center at Texas Tech University for their support of this
research. I also would like to thank Mr. Matthew Mason for his help with the
experiments and other support. Finally, I would like to thank my wife for her
support and help during the pursuit of this endeavor.
TABLE OF CONTENTS
ACKNOWLEDGEMENTS II
ABSTRACT v
LIST OF TABLES vll
LIST OF FIGURES vill
CHAPTER 1
1 INTRODUCTION 1
L1 Background 1
^.2 Literature Review 3
1.2.1 Simulators 3
1.2.2 Measurements 8
1.2.3 Numerical Analysis 11
1.2.4 Conclusions and Objectives 14
2 EXPERIMENTAL SETUP 16
2.1 Design Criterion 16
2.2 Simulator 17
3 EXPERIMENTAL PROCEDURE 20
3.1 Principles of Operation 20
3.2 Procedures 25
3.3 Flow Visualization 25
3.4 Measurement Techniques 26
3.5 Uncertainty Analysis 27
4 RESULTS AND DISCUSSION 29
4.1 Flow Visualization 29
4.2 Pressure Measurements 46
4.2.1 Models 46
4.2.2 Orientation 47
1.1 Stationary Force Distributions 49
III
4.2.3 Cube Model 53
4.2.4 Cylinder Model 65
4.3 Transient Analysis 75
4.3.1 Transient Cube 76
4.3.2 Transient Cylinder 90
4.4 Statistical Analysis 104
5 CONCLUSIONS AND RECOMMENDATIONS 107
5.1 Conclusions 107
5.2 Recommendations I l l
REFERENCES 112
APPENDIX A 115
IV
ABSTRACT
A Ward-type tornado simulator has been built using a configuration of 16
slotted jets instead of a rotating screen to create the required far field circulation
needed to produce a tornado4ike vortex. Flow visualization data, velocity data
and pressure data were all obtained using the simulator. The produced vortices
observed ranged from a laminar, rope-like, single-celled vortex to a turbulent,
much larger diameter, two-celled vortex. Helium bubbles were used to visualize
the vortices in the convergent region of the tornado simulator. At a=0.5, the low
swirl ratios (the ratio of the tangential flow rate to the updraft flow rate) calculated
were s=2.23 and at a=1 s=1.51. The high swirl ratios calculated were s=8.03 at
a=0.5 and s=6.72 at a=1. The swirl ratios calculated are unique to the TTU TVS
II.
The initial vortex configuration in the TTU TVS II was that of a single-
celled vortex. During flow visualization, as the swirl ratio was Increased, a
breakdown bubble was observed moving down the vortex core region toward the
surface of the simulator. Once the breakdown bubble has traversed the vortex
core to the surface of the simulator, the vortex is defined as two-celled. The TTU
TVS II was capable of producing single-celled and two-celled vortices.
Pressure data was obtained on cubical and cylindrical models that were
positioned at various radial locations within the simulator. The models were also
subjected to moving tests through the TTU TVS II In order to compare the
stationary data to the moving data. Using the pressure data, non-dimensional
force coefficients were calculated and contour plots of the force coefficients on
the cube and cylinder were generated for the stationary tests while, for the
moving tests, specific points on the models were chosen, and the force
coefficients at these points were plotted as a function of position In the TTU TVS
II. The stationary tests show that both the cube and the cylinder models
experience flow regimes at different points In the TTU TVS II similar in pattern to
those induced by boundary layer-type flows, but mainly towards the outer regions
ofthe simulator in low swirl cases (s=2.23 and s=1.51). Also for the cylinder, the
contour plots indicate a horseshoe vortex forms around the cylinder. At the
center of the simulator, both the cylinder and the cube disrupt the flow field
significantly, and at this point, the flow field is very complex and at the present
time the experimental equipment and data are not sufficient to quantify the flow In
this region. The leading and trailing edges of the roof as well as the leading and
trailing sides of each model were chosen and force coefficients were calculated
and plotted as a function of radial position In the TTU TVS II for the moving tests.
Each of these moving tests had approximately the same trends for the leading
edge and side and the trailing edge and side with a few exceptions. The
stationary test data followed the trends of the moving test data In most cases
tested. This would mean that less significance could be placed on the much
more complicated moving tests and more significance on the less complicated
stationary tests In future testing
Limited statistical analysis was also performed on the obtained data sets.
This showed that standard deviation for all cases Is very small, so the distribution
should be concentrated towards the center of the normal distribution. Most of the
skewness values are negative Indicating the normal distribution is skewed to the
right of the centerllne and slow, Infrequent variations In pressure below the mean.
Very high kurtosis values like the ones shown for the center of the roof of the
cylinder at the center of the simulator indicate an Increase In the high-frequency
content ofthe fluctuating pressure signals read.
VI
LIST OF TABLES
0-1: Likely range of non-dimensional parameters 20
3-2: Velocity, Turbulence Intensity 24
3-3: Calculated Swirl Ratios 24
4-1: Scale Factors Between Simulated Vortex and Actual Vortex 47
4-2: Velocity Scale Factor Range In TTU TVS II 75
4-3: Cube Model Percent Error from Average Velocity 76
4-4: Cylinder Model Percent Error from Average Velocity 90
4-5: Statistical Values at Center of Roof on Cube and Cylinder at Center of
Simulator 105
|4-6: Statistical Values at Center of Roof on Cube and Cylinder at 2.0*ro in
Simulator 105
4-7: Statistical Values on North Face of Cube and Cylinder at Center of Simulator
105
4-8: Statistical Values on North Face of Cube and Cylinder at 2.0*ro in Simulator
106
4-9: Statistical Values on South Face of Cube and Cylinder at Center of
Simulator 106
|4-10: Statistical Values on South Face of Cube and Cylinder at 2.0*ro In
Simulator 106
VII
LIST OF FIGURES
n- l : Ward-Type Tornado Simulator (Ward, 1972) 4
2-1: Texas Tech University Tornado Vortex Simulator II 19
2-2: Dimensions of TTU TVS II (Plan View) 19
0-1: Jet Velocity Profile (a=0.5, Low Swirl) 22
0-2: Jet Velocity Profile (a=0.5, High Swirl) 22
0-3: Jet Velocity Profile (a=1. Low Swirl) 23
0-4: Jet Velocity Profile (a=1. High Swirl) 23
f^-^•. start of Vortex Formation (a=0.5) 30
[4-2: Progression of Vortex Formation (a=G.5) 31
n-3: Single-Celled Vortex (a=1, LowSwIri, s=1.51) 31
M : Scale of Inner Cone of Single-celled Vortex (a=0.5. Low Swirl, s=2.23).... 32
|4-5: Scale of Single-Celled Vortex (a=1, Low Swirl, s=1.51) 33
^-6: Vortex with Helium Bubbles and Smoke (a=0.5, Low Swirl, s=2.23) 33
n-7: Two-Celled Vortex (a=1, High Swirl, s=6.72) 34
n-8: Scale of Two-Celled Vortex (a=0.5, High Swirl, s=8.03) 35
|4-9: Flow Visualization with Cube in Center (a=0.5, Low Swiri, s=2.23) 36
|4-10: Flow Visualization with Cube in Center (a=1, Low Swirl, s=1.51) 36
|4-11: Flow Visualization with Cube in Center (a=1, Low Swirl, s=1.51) 37
14-12: Flow Visualization with Cube Offset (a=0.5, Low Swirl, s=2.23) 38
n-^3: Flow Visualization with Cube Offset (a=0.5, Low Swirl, s=2.23) 38
14-14: Flow Visualization with Cube Offset (a=1, LowSwIri, s=1.51) 39
[4-15: Flow Visualization with Cube In Center (a=0.5. High Swiri, s=8.03) 40
[4-16: Flow Visualization with Cube in Center (a=1, High Swirl, s=6.72) 41
^-17: Flow Visualization with Cube Offset (a=0.5, High Swirl, s=8.03) 41
[4-18: Flow Visualization with Cube Offset (a=0.5. High Swiri, s=8.03) 42
^-^9•. Flow Visualization with Cube Offset (a=1, High Swiri, s=6.72) 42
p -20: Flow Visualization with Cylinder In Center (a=1, LowSwIri, s=1.51) 43
Vlll
14-21: Flow Visualization with Cylinder in Center (a=1, High Swiri, s=6.72) 44
F -22: Flow Visualization with Cylinder Offset (a=1, Low Swiri, s=1.51) 45
P4-23: Flow Visualization with Cylinder Offset (a=1. High Swiri, s=6.72) 45
FI-24: Schematic of Model Cube and Cylinder 46
pi-25: Schematic of Surface of Tornado Simulator 48
[4-26: Positions where Models were Tested in Simulator 49
M 7 : Surface Pressure Profile for a=0.5, s=2.23 51
Ft-28: Surface Pressure Profile for a=0.5, s=8.03 51
[4-29: Suri ace Pressure Profile for a=1, s=1.51 52
[4-30: Surface Pressure Profile for a=1, s=6.72 52
^ -31 : Exploded View of Cubical Model 54
^-32: Force Coefficients on Cube at 2.0*ro (Point 1) in Simulator (a=0.5, s=2.23)
56
Fl-33: Force Coefficients on Cube at LOVo (Point 6) in Simulator (a=0.5, s=2.23)
57
[4-34: Force Coefficients on Cube at 2.0*ro (Point 1) In Simulator (a=0.5, s=8.03)
59
[4-35: Force Coefficients on Cube at 0.5*ro (Point 10) In Simulator (a=1, s=6.72)
59
Fl-36: Force Coefficients on Cube at 0.25*ro (Point 12) In Simulator (a=0.5,
s=8.03) 61
[4-37: Force Coefficients on Cube at 0.125*ro (Point 18) in Simulator (a=0.5,
s=2.23) 62
^-38: Force Coefficients on Cube at 0.0625*ro (Point 15) In Simulator (a=0.5,
s=2.23) 63
pt-39: Force Coefficients on Cube at Center (Point 16) of Simulator (a=1, s=6.72)
64
[4-40: Schematic of Exploded Cylinder 65
IX
M l : Force Coefficients on Cylinder at 2.0% (Point 1) In Simulator (a=1, s=6.72)
67
14-42: Force Coefficients on Cylinder at I.OVo (Point 6) in Simulator (a=0.5,
s=8.03) 67
14-43: Force Coefficients on Cylinder at 0.5*ro (Point 10) In Simulator (a=0.5,
s=2.23) 68
M 4 : Force Coefficients on Cylinder at 0.25*ro (Point 12) in Simulator (a=1,
s=1.51) 68
Fi-45: Force Coefficients on Cylinder at 0.125*ro (Point 14) in Simulator (a=0.5,
s=2.23) 70
[4-46: Force Coefficients on Cylinder at 0.125*ro (Point 14) in Simulator (a=1,
s=6.72) 70
|4-47: Force Coefficients on Cylinder at Center (Point 16) in Simulator (a=0.5,
s=2.23) 72
[4-48: Force Coefficients on Cylinder at Center (Point 16) in Simulator (a=1,
s=6.72) 73
F*-49: Force Coefficients on Cylinder at 0.0625*ro (Point 17) In Simulator (a=0.5,
s=8.03) 73
14-50: Force Coefficients on Cylinder at 0.0625*ro (Point 17) In Simulator (a=1,
s=1.51) 74
^ - 5 1 : Force Coefficients at Leading Edge on Roof of Cube (a=0.5, s=2.23) 78
[4-52: Force Coefficients on Trailing Edge on Roof of Cube (a=0.5, s=2.23) 79
[4-53: Force Coefficients on Center of Leading Face of Cube (a=0.5, s=2.23).. 79
[4-54: Force Coefficients on Center of Trailing Face of Cube (a=0.5, s=2.23)... 80
|4-55: Force Coefficients on Leading Edge of Roof of Cube (a=0.5, s=8.03) 81
[4-56: Force Coefficients on Trailing Edge of Roof of Cube (a=0.5, s=8.03) 82
p^-57: Force Coefficients on Center of Leading Face of Cube (a=0.5, s=8.03).. 82
^-58: Force Coefficients on Center of Trailing Face of Cube (a=0.5, s=8.03)... 83
P4-59: Force Coefficients on Leading Edge of Roof of Cube (a=1, s=1.51) 84
Ft-60: Force Coefficients on Trailing Edge of Roof of Cube (a=1, s=1.51) 85
[4-61: Force Coefficients on Center of Leading Face of Cube (a=1, s=1.51) 85
pt-62: Force Coefficients on Center of Trailing Face of Cube (a=1, s=1.51) 86
M 3 : Force Coefficients on Leading Edge of Roof of Cube (a=1, s=6.72) 87
^-64: Force Coefficients on Trailing Edge of Roof of Cube (a=1, s=6.72) 87
(4-65: Force Coefficients on Center of Leading Face of Cube (a=1, s=6.72) 88
pi-66: Force Coefficients on Center of Trailing Face of Cube (a=1, s=6.72) 88
14-67: Force Coefficients on Leading Edge of Roof of Cylinder (a=0.5, s=2.23) 92
[4-68: Force Coefficients on Trailing Edge of Roof of Cylinder (a=0.5, s=2.23). 92
fA-69: Force Coefficients on Leading Side of Cylinder (a=0.5, s=2.23) 93
Ft-70: Force Coefficients on Trailing Side of Cylinder (a=0.5, s=2.23) 93
[4-71: Force Coefficients on Leading Edge of Roof of Cylinder (a=0.5, s=8.03) 95
|4-72: Force Coefficients on Trailing Edge of Roof of Cylinder (a=0.5, s=8.03). 95
[4-73: Force Coefficients on Leading Side of Cylinder (a=0.5, s=8.03) '96
[4-74: Force Coefficients on Trailing Side of Cylinder (a=0.5, s=8.03) 96
[4-75: Force Coefficients on Leading Edge of Roof of Cylinder (a=1, s=1.51).... 98
[4-76: Force Coefficients on Trailing Edge of Roof of Cylinder (a=1, s=1.51) 98
[4-77: Force Coefficients on Leading Side of Cylinder (a=1, s=1.51) 99
[4-78: Force Coefficients on Trailing Side of Cylinder (a=1, s=1.51) 99
^-79: Force Coefficients on Leading Edge of Roof of Cylinder (a=1, s=6.72).. 101
[4-80: Force Coefficients on Trailing Edge of Roof of Cylinder (a=1, s=6.72)... 101
M 1 : Force Coefficients on Leading Side of Cylinder (a=1, s=6.72) 102
Ft-82: Force Coefficients on Trailing Side of Cylinder (a=1, s=6.72) 102
XI
CHAPTER 1
INTRODUCTION
1.1 Background
In an average year, 800 tornadoes are reported nationwide, resulting In 80
deaths and over 1500 Injuries [1]. This makes the study of tornadoes very
important. Waterspouts, fire whiris, dust devils, and steam devils are other types
of small but intense vortical flows. A tornado Is defined as being a violently
rotating, tall, narrow column of air, averaging about 100m In diameter [2].
Tornadoes are produced within cumuliform clouds. Visually, a tornado consists
of a funnel-shaped cloud protruding out of the bottom of the cumuliform cloud
and is generally accompanied by a swiriing cloud of dust and debris which come
from the ground below the tornado. A funnel cloud is a tornado that does not
reach the ground. Tornadoes are classified in fluid flow dynamics as vortices,
which are flows with associated core of concentrated rotation. Tornadoes come
in many different shapes and sizes. The shapes range from a thin rope-like
shape to a large cone shape to multiple-vortices spinning around what appears
to be a central axis. Five discrete stages of the tornado life cycle are described
by Davies-Jones [2].
1. The dust-whiri stage, when the first signs of circulation are visible,
and a small vortical protrusion appears In the cloud above.
2. The organizing stage is the stage where the increase In Intensity of
the tornado is evident by an overall downward descent of the
funnel.
3. The mature stage is the stage where possible damage Is most
Intense. It is characterized by the funnel reaching its greatest width
and being almost vertical.
4. The shrinking stage is the stage in which the tornado starts to loose
energy. It Is Indicated by an Increase In funnel tilt, a narrowing
damage swath, and decreasing funnel width.
5. The decay stage Is marked by the vortex being stretched into a thin
rope shape and becomes greatly contoured before dissipating.
The parent storm of a tornado Is two orders of magnitude In horizontal
dimension larger than the core of a tornado and its total energy and circulation
greatly exceed the energy and circulation associated with the tornado according
to Davies-Jones [2]. This Indicates that In order to find and predict
tomadogenesis, a thunderstorm with sufficient energy to produce a tornado must
be found. These thunderstorms are typically generated when warm, moist air
flows Into a building storm at low levels and rises through the storm. Evaporating
precipitation cools the drier air that surrounds the updraft of the storm, and this
cooler air sinks towards the ground. As it reaches the ground and starts to
spread out, it encounters the rising, warm, moist air and is drawn around the
warm air to form what Is now known as the leading edge of a thunderstorm
downdraft. As the downdraft gust propagates, It displaces and lifts the moist,
warm air In its path which feeds the generation of the thunderstorm. This cycle
continues to generate energy in the thunderstorm and eventually, If the
conditions are right, the updraft begins to rotate, and a tornado Is formed.
The problem with studying tornadoes in nature is the unpredictability and
the extreme danger of severe storms. The three other methods for studying
tornadoes are analytical work, numerical modeling, and experimental simulation.
Analytical work and numerical modeling are very difficult to perform due to the
nonlinearlty and uncertainty in the modeled equations and the lack of knowledge
of detailed real-worid boundary conditions. For such reason, a number of
different tornado simulators have been constructed over the last thirty to forty
years to try and mimic a tornadic wind field. These simulators try to create flows
that are dynamically and geometrically similar to natural atmospheric flow. YIng
and Chang presented one of the first known modern tornado vortex simulators in
1969 [3]. They were followed by Ward In 1972 [4], Davies-Jones In 1973 [5],
Jlschke and Parang in 1974 [6], and Church, Snow and Agee In 1977 [7], Snow,
Church and Barnhart In 1979 [8], Church, Snow, Baker and Agee In 1979 [9], and
others since 1979. It was concluded by Davies-Jones after comparing different
configurations of tornado vortex simulators that the most appropriate apparatus
for tornado modeling is the Ward-type simulator developed by Ward In 1972 [4].
At Texas Tech, a Ward-type simulator has been constructed, but with slotted jets
to provide tangential acceleration instead of a mesh screen used In previous
simulators. These slotted jets allow for much easier access to the regions of
interest In the convergence region than the mesh screen does for obtaining
quantitative and qualitative data, namely pressure measurements, velocity
measurements, and flowfield visualization.
1.2 Literature Review
1.2.1 Simulators
Quite a bit of research went on in the 1960-1980's to produce an accurate
depiction of an actual tornado with the help of experimental tornado simulators.
Some of the first researchers to build a modern tornado vortex simulator were
YIng and Chang In 1969 [3]. They built an eariy model of what would later
become known as a Ward-type tornado simulator. YIng and Chang recognized
that there were two essential factors In producing a tornado vortex near the
ground, circulation and updraft. They produced circulation by rotating a
cylindrical wire screen, and they produced updraft by a separate exhaust fan
located at the top of their simulator. This configuration allowed YIng and Chang
to independently control the circulation and the updraft which, in later work,
proved essential to the control of the vortex size. YIng and Chang concluded
several things. First, that they could provide a controllable constant circulation
along the vortex axis with the rotating screen without Introducing an evident
secondary fiow, and that a complex buoyant effect could be avoided with the
suction fan located far above the hood opening. Second, that the pressure Is
neariy constant vertically in the turbulent ground boundary layer except In the 3
region of the vortex core. Third, that it was not possible to carry out
measurements with probes at the present time in the complex flow at the foot of
the vortex without Introducing large errors. Fourth, that a successful mechanical
simulation of tornado-like vortices was evident by both the visual vortex core and
the reverse-flow funnel demonstrated with smoke visualization. Finally, that
above the boundary layer, the static pressure distribution and the tangential
velocity components depend mainly on the radius from the center of the
simulator and the circulation and only slightly on the vertical distance of the
confluent zone except for the vortex core region.
In 1972, Ward developed a tornado vortex simulator that would become
the standard for most experimental tornado research to date [4]. Ward's
apparatus (see Figure 1.1) was much the same as YIng and Chang, except that It
used a flow-straightening device at the top of the chamber that removed the
tangential component vorticity.
/ COLLECTION AREA
HONEYCOMB SECTION ^ ^ / I / 4 INCH MESH \ \ 3 / 4 INCH T H I C K ;
y EXHAUST
FAN
t CONVECTION CELL t
'A ^CONVERGENT \^ %^
ROTATING SCREEN
l / CONFLUENT
ZONE
/ DIRECTION
VANE I I- I FTH SCALE
Figure 1-1: Ward-Type Tornado Simulator (Ward, 1972)
Ward realized that there were three characteristic features of tornadoes
that he could simulate In a laboratory system. The three features were:
1. Characteristic surface pressure profile.
4
2. Bulging deformation on the vortex core.
3. Multiple vortices in a single convergence system.
He found that (1) and (3) can be produced only when the diameter of the updraft
column Is greater that the depth of the Inflow layer in his model. Ward concluded
that the extreme radial pressure gradient accounts for centripetal acceleration,
and that air which was at a significantly higher potential temperature when the
vortex increases in scale could subside motion along the central axis. He also
said that these factors ((1) and (3) above) do not exist before vortex formation.
Ward also concluded that as a vortex forms, there is a large Influx of radial
momentum which can produce a force field such that a portion of the fluid mass
is required to converge against opposing centrifugal force plus any net outward
pressure.
Davies-Jones related the significance of core radius dependence to swiri
ratio In 1973 [5]. Davies-Jones used a Ward-type simulator to show that the non-
dimensional radius of the turbulent core in a tornado simulator Is primarily a
function of swiri ratio. This paper reinterprets Ward's data on turbulent core
radius, and concludes that volume flow rate is a much more important factor than
radial momentum flux In the production of Intense atmospheric vortices. Davies-
Jones also states that high-volume flow rate Is required for the production of
concentrated vortices for a given circulation and updraft radius. Also in 1974,
Jischke and Parang [6] showed that tornado-like vortices simulated In Ward-type
simulators show a systematic increase in core radius with Increased imposed
swiri and that an instability at a critical value of the imposed swiri causes the
usual single-celled vortex to undergo a transition to a two-celled vortex
configuration.
Church, Snow, and Agee created a four-meter wide and seven-meter tall
Ward-type tornado vortex generator In 1977 at Purdue University [7]. They
focused on laboratory simulation and study of tornado-like vortex features, with a
primary emphasis on quantitative investigation Into the nature and cause of the
multiple vortex phenomenon. The Purdue simulator demonstrated the ability to
achieve vortex breakdown and multiple vortex formation, and offered the
possibility to obtain detailed Information about flow fields for various transitional
stages of vortex appearance and formation. This paper was followed up In 1979
with a second paper about the Purdue tornado simulator by Snow, Church, and
Barnhart [8]. They performed a series of laboratory experiments In the tornado
vortex simulator to obtain a better physical understanding of the mechanisms
producing the surface pressure fields recorded on barograph traces by Fujita [10]
and Ward [4]. Snow, Church, and Barnhart found that the surface pressure field
In the convergent region outside the central core is determined by two processes.
One Is radial Inertlal forces acting to decelerate the inflow by establishing a
region of higher pressure about the centerllne, and the other is the dynamic
pressure field Induced by the conservation of angular momentum acting to
produce a region of lower pressure about the centeriine. They also discovered
that the largest central-pressure deficits were found In single-celled vortices
characterized by Intermediate swiri ratio values, and that when two-celled
structures developed, the lowest pressures were found off-axis of the centeriine
in an annular region. They suggested that there Is a strong evidence of the
existence of a dynamically induced downdraft In the two-celled vortex.
Swiri ratio is one of the most important factors in tornado vortex
simulation. Church et al. helped to quantify this ratio In 1979 with the Purdue
tornado simulator [9]. They used a Ward-type simulator to observe five different
vortex configurations by varying the swiri ratio, the radial Reynolds number, and
the aspect ratio:
1. The single, laminar vortex or single-celled vortex.
2. The single-celled vortex with the upper turbulent region separated
from the lower laminar region by a breakdown bubble.
3. A fully developed turbulent core, where the breakdown bubble
penetrates to the bottom of the experimental chamber, joining the
two-celled vortex.
4. Vortex transition to two Intertwined helical vortices.
5. Examples of higher order multiple-vortex configurations that form in
the core region.
They showed that as the angular momentum Increased, effectively Increasing
swiri ratio, the vortex evolved from a single-celled vortex to a two-celled vortex to
a multiple vortex configuration. This evolution Is characterized by the single-
celled vortex developing a "separation bubble" at the top of the vortex that
propagates towards the surface of the simulator as the swiri ratio Increased until
the bubble touches the surface and the vortex Is then characterized as two-
celled. They also Indicated that there was a downdraft in the center of the vortex
that penetrates to the level of the breakdown bubble and to the surface of the
simulator when the separation bubble touches the surface. It was also
concluded that the swiri ratio Is the parameter that primarily determines the core
configuration, not the aspect ratio or the radial Reynolds number.
The tornado simulator at Purdue University was updated in 1987 by Snow
and Lund [11]. The primary objective of the updated chamber was to construct
an apparatus suitable to take measurements of the radial and tangential
components of velocity in and near the core of tornado-like vortices using a Laser
Doppler Velocimeter (LDV). This simulator also used a vane assembly to create
swiri on the Incoming flow rather than using the traditional rotating wire screen.
In 1993, the American Geophysical Union published The Tornado: Its
Structure, Dynamics, Prediction, and Hazards' [12]. This book contains a vast
amount of information about tornadoes, and has a paper by Church and Snow on
laboratory models of tornadoes [13]. In this paper. Church and Snow compare
current and past tornado vortex simulators. All of the working tornado vortex
chambers discussed in this paper are variants of the Ward-type simulator and
use a system of vanes to produce circulation rather than a wire screen. The
working models of tornado simulators discussed are at Purdue University [11],
where the goal of the research was to Implement a LDV system to an updated
simulator, University of Oklahoma [14], where the goal of the research was to
quantify the effects of surface friction on rotating fluids, Kyoto University [15],
where the research goal was to create multiple vortices and to study the
characteristics of tornado-like vortices on various surfaces, and at Miami
University in Ohio, where the goals were to study tornado-like vortices.
Davies-Jones compared different configurations of tornado simulators In
1976 [16]. His purpose was to comment critically on the relevancy of laboratory
experiments to tornadoes, and to assess their contributions to current knowledge
of tornado dynamics. Davies-Jones concluded that in 1976, high quality
measurements were still difficult to make due to the influences of probe
interference, vortex wander and extraneous perturbations. He also concluded
that Ward's model was probably the most realistic model as of 1976 due to Its
large exhaust radius and high Reynolds number which at a low aspect ratio and
a low swiri ratio appear to be the conditions necessary to simulate a tornado
vortex.
1.2.2 Measurements
There are two main types of data taken In a tornado vortex simulator,
velocity and pressure. Both of these measurements can be taken with or without
objects inside the flow field. Intrusive velocity measurements are taken using a
thermal anemometry system; within the past few decades, laser Doppler
velocimeters (LDV) and particle image veloclmetry (PIV) systems have been
employed due to their 'unobtrusive' nature. Lund and Snow used the second
generation Purdue University tornado vortex simulator to take LDV
measurements in tornado-like vortices [17]. They discussed radial and vertical
profiles of radial and vertical velocity component measurements and derived
vertical velocity components. Lund and Snow found that vertical and radial
profiles of radial and tangential velocity components reveal characteristic
boundary layer and vortex flow features. They found that the greatest tangential
speeds were in an annular volume of small radius and small radial width but
significant vertical extent, with large centrifugal accelerations and largest
pressure gradients occurring well aloft. They showed that LDV measurements
are an accurate means of collecting quantitative data about critical flow structure,
8
but vortex wander hinders accurate measurement of the innermost core. Fiedler
and Rotunno theorized in 1985 that the most Intense laboratory vortex occurs
when the vortex Is in the form of an end-wall vortex or 'supercritical' vortex [18].
This is the vortex that forms upstream of the vortex breakdown 'bubble', and will
be described at length in section 4.1. They came up with a model for the
maximum Intensity of the vortices by modeling the end-wall vortex and finding the
criterion for vortex breakdown. Cleland performed axial vertical velocity
measurements of simulated super-critical tornado-like vortices in the Miami
University Tornado Vortex Chamber In 2001 [19]. The results suggest a strong
correlation between swiri ratio and super-critical Inner core region diameter, and
a breakdown of super-critical structure far below vortex breakdown.
Another important quantitative measurement in tornado simulator research
Is pressure distribution. Centeriine pressure distributions in a tornado simulator
are obviously an important starting point. Church and Snow obtained axial
pressure measurements In tornado-like vortices for two purposes [20]. First, they
wanted to determine how the magnitude of the central pressure deficit in a
columnar vortex varies with height, and second, to determine what functional
relationships exist between these deficits and the dynamic and geometric
parameters characterizing the flow. They presented vertical profiles of central
pressure deficits for a representative number of laminar and turbulent (single-
celled and two-celled) tornado-like vortices, showing that the variation of the
central pressure with the height is very complicated. They also found that the
largest pressure deficits in the low-swiri vortices are slightly above the surface of
the simulator, not on the surface, and that the low-swiri vortices have generally a
greater central pressure deficit than that of moderate to high-swiri events.
Pauley followed Church and Snow with measurements of axial pressures
in two-celled tornado-like vortices [21]. His goal was to better define the vertical
momentum balance in the cores of two-celled laboratory vortices, where two-
celled vortices Is defined as one with a stream surface dividing an outer cell of
swiriing inflow and up flow from an inner cell which may have down flow near the
axis. Pauley found that the axial pressure increased with height downstream
(above the bubble) of the vortex breakdown. He also found Indications through
visualization that downstream flow of breakdown is two-celled everywhere, and
that the strongest axial down flow occurred at the middle levels of the vortex.
Jlschke and Light used a modified Ward-type tornado simulator to study
interaction of model structures and tornadic flow fields [22]. They took
measurements of pressure, with and without swiri, on the surfaces of a
rectangular model. Jischke and Light's experiments showed that when
compared with ordinary boundary layer flow, the addition of swiri to flow could
significantly change the forces and moments experienced by the model. They
concluded that location of the model with respect to the tornado vortex and Its
orientation are very important factors in a tornado's capacity for damage In
addition to the tornado's maximum wind speed. Jlschke and Light next studied
tornadic wind loads on a cylindrical structure [23]. The cylindrical structure was
meant to model a nuclear reactor containment building. It was a circular cylinder
with a hemispherical roof. They compared surface pressure coefficients for swiri
angles of 0° and 45° with the model In the convergent zone of the simulator, and
with the model at the boundary of the convergent and convective zones. They
found that when the model is oriented In the convergent flow region of the
simulator, a region where the vertical velocity component is small, the results are
similar to the results of an infinite circular cylinder with circulation. When the
model is near the boundary of the convergent and convective zones of the
simulator, the vertical velocity components of the tornado4ike vortices induce a
circulation about the model which leads to an asymmetric pressure distribution.
That results in forces on the building having both drag and side force
components.
Chang used an indirect approach of using model simulation in laboratory
experiments In 1971 to study tornado wind effects on buildings and structures
[24]. Chang wanted to establish the dynamic similarity of the vortex that was
experimentally created to a typical tornado near and In the ground boundary
10
layer. He used a cubical model of a building fitted with distributed pressure taps
that was placed In a fixed position inside the tornado-like vortex. Chang
performed pressure tests for two cases and at two locations. He concluded that
experimental tornado vortex modeling to test wind loading on buildings was
feasible, and accurate representation of dynamic and kinematic effects of full-
scale tornado wind loadings on real buildings, and that pressure distributions
show the combined effects of dynamic pressure and suction.
Bienkiewicz and Pragnesh studied the effects of swiri ratio and surface
roughness on the flow generated by a Ward-type simulator and on building
loading [25]. They concluded that surface roughness has a major effect on the
flow characteristics of a vortex; moderate roughness delayed the transition to and
development of multiple vortices at moderate swiri ratios. Bienkiewicz and
Pragnesh also found that the swiri ratio highly Influences the mean pressure
coefficient on the roof of a building.
Wang also built a working Ward-type tornado vortex simulator, the TTU
TVS I [26]. He tested scale models of generic cubical and cylindrical structures
In this simulator utilizing stationary and transient tests. He found that the roof of
the cylinder encountered more suction than the roof of the cube and that the
pressure fluctuated by as much as 150% between stationary and transient
testing of the models. Wang also found that the pressure distributions for the
dynamic tests he performed were of similar magnitude as the stationary tests he
performed.
1.2.3 Numerical Analvsis
Although the work being done currently at TTU with the tornado vortex
simulator is strictly experimental, it should be noted that numerical calculations
also play an Important role in the worid of tornado study. The fields of
experimental fluid dynamics and computational fluid dynamics are very useful
when combined together. Each field has Its own strengths and weaknesses, so
by exploiting both, one can draw from the strengths of each. Rotunno studied
tornado-like vortex dynamics with fine resolution calculations by using an
11
axisymmetric numerical model of the flow within a Ward-type tornado simulator
[27]. His results Indicated that the swiri ratio was the single most Important
parameter In governing the structure of the vortex. Rotunno's model was
consistent with laboratory experiments performed previously by others. He found
that the boundary layer separation at low swiri, a high shear core wall
surrounding a relatively stagnant inner region, and vortex breakdown and
transition to turbulence were all observed and simulated. Hariow and Stein
obtained numerical solutions for tornado4lke vortices using a high speed
computer In 1974 [28]. They did not introduce any special procedures to force
the occurrence of a single-celled or two-celled vortex, however, numerous
examples of single-celled and two-celled vortices were obtained In the range of
parametric variations Investigated. Hariow and Stein concluded that for the first
time numerical calculations performed had shown possible variations In vortex
structure without the requirements for special boundary conditions In the
numerical model to force previously expected results.
Nolan and Farrell studied the structure and dynamics of axl-symmetric
tornado-like vortices with a numerical model of axl-symmetric incompressible
flow [29]. They agreed with previous tornado research and found that the
angular momentum of the background rotating wind field and the turbulent eddy
viscosity, a value that was not determined, entirely determines the structure of a
tornado. Nolan and Farrell also stated that the structure and dynamics of actual
tornadoes will depend crucially on the details of their turbulent swiriing boundary
layers.
Wicker and Wilhelmson studied tornado genesis within a supercell with a
three-dimensional numerical simulation using a two-way interactive nested grid
[30]. During the 40-mlnute simulation, two tornadoes grow and decay within the
storm's mesocyclone, each with a life of about 10 minutes. Winds exceeding
60m/s and 0.018kPa/m of horizontal pressure gradients were recorded for the
tornadoes. When compared to Doppler and field observations of supercells and
tornadoes, the simulated storm evolution showed many similar features.
12
Lewellen and Lewellen used large eddy models to simulate a tornado's
interaction with surface structure [31]. The review by Davies-Jones In 1986 was
consistent with the Lewellens' findings In that the swirl ratio variation shows that
the average flow transforms from one with a vortex breakdown above the surface
for low swiri to a two-celled flow on the surface at moderate to high swiri. They
also showed with time averaged velocity distributions that the Interaction of the
tornado with the surface intensifies the low level vortex for all values of swiri.
Selvam has also modeled tornado forces on buildings [32]. He
encountered difficulties in the imposition of boundary conditions, selecting a
turbulence model, and in numerical convergence. He derived a solution
procedure to solve the pressure correction equations. Selvam found that the
forces created on a building's roof were more than five times larger than the
straight boundary layer flow In the forced vortex region and of the same order in
the free vortex flow. Selvam and Millett modeled tornado-structure interaction
with a cubical building using finite-differences to solve the RANS equation and
large eddy simulation equation turbulence model [33]. They found that a
translating tornado produces 45 percent greater overall forces on the walls and
100 percent greater overall forces on the roof than a quasi-steady wind does.
They also found that these forces change magnitude and direction quickly when
the core of the tornado is near the building, and that the localized suction
pressures on the building envelope are generated in multiple locations and are
greater than those in a straight line wind.
Flow in the surface boundary layer beneath a Rankine vortex using a
numerical technique was studied In three-dimensions by Chi and JIh [34]. They
derived generalized equations of motion based on dimensionless vorticity,
stream function and circulation. They found that their method of using Gauss-
Seidel's Iteration procedure to solve the equations assuming a uniform effective
viscosity and a Rankine-type intense vortex at the upper boundary was stable in
the range of Reynolds numbers for natural tornadoes.
13
Howells and Smith described an axl-symmetric numerical vortex model
suitable for modeling Intense tornadic activities In 1983 [35]. They used a
stretched grid in the radial direction to provide economical resolution of the vortex
core and the rotating cloud updraft. Howells and Smith found that it was
dynamically possible to generate a relatively narrow vortex in a suitable
background field of ambient rotation that was driven by a much broader field of
buoyant forcing aloft.
Smith also attempted to numerically model tornado-like vortices and
quantify the effects of boundary conditions [36]. He examined the boundary
conditions for Rotunno's numerical model to simulate tornado-like vortices
focusing on the lateral boundary condition for tangential velocity and the upper
boundary condition for radial and tangential velocity, to determine if either had
any significant Impact on vortex development. The presence and absence of the
flow-stralghtening baffle are attempted to be simulated by the upper boundary
conditions. He found that at what he considered low swiri ratios (s=0.87), the
upper boundary condition had a very distinct Impact on the single-celled vortex
by producing changes In the pressure field that Intensified the vortex. At higher
swiri ratios, the upper boundary condition did not appear to have significant
Impact on the development of the vortex. As for the lateral boundary condition,
Smith found that it did not have a significant impact on the development of the
vortex.
1.2.4 Conclusions and Objectives
Ward-type tornado simulators have long been used to study tornado-like
vortices due to their realistic modeling capabilities. The parameters that affect
the type of vortex formed are swiri ratio and aspect ratio. Reynolds number does
not have much influence on the tornado vortex as long as the flow Is turbulent.
As the swiri ratio Is Increased, the vortex starts to break down from a single-
celled vortex to a two-celled vortex and eventually to multiple vortices. Velocity
and pressure measurements have been performed inside the flow field of
tornado simulators as well as on different models which are placed Inside the
14
flow field. These models show that the flow field In and around a tornado-like
vortex Is very different from traditional boundary-layer flow.
The objective of this research is to further develop a Ward-type tornado
simulator based on a previous model at Texas Tech University which utilized
slotted jets to provide tangential flow rather than a rotating screen or vanes. The
new simulator will be used to quantify vortices formed. Selected configurations
of boundary conditions will then be used to obtain flow visualization data, velocity
data, and pressure data inside the tornado vortex chamber as well as on scaled
models of generic structures both stationary and transient in nature.
15
CHAPTER 2
EXPERIMENTAL SETUP
2.1 Design Criterion
In order to reproduce a tornado In the laboratory, two basic sources of
energy are needed to provide the updraft and circulation. The updraft usually
comes by means of a blower on the top of what will become the tornado chamber
that pulls air out of the chamber. Circulation Is essentially the fluid dynamics
equivalent of angular acceleration. Traditionally, with Ward-type simulators, the
circulation Is caused by a rotating screen or more recently, turning vanes. These
both create sufficient circulation required to generate and maintain single or
multiple vortices and provide a great flow field to perform the task of flow
visualization. The problem with both lies in the actual testing of the flow field.
When the screen or vanes are rotating. It becomes very difficult to place
measurement devices to measure pressure or velocity in the regions of interest
in the flow field.
The Texas Tech University Tornado Vortex Simulator II (TTU TVSII)
eliminates the Impediment of the rotating screen or vanes. For the pressure
testing, the pressure distribution on various models of structures would be
performed while the structures were stationary In specific locations and while the
structures were being translated through the flow field that was created by the
simulator. Instead of using the rotating screen or vanes, slotted jets are used.
The jets protruded from a plenum that was pressurized by a blower with
independent control from the updraft blower. A one-quarter inch slit was cut Into
the jets, and the silt extended from the surface of the simulator to the roof of the
simulator. Using these jets provided access to the regions where the flow field
would be tested. The downside to the slotted jet approach was the quantification
of the actual circulation. With a rotating screen or vanes, the circulation that Is
produced Is very uniform around the perimeter of the simulator. With the slotted
jets, the circulation Is not as uniform because the jet velocity decays and diffuses
16
In different directions as a function of distance from the jet exit. The simulator's
dimensions were based on the likely atmospheric range as mentioned by
Church, et al. [9]. The two most important geometric ratios are the aspect ratio
and the swiri ratio. Church, et al. [9] reported that the aspect ratio of a natural
tornado is between 0.2 and 1, and that the swiri ratio (based on a uniform flow) of
a natural tornado varies between 0.05 and 2. Swiri ratio for this simulator will be
discussed In section 3.1.
Three major regions of fluid flow exist In the tornado simulator, the
confluence region, the convergence region, and the convection region. The
confluence region is the region where the fluid flow first enters the tornado
simulator between the roof and the surface. The convergence region is the
region of the simulator where the circulating and updraft flow converge to create
a vortex. This is located at the very center of the simulator between the surface
and the roof. The convection region of the simulator Is the region In which the
vortex propagates up and out of the simulator. These sections are discussed at
length by Wang [26].
2.2 Simulator
Wang at Texas Tech University built the first TTU Tornado Vortex
Simulator and quantified the flow characteristics of the simulator [26]. This
simulator is the first known simulator to use the slotted jet approach rather than a
rotating screen or vanes to provide circulation. Although this simulator served as
a great proof of concept, Its shortcoming was its small size. The original TTU
Tornado Vortex Simulator had a radius to jets of only 0.5m. This radius did not
provide a sufficient confluence zone for the Incoming flow to develop
independently of the slotted jets. In order to remedy this problem, the Texas
Tech University Tornado Vortex Simulator II (TTU TVSII) was constructed. As
mentioned before, a series of sixteen equally spaced slotted jets (50.8mm
diameter pipe with 6.35mm wide silt cut axially) were placed radially around and
protruding from a large (2mx2mx0.6m) plenum. The slots that were cut Into the
pipe spanned the length between the surface of the simulator and the roof where
17
the updraft hole was located. This plenum was supplied with a variably
controllable supply of air by means of a blower. To quantify the flow rate of the
air entering the plenum, an orifice plate was constructed and calibrated according
to ASME specifications to be placed between the blower and the plenum. The
surface of the test section of the simulator was located above the plenum and
could be adjusted up and down as the radial updraft hole was kept constant to
control the aspect ratio. The test section Itself was two meters in diameter.
Above the surface of the test section was a roof and in the center of the roof a
radial updraft hole that led Into a convection chamber. The updraft hole Is fixed
with a 0.381m diameter. The convection chamber was made of a cylindrical
piece of plexiglass and measured one meter in diameter. At the top of the
convection chamber Is the flow-stralghtening device that Ward [4] recommended
that eliminates the fan blower's vorticity from the tornado-like vortex. The flow
straightening device is simply a honeycomb grid approximately 76.2mm in length.
Atop the honeycomb grid is the blower that creates suction in the convection
chamber and creates the needed updraft flow. Figure 2-1 Is a schematic of the
Texas Tech University Tornado Vortex Simulator II, and Figure 2-2 is a
schematic of the dimensions of the simulator. The height of the convection
chamber Is 0.813m and the distance from the roof to the surface Is variable
between 0.064 and 0.1905m for an aspect ratio of one-half and one, respectively.
18
16 Slotted Jets
Roof
Surface
Vortex Blower
Oriface Plate
Hnnpyrnmh
Convection Chamber
Plenum
Figure 2-1: Texas Tech University Tornado Vortex Simulator II
North
West East
South
Figure 2-2: Dimensions of TTU TVS II (Plan View)
19
CHAPTER 3
EXPERIMENTAL PROCEDURE
3.1 Principles of Operation
There are many forms of tornado simulators In the present day. Most are
designed to produce a visible flow field of a vortex that looks like a tornado, but
not all capture the atmospheric complexities associated with an actual tornado
that will lead to meaningful quantification of the flow characteristics. The
traditional Ward-type simulators with Independent control over the tangential and
the updraft flow rates can replicate a scaled down version of the atmospheric
conditions present when a tornado is formed. As previous studies have shown,
the parameters that govern tornadic flow in the atmosphere are the three non-
dimensional ratios, aspect ratio, a, swiri ratio, s, and radial Reynolds number,
Ror. In order to achieve dynamic and geometric similarity, the experimental
aspect and swiri ratio must be comparable to the natural aspect and swiri ratios.
Studies by Church et al. [9] list the typical characteristics of actual rotating
thunderstorm-tornado cyclone system as shown In Table 3-1.
Table 3-1: Likely range of non-dimensional parameters
Dimensionless Group Likely Atmospheric Range
Aspect Ratio (a)
Swirl Ratio (s)
Radial Reynolds Number
(Re,)
0.2-1
0.05-2
10^-10^^
The aspect ratio, a. Is given by the equation:
a = • (3.1)
where h Is the height measured in the convergent region of the simulator
between the surface and the roof, and ro is the radius of the updraft hole. On the
TTU TVS II, the aspect ratio Is adjustable by moving the ground surface up or
20
down to achieve the desired ratio. The two ratios that were utilized for this study
were a=0.5 and a=1 which fall In the range that Church, et al. [9] described.
The swiri ratio is another of the important non-dimensional parameters
that dictates the type of flow field that will occur in the simulator. The swiri ratio Is
defined as the ratio of tangential flow rate to updraft flow rate. It is given by:
F r s = "- (3.2)
2Q ^ '
where F Is the circulation, ro Is the radius of the updraft hole, and Qup is the
volume flow rate through the updraft hole. The circulation, F, evaluated at the
slotted jets is not as simple to calculate as It Is for a rotating screen, because the
jet velocity profile Is a function of the nozzle height and distance from the jet exit.
In the case of the rotating screen, the circulation can be directly related to the
speed of the rotating screen if It Is assumed a perfect coupling exists between
the incoming air and the rotating screen. The circulation is defined as the cyclic
integral of the tangential velocity dotted with a differential arc length of the curve
for this work. In order to calculate the circulation in the TTU simulator, a single-
channel hot film probe was used to measure the jet velocity profile as a function
of height and position. The following relation was used to determine the
circulation:
T = <^Vds^n^^V(x)-dx (3.3) 0
where V(x) is the mean jet velocity obtained using a curve fit of measured
velocities as a function of distance from the jet, / Is the distance between jets,
and n is the number of jets. The jet velocity profiles were obtained by sampling
eight axial positions three times a piece and averaging. This was done at three
different heights, low, mid, and high, for each case, then the low, mid and high
were averaged and a trendllne was fitted to the overall averaged profiles for each
case (Figure 3.1, Figure 3.2, Figure 3.3, and Figure 3.4).
21
0.15 0.2 Rasition (m)
o
0
A
+
Low Height
Mid Height
Hgh Height
Average
-Fbwer (Average)
0.35
Figure 3-1: Jet Velocity Profile (a=0.5, Low Swirl)
0.15 0.2
Position (tn)
o
a
A
+
Low Height
Md Height
High Height
Average
-RDwer
0.35
Figure 3-2: Jet Velocity Profile (a=0.5, High Swirl)
22
o
n
A
+
Low Height
Md Height
High Height
Average
-Fbwer
0.2
Position (m)
0.3 0.4
Figure 3-3: Jet Velocity Profile (a=1. Low Swiri)
18
16
14
~ •'2
1 . 10
o <u >
o \ + \ A ^
A
..ft
o
v=0.6136x-°^93
T 8 - *
o
D
A
+
Low Height
Mid Height
High Height
Average
-Rower
0.05 0.1 0.15 0.2
BDsition (nfi)
0.25 0.3 0.35
Figure 3-4: Jet Velocity Profile (a=1. High Swiri)
The equation for the trendline Is integrated using equation 3.3 above to find the
circulation.
Once the circulation Is known, the only other parameter that needs to be
calculated to obtain the swiri ratio is the updraft volume flow rate, Qup. The
23
updraft volume flow rate was determined using the single-nomer hot-film to
traverse the updraft hole to obtain velocities across the updraft hole. For
comparison. Table 3.2 shows the updraft velocity across the updraft hole of the
simulator and the turbulence intensity at each of these points.
Table 3-2: Velocity, Turbulence Intensity Radius, (mm) 0.000 12.700 25.400 38.100 50.800 76.200 101.600 127.000
a=1/2, low swirl Velocity, (m/s)
Turbulence Intensity
15.117
18.553
14.499
18.810
14.199
17.664
12.873
26.487
14.183
22.710
12.322
28.765
12.698
22.600
12.787
29.674
190.500
10.162
41.816
3=1/2, liigh swirl Velocity, (m/s)
Turbulence Intensity
10.430
31.330
10.839
30.969
10.320
34.407
10.424
30.487
9.840
30.798
9.542
33.580
8.845
31.031
8.769
33.922
8.171
32.845
a=1, low swirl
Velocity, (m/s) Turbulence
Intensity
12.163
14.637
11.913
16.049
13.993
22.349
13.492
17.379
12.946
16.142
14.729
16.615
14.363
17.120
14,789
21.029
13.018
27.164
a=1, tiigh swirl
Velocity, (m/s) Turbulence
Intensity
7.978
31.581
8.386
36.889
9.814
34.250
9.394
34.310
8.796
35.790
8.379
36.413
8.942
33.739
8.344
33.692
7.551
34.165
These velocities were then multiplied by annuli of the updraft hole and
summed to obtain a volume flow rate. To ensure repeatability of swiri ratio each
time the simulator was used, an orifice plate between the blower and the plenum
was attached to a manometer and monitored. Using the manometer, the change
In pressure across the orifice plate could be adjusted to a precise, predetermined
value each time testing was to be done. Using the data acquired, swirl ratios
were found and are presented In Table 3.3, and are unique to the TTU TVSII.
Table 3-3: Calculated Swirl Ratios
Swir l Ratio Aspect Ratio
a=0.5 a=1
Low Swir l 2.23 1.51
High Swir l 8.03 6.72
The radial Reynolds number, Rer, Is defined by Church et al. [10] as the
volume flow rate per axial length, q, divided by two times pi times the kinematic
24
viscosity of the fluid in question. The axial length Is described as h, the depth of
the convergence region. The equation for radial Reynolds number is:
27IV
Even though It Is typically discussed as an Important ratio in tornado vortex
generation, Ward [4] states that it was not an important parameter in determining
the type of vortex that is developed as long as the flow field Is turbulent.
3.2 Procedures Two main types of vortices were studied In the TTU tornado simulator.
The first is called a single-celled vortex and the second a two-celled vortex. The
swiri ratios calculated above correspond to a single-celled and a two-celled
vortex for the low swiri and the high swiri, respectively. The single-celled vortex
is characterized by a thin, rope-like, vortex, which some refer to as laminar [9].
The two-celled vortex is characterized by a much larger vortex that has a stream
surface dividing an outer cell of swiriing in-flow and up-flow from an Inner cell In
which down flow near the axis exists. The first step In quantifying the swiri ratios
that would be used In the study was to visualize each type of vortex and record
the pressure change across the orifice plate at which the vortex occurred. This
ensured that the same type of vortex could be obtained for each test, even when
no flow visualization material was used. It is also notable that the vortex rotates
in a counter-clockwise direction throughout all tests. The flow field created
consists of two main components. The tangential component of the flow field Is
the flow that Is rotating In a horizontal plane around the model. The radial
component of the flow field is the flow coming in from the sides of the simulator in
a vertical plane.
3.3 Flow Visualization
Flow visualization is essential to determine what type of vortex is being
formed in a tornado simulator. Four visualization methods were used to study
the vortex: helium bubbles, "smoke," a fog generator, and liquid nitrogen. The
helium bubbles method uses a helium bubble generator to create uniformly sized
25
bubbles that are neutrally buoyant. This method turned out to be the best of the
three ways to visualize the vortex due to control of location and quantity of
bubbles introduced. When performing the flow visualization on the TTU tornado
vortex simulator, neutrally buoyant helium bubbles from a helium bubble
generator were Introduced at the surface of the simulator with effectively no
vertical or radial velocity component. The bubbles are Illuminated with the aid of
an arc lamp. The bubbles were swept through the convergence region of the
simulator up into the convective chamber and out of the chamber through the
updraft fan. The "smoke" methods created visualization material that could not
be reasonably controlled. Visualizing the vortices using liquid nitrogen and fog
from the fog generator has shown promise, as some control of the quantity
introduced is available, but at this time problems exist for controlling the amount
of liquid nitrogen or fog to use for optimum visualization.
3.4 Measurement Techniques
To measure velocity, a TSI IFA 300 constant temperature anemometer
system was utilized. In order to control the IFA 300, TSI's ThermalPro software
was used. The IFA 300 constant temperature anemometer measures velocities
by means of a single-channel hot-film probe. The probe was controlled by a
dual-axis linear traverse and was connected to the IFA 300 system via a thirty
meter cable. All velocities reported were obtained using a frequency of 1000 Hz
and four kilopolnts per channel (4000 points). Velocity measurements are an
Integral part of the data which was collected and great care was taken to obtain
the velocities without disturbing the flow regime. However, the hot-film
anemometry system is very intrusive, especially In a rotating flow such as this, so
velocity measurements are subjected to interference. Due to these limitations,
only a jet exit velocity profile and the updraft hole velocity profile were obtained
using the hot-film anemometry system.
Pressure measurements were performed In the tornado vortex simulator
using of a Scanlvalve DSM 3000 system and ZOC 33/64Px and ZOCEIM
26
scanning modules. For the stationary tests performed, only the ZOC 33/64Px
scanning module was needed, but for the dynamic moving tests, the ZOCEIM
module was employed to serve as a precision timer so that the velocity of the
model could be obtained as it traversed the simulator. The ZOC 33/64Px
scanning modules consists of 64 piezoreslstive sensors that are activated via
pneumatic switching by the DSM 3000 CPM which is the pressure distribution
control module. The ZOCEIM module was calibrated to read a voltage signal
when a switch Is tripped. During the dynamic moving tests, the switch was
tripped twice in order to obtain a starting and a stopping point. All pressure
measurements were performed at 300 Hz for six seconds, but the dynamic
moving tests will have a slightly reduced time frame.
3.5 Uncertainty Analvsis Uncertainty analysis was calculated for the swiri ratio and the stationary
force coefficient. Uncertainty is limited to man-made and instrumental
uncertainty due to the fact that there Is no method at this time to determine the
flow-induced uncertainty. The method used to calculate uncertainty was the
general error propagation equation:
"/ = z y, (3.5)
The uncertainty in the swiri ratio is given by:
w, =. ds
— I
ar
ds
52, Qup
up J
(3.6)
where Ur=UQup=0.5% as reported by the manufacturers calibration. The
uncertainty in swiri ratio Is approximately 5%.
The uncertainty in force coefficient is:
(dC, Ur = .
dp -1-
dC.
dw ) -H
dc. ydA, J
dCj \ 2
(3.7)
27
where Up=0.2%, Uw=0.5%, and UAI,2=1%- Up and Uw were obtained by
manufacturer's specifications, Uai,2 was obtained by direct calculation. This
makes the uncertainty in force coefficient 1.16%.
28
CHAPTER 4
RESULTS AND DISCUSSION
For this tornado simulation study, flow visualization and pressure
measurements on model structures were performed to determine fluid structure
Interactions. This is the first step in ultimately designing safer structures. For
flow visualization, helium bubbles and various other flow visualization materials
were introduced in the flow field of the TTU TVSII and pictures were taken to
qualitatively show what type of flow existed Inside the TTU TVSII. These pictures
showed various types of tornado vortex structures.
To quantitatively examine the TTU TVSII, pressure measurements were
taken on two different scale models of structures, one being a cube the other a
cylinder. These pressure measurements were then converted into force
coefficients and plotted In contour plots of each structure at various radial
locations Inside the simulator. Also, in an attempt to model a natural tornado as
precisely as possible and to compare with stationary tests, moving tests were
performed with each model where the model was traversed across the simulator
while simultaneously collecting pressure measurements. These pressure
measurements were converted to force coefficients, and plotted as a function of
radial position in the simulator.
4.1 Flow Visualization Flow visualization is a very Important process to qualitatively determine
what type and direction of flow is present in any fluid flow field. As previously
stated in section 3.3, different types of flow visualization techniques were utilized
in the TTU tornado vortex simulator, but helium bubbles proved to be the best
means of visualizing the flow field.
The flow field of the TTU tornado simulator behaves much like other
simulators before [3, 6, 7, 8]. When the swiri ratio is very small, on the order of
zero, the fluid Inside the tornado simulator does not experience much tangential
29
velocity, only updraft velocity. For this reason, the flow in the convergence
region acts in much the same way as the flow In a conventional wind tunnel with
a horizontal velocity component until It reaches the updraft hole. As the fluid
moves horizontally under the region of the updraft hole the flow changes from
mostly radial to mostly vertical. The fluid is then expelled out of the top of the
convection chamber. As swiri is slowly Imparted on the fluid, the fluid starts to
circulate tangentially as it moves toward the updraft hole. When the circulating
fluid Interacts with the updraft, it is lifted through the updraft hole stretching Its
vorticity vertically. This is the start of the formation of the tornado vortex. Figure
4-1 shows a slight protrusion of bubbles from the top surface of the simulator
which is where the updraft hole Is located. The updraft flow still dominates the
circulating flow.
'^mm
Figure 4-1: Start of Vortex Formation (a=0.5)
30
ism l^ssJS:^^n;^''.:2..i,^a»^^;^>^;^>y-';t^ mmm MM ^TJ'jC^Sg^S^^^^I^^SS ***^^^*'^*^'^^i»i7iM?5S*ii^li'SiRfii
HP M%*PW*< S8P
i
Figure 4-2: Progression of Vortex Formation (a=0.5)
Figure 4-3: Single-Celled Vortex (a=1, Low Swirl, s=1.51)
31
As the swiri ratio is increased, the vortex propagates downward toward the
surface of the tornado simulator, eventually reaching the surface. Figure 4-2
shows the progression as the swiri ratio increases and the vortex moves from the
updraft hole to the surface of the simulator. Figure 4-1 and Figure 4-2 are shown
at an aspect ratio of a=0.5; this type of vortex is called a single-celled vortex.
Figure 4-3 shows a single-celled vortex at an aspect ratio a=1. The Inner core
diameter of these single-celled vortices ranged from ten to fifteen millimeters in
width as shown In Figure 4-4 and Figure 4-5 for the aspect ratio of one-half and
one, respectively. (Please note that the ruler scale is inches.) Figure 4-6 shows
two different vortices, each at an aspect ratio of one-half and low swiri ratio,
visualized with helium bubbles and fog from a fog generator. Notice that the
vortex of bubbles has a much smaller diameter that the vortex of smoke. This is
due to the fact that the bubbles converge to the inner vortex core and the smoke
defines the outer region of the vortex core which has an estimated diameter of
approximately 50mm.
Figure 4-4: Scale of Inner Cone of Single-celled Vortex (a=0.5. Low Swirl, s=2.23)
32
'• , O L . -'U^'l
, - _ _ ,
Figure 4-5: Scale of Single-Celled Vortex (a=1, Low Swirl, s=1.51
Figure 4-6: Vortex with Helium Bubbles and Smoke (a=0.5, Low Swirl, s=2.23)
As the swiri ratio is increased, i.e., the ratio of the tangential flow rate to
the updraft flow rate increases, the tangential flow rate starts to have a
dominating effect on the updraft flow rate meaning that the tangential flow rate
has more of an influence on the flow field than the updraft flow rate. When this
happens, an adverse pressure gradient starts to occur spawning the 'breakdown
bubble' at the top of the vortex. This 'bubble' forms the boundary between the
supercritical flow upstream and the subcritical flow downstream. The
supercritical flow upstream of the breakdown bubble is very similar. In
appearance, to the single-celled vortex described eariier. However, the 33
subcritical flow downstream is tripped by the bubble and appears to be turbulent.
As the tangential flow rate is increased, the bubble moves down the vortex
toward the surface of the simulator. This causes a deceleration In the axial
direction of the vortex inner core and eventually, an actual down draft in the very
center of the vortex. This central down flow region is surrounded by the vertical
vorticity of upflow. When the breakdown bubble reaches the surface of the
simulator, the core of the vortex expands radially and the down flow in the central
core penetrates to the surface. When the combination of the updraft and
downdraft are each present at the same time, and the breakdown bubble has
penetrated the vortex core to the surface of the simulator, the vortex Is defined as
two-celled. Figure 4-7 shows a two-celled vortex.
Figure4-7: Two-Celled Vortex (a=1. High Swirl, s=6.72)
A two-celled vortex is much larger in diameter that a single-celled vortex.
In the TTU tornado simulator, the two-celled vortices generally ranged from 80 to
100 millimeters as shown In Figure 4-8. (Note that the scale on the ruler Is
measured In Inches.)
34
Figure 4-8: Scale of Two-Celled Vortex (a=0.5, High Swirl, s=8.03)
Another interesting study with flow visualization Is the Interaction of the
fluid flow field with structures. Two generic structures, one a 30 millimeter cube
and the other a 30 millimeter cylinder were place In different positions Inside the
tornado simulator. These structures combined with the flow visualization material
provided Images of the fluid-structure interaction in the fluid flow field inside the
TTU tornado vortex simulator. In all Images, the helium bubbles were introduced
on either side of the cube. Figure 4-9 shows the cubical model in the center of
the TTU tornado vortex simulator at low swiri and an aspect ratio of one-half.
Figure 4-10 and Figure 4-11 show the same cubical model In the center of the
tornado simulator at low swiri and an aspect ratio of one. These Image shows
that the single-celled vortex is severely disrupted by the placement of the cube.
The single-celled vortex is evident in all three figures, but it is disrupted by the
sharp edges on the cube.
35
Figure 4-9: Flow Visualization with Cube in Center (a=0.5, Low Swirl, s=2.23)
Figure 4-10: Flow Visualization with Cube in Center (a=1, Low Swirl, s=1.51)
36
Figure 4-11: Flow Visualization with Cube in Center (a=1, Low Swirl, s=1.51)
Figure 4-12 and Figure 4-13 show the cubical model in the tornado simulator
slightly off center (0.5*ro to center of cube) at low swiri, s=2.23, and an aspect
ratio of one-half. These images show that there is a definite single-celled vortex
present even with the disruption in flow caused by the cubical model.
37
Figure 4-12: Flow Visualization with Cube Offset (a=0.5, Low Swirl, s=2.23)
Figure 4-13: Flow Visualization with Cube Offset (a=0.5. Low Swirl, s=2.23)
38
Figure4-14: Flow Visualization with Cube Offset (a=1, Low Swirl, s=1.51)
Figure 4-14 shows four different positions of the cubical model In the
tornado simulator at an aspect ratio of one and low swiri. As the cubical model
moves away from the center of the tornado simulator, more of the helium bubbles
are swept up through the updraft hole without being caught in the vortex that Is
formed (note that the bubble nozzles are also moving). As the cubical model
gets closer to the center of the tornado simulator, more of the bubbles are pulled
into the visualized vortex. This is due to the fact that the farther away from the
center of the tornado simulator In the convergence region the cube gets, the
more the flow behaves like boundary layer flow. This is because the radial flow
component of velocity greatly supercedes that of the tangential flow component
of velocity at low swiri ratios. At high swiri ratios, when the vortex is considered 39
two-celled. The vortex does not seem to be neariy as disrupted by the cubical
model as when the vortex is single-celled (Figure 4-15 and 4-16). This Is
because the tangential flow component dominates over the radial flow
component for the high swiri case. This means that the larger vortex Is being
more influenced by the tangential velocity component than the radial velocity
component of the fluid flow.
Figure 4-15: Flow Visualization with Cube in Center (a=0.5. High Swirl, s=8.03)
When the cube Is offset as in Figure 4-17, Figure 4-18, and Figure 4-19
the helium bubbles are caught in the tangential flow rather than the radial flow.
These figures show that In the high swiri case, the vortex Is not very disrupted by
the presence of the model.
40
Figure 4-16: Flow Visualization with Cube in Center (a=1. High Swirl, s=6.72)
Figure 4-17: Flow Visualization with Cube Offset (a=0.5, High Swirl, s=8.03)
41
Figure 4-18: Flow Visualization with Cube Offset (a=0.5. High Swirl, s=8.03)
Figure 4-19: Flow Visualization with Cube Offset (a=1, High Swirl, s=6.72)
42
Another Interesting model to observe along with the flow visualization
material In the TTU tornado simulator is a generic model of a cylinder. Due to Its
circular geometry, the fluid flow field around the cylinder is more uniform than the
flow field around the cubical model. When the cylinder is in the center of the
tornado simulator, a vortex continues to form above the cylinder for both high and
low swiri ratio cases. The vortices that form above the cylinder are much more
consistent with the vortices formed when no model Is present than the vortices
that form above the cubical model. Figure 4-20 and Figure 4-21 show the
cylinder In the center of the tornado simulator at low swiri and high swiri,
respectively. It Is cleariy evident that the vortices above the cylinder are not
neariy as disrupted as the vortices above the cubical model.
Figure 4-20: Flow Visualization with Cylinder in Center (a=1. Low Swirl, s=1.51)
43
Figure 4-21: Flow Visualization with Cylinder in Center (a=1, High Swirl, s=6.72)
The flow around the cylinder also encounters little disruption when the
cylinder is offset from the center (Figure 4-22 and Figure 4-23). The vortical flow
sweeps the bubbles that are on the outside of the cylinder around the cylinder
following the cylinder's circular profile and into the vortex Itself. This happens for
low and high swiri ratio, but is much more pronounced in the high swiri ratio case.
44
Figure 4-22: Flow Visualization with Cylinder Offset (a=1, Low Swirl, s=1.51)
Figure 4-23: Flow Visualization with Cylinder Offset (a=1, High Swirl, s=6.72)
45
4.2 Pressure Measurements
4.2.1 Models
As used with the flow visualization, two generic models were constructed
to study the fluid-structure interaction in the Texas Tech University Tornado
Vortex Simulator. One Is a thirty millimeter cube with a flat roof and the other is a
thirty millimeter in diameter cylinder (Figure 4-24). The cube had a total of sixty-
four pressure taps distributed across its faces, 16 evenly distributed on the top
face and 12 on each side. The cylinder had a total of 39 pressure taps, seven on
the top and 32 arranged In vertical rows of four every 45 degrees around the
perimeter of the cylinder. The pressure taps on each model consisted of 1.02mm
Internal diameter tubing that was 200mm long. This tubing was glued from the
inside In the holes drilled Into the models and any excess tubing protruding from
the models was carefully trimmed off with a razor blade.
30mm Cube
Figure 4-24: Schematic of Model Cube and Cylinder
In order to compare the models tested with actual full-size buildings, a
scale ratio has to be obtained. One parameter that Is known from the flow
visualization is the inner core diameter of the vortices. According to Church [7],
the core diameter for a typical F2 tornado is estimated to range between 10 and
150 meters. Using this range coupled with the fact that the TTU tornado vortex
simulator produces vortices ranging on average from 12.5 mm for the single-
46
celled vortex to 90 mm for the two celled vortex, a scale factor range can be
calculated (Table 4-1). At the low-scale range, the model of the cube and
cylinder correspond to a structure similar in size to a storm shelter at 3.33m by
3.33m. At the high-scale range, the models correspond to a tremendous 360m
by 360m structure.
Table 4-1: Scale Factors Between Simulated Vortex and Actual Vortex
TTU Tomado Vortex Simulator Vortex Diameter (m)
0.0125-0.090
Typical F2 Tomado in Nature (m)
10-150
Scale
-1:100-1:10000
4.2.2 Orientation
The orientation of the models with respect to the vortex must be
established for both the stationary and moving tests when testing the two models
to obtain the pressure distribution. The sides on each model were labeled north,
south, east, west, and top, and the models were positioned In the simulator
consistently each time tests were taken. The model was always positioned
starting on the north side of the simulator with the north face of each model
facing north. The pressure tests were performed with the models advancing from
north to south. Pressure data was obtained for model locations between +2.0*ro
of the updraft hole, with a total of thirty-one positions. A schematic of the surface
of the tornado simulator is shown in Figure 4-25, and the different positions that
the models were tested at are shown in Figure 4-26. Note that more locations
near the center of the simulator were used.
47
North
West East
South
Figure 4-25: Schematic of Surface of Tornado Simulator
48
West
Position North
South
East
Figure 4-26: Positions where Models were Tested in Simulator
4.3 Stationary Force Distributions
The cubical and cylindrical models were positioned at each of the 31
positions described above for each aspect ratio and swiri ratio. Through these
tests, differential pressures were obtained for each of the pressure taps on the
two models. Pressure coefficients are reported for most tornado vortex simulator
studies. Pressure coefficients are dimensionless due to the fact that a differential
pressure Is divided by some dynamic reference pressure. Originally, for this
study, pressure coefficients were to be used. The reference pressure that would
non-dimensionalize the differential pressure was obtained by use of a pitot static
tube placed at the center of the updraft hole facing downward toward the surface
of the tornado simulator. This configuration gave valid results when the swiri was
very low, but as the swiri increased, downdrafts started to occur at the center of
the updraft hole that resulted In negative pressures as measured by the pitot
static tube (e.g., not the dynamic pressure).
Due to the fact that the dimensionless pressure coefficient was Invalid,
force coefficient calculations were obtained for each pressure tap on the models. 49
The force coefficients were generated by non-dlmensionallzing the recorded
differential force with a momentum Integral relation. The force coefficients were
calculated with the equation
c,-'-y (4^1) p^w A
where p is the density of air, w is the axial velocity component at the center of the
updraft hole and A is the frontal area of the model. The F-F^ef term In the
numerator is calculated by the equation
where prPret is the differential pressure obtained with the Scanlvalve pressure
measurement system at each pressure tap minus the differential pressure
obtained with the Scanlvalve pressure measurement system on the surface of
the simulator at each point studied, and /4,is the small area associated with each
pressure tap. The surface pressure profiles (gage pressures) relative to the local
atmospheric conditions used in calculations are shown In the following figures
(Figure 4-27, Figure 4-28, Figure 4-29, and Figure 4-30).
50
Figure 4-27: Surface Pressure Profile for a=0.5, s=2.23
-50 -
-100 -
i^-150
"-200
-250
-300
-350 -2
1
.
-
1
1 1 1
~~^^--..^^
\ \
1 1 1
^-^^ y
• 1
^
1
-1.5 -0.5 0 r/ro
0.5 1.5
Figure 4-28: Surface Pressure Profile for a=0.5, s=8.03
51
Figure 4-29: Surface Pressure Profile for a=1, s=1.51
Figure 4-30: Surface Pressure Profile for a=1, s=6.72
52
For the cubical model, the top face area was divided Into sixteen equal
areas of 56.25mm^ each and each side area was divided Into twelve equal areas
of 75mm^ each. Likewise, for the cylinder, the body was divided into thirty-two
equal areas of 88.36mm^ each, and the top face area was divided Into seven
different regions of 80mm^ each. Using the calculated areas, the product of each
gage pressure calculated at each pressure tap and the corresponding calculated
area was obtained. This produced a force (in Newtons) at each finite area on the
two different models. To non-dimensionalize this force, a momentum Integral
relation was needed. The most accurate velocity measurement which could be
obtained and was repeatable was the updraft hole velocity at its center point.
Using the updraft velocity at each swiri ratio and aspect ratio, the area of the
frontal face of each model, and the density of air, a reference force was
calculated that would non-dimensionalize the directly calculated forces on the
models. These force coefficients were plotted using MatLab on contour plots. It
should be noted that the contour plots represent the area on the models bounded
by the pressure taps, not the actual area of the model. It Is also notable that the
vortex rotates In a counter-clockwise direction throughout all tests. The flow field
created consists of three main components, tangential, radial, and axial. The
tangential component of the flow field Is the flow that is rotating In a horizontal
plane around the model. The radial component of the flow field Is the flow
coming in from the sides of the simulator In a vertical plane. The axial
component of flow is the flow moving up through the convection region of the
simulator. It should also be noted that a representative selection of figures has
been chosen for these results and discussion. Appendix A contains figures for
each position described at each swiri and aspect ratio.
4.3.1 Cube Model
Contour plots of force coefficients as a function of position on the cubical
model were generated for this study. Each figure will contain a set of five
different contour plots that correspond to each face of the cube in an exploded
view. Figure 4-31 shows a map of where each face is plotted In the exploded
53
view of the cube. This is held constant throughout all figures and also, the word
vortex has been added to each figure representing the orientation of the model
with respect to the vortex center. It should be noted that each face Is rotated
outward on Its adjacent axis with the top face for the exploded view of the cube
and that the contour plots generated are bounded by the matrix of pressure taps
on the cube.
North Face
Vortex Rotation Direction
infest Face Top Face East Face
South Face
Figure 4-31: Exploded View of Cubical Model
Figure 4-32 and Figure 4-33 show the exploded view of the cube with
contour plots of force coefficients for each face at position 1 (2.0*ro) and 6
(1.0*ro), respectively, in the simulator at a=0.5, s=2.23. At these locations In the
simulator, the greatest force coefficients on the cube are on the side faces, with
the north face having the greatest force coefficients In magnitude. In the case of
low aspect ratio, a=0.5, it can be seen that the values on the north face of the
cube are greater In magnitude than the values on the other faces of the cube.
This could indicate a possible separation of flow from the cube at the top, east
54
and west faces. The force coefficients on these specific model plots are of the
same pattern as force coefficients on model plots that are in a boundary layer
type flow with positive pressures on the north face, much lower pressure along
the top face, and slightly lower pressures along the south face. This Implies that
the flow around the cubical model is dominated by the radial flow coming in from
the outside of the simulator rather than the tangential flow rotating inside the
simulator In a horizontal plane Indicating that the radial flow velocity component
has the greater contribution to the pressure distribution on the cubical model than
the tangential flow. Although the magnitudes of the values on each of the side
faces for the high aspect ratio, a=1, cases are closer to being equal to each other
than the values for the a=0.5 case, the same type of patterns exist on the high
aspect ratio case, a=1, as did on the low aspect ratio case, a=0.5, indicating
again, a possible pattern of force coefficients that look like the pattern for
boundary layer type flow force coefficients on a similar model. This Is a very
typical pattern of force coefficient on the cube when the cube Is positioned
greater than 0.5*ro from the center of the simulator In the low swiri case (s=2.23
and s=1.51), and occurs when the cube Is positioned on either side of the
simulator, the north or the south side. It should be noted at this point that some
of the figures showing contour plots of force coefficients on the cubical model
with all positive force coefficients. This Is due to the fact that the local surface
pressure (measured without a model present) Is lower than the pressure
obtained at any position on the cube. A possible reason for this is that the flow is
disrupted In such a way that the local velocity Is smaller when the model is
present in the simulator than when the model Is not present. This lower local
velocity would cause a greater local static pressure and therefore the calculated
pressure coefficients would be positive rather than negative.
55
Figure 4-32: Force Coefficients on Cube at 2.0*ro (Point 1) in Simulator (a=0.5, s=2.23)
56
XlO'
3
2.5
2
1.5
\ o
/ i/
Figure 4-33: Force Coefficients on Cube at 1.0*ro (Point 6) in Simulator (a=0.5, s=2.23)
Figure 4-34 and Figure 4-35 show the exploded view of the cube with
contour plots of force coefficients for each face at position 1 (2.0*ro) at a=0.5 and
s=8.03 and at point 10 (0.125*ro) in the simulator at a=1 and s=6.72, respectively.
Both plots show the greatest force coefficients acting on the north and east faces
of the cube. This would indicate a combination of radial and tangential flow
components contribute significantly to the forces acting on the cube. The lower
force coefficients on the south and west faces indicate a possible separation from
the north-west and south-east corners of the cube. This pattern of force
coefficient on the cube is the same for the high swiri (s=8.03 and s=6.72) cases
from when the cube Is between 2.0*ro and 0.5*ro from the center of the simulator.
This pattern of force coefficients is also present when the cube is at a=0.5,
s=2.23 and a=1, s=1.51 at 0.25*ro in the tornado simulator. The force
coefficients on the cube at these two lower swiri ratios when the cube Is much
closer to the vortex in the tornado simulator behave much like the force
57
coefficients on the cube at higher swiri ratios when the cube Is much farther away
from vortex. This Is due to the fact that the circulation Is much greater In the high
swiri (s=8.03 and s=6.72) cases than in the low swiri (s=2.23 and s=1.51) cases
causing the flow field to rotate less at the outer regions of the simulator In the low
swiri (s=2.23 and s=1.51) cases than It does in the high swiri (s=8.03 and s=6.72)
cases. As a consequence, the cube must be much closer to the vortex,
approximately 0.25*ro away from the center of the simulator. In the low swiri
(s=2.23 and s=1.51) cases to experience the same force coefficients exerted on
it during the high swiri (s=8.03 and s=6.72) cases when it Is much farther away
from the vortex, approximately 0.5-2.0*ro away from the center of the simulator.
It should be noted that the plots shown are for the cube positioned on the north
side of the vortex. When the cube Is positioned the same distance away on the
south side, the faces with the greatest force coefficients acting on them will be
the mirror Image. Therefore, since these plots show the greatest force
coefficients acting on the north and east faces of the cube with possible
separations at the northwest and southeast corners of the cube, the plots of the
cube when it Is oriented on the south side of the vortex show the greatest force
coefficients acting on the south and west faces of the cube with possible
separations at the northwest and southeast corners of the cube.
58
3
2.5
2
1.5
°^y/ — 0 . 5 6 - ^
\ /y
o
y^
3
2.5
2
1.5
1
V 1 \yc y ^0.74-
/
y /p.T^s;^
Figure 4-34: Force Coefficients on Cube at 2.0% (Point 1) in Simulator (a=0.5, s=8.03)
2.5
2
1.5
1
--0 05 / /
- - 0 . 1 - ^ ^
V
/
•':5-^
^ . 1 5 "
xlO-
rf 1 ] /^^
z '
r
— 1 -
A''
\ \ \ \ \'<' VKA \ ioXX ^-^S \ —oS /
::—-^-^
/ n , V c.'
s\ \ \ \ \
/ <o
\ 9'l
\\]
N ^ /
" / \ / ^
yyy^ ^ - -0.1 • — ^ -0.15
2 Vortex
Figure 4-35: Force Coefficients on Cube at 0.5*ro (Point 10) in Simulator (a=1, s=6.72)
59
Figure 4-36 and Figure 4-37 show the contour plots of force coefficients
along each face of the cube at 0.25*ro (Point 12) in the simulator at a=0.5 and
s=8.03 and at 0.125*ro (point 18) at a=0.5 and s=2.23, respectively. These plots
are of the cube at different swiri ratios and on different sides of the vortex, but
they show the same pattern of force coefficients emerging on opposite sides of
the cube. In each case, the greatest force coefficients now occur on the east and
west faces of the cube for the s=8.03 and s=2.23 cases, respectively. For the
case when the cube is at 0.25*ro (point 12) In the simulator (cube on the north
side of the vortex), the greatest force coefficients are located on the east face of
the cube and all other faces experience significantly lower force coefficients,
possibly due to flow separation on these faces of the cube. For the case when
the cube Is at 0.125*ro (point 18) in the simulator (cube on the south side of the
vortex), the greatest force coefficients are located on the west face of the cube
and all other faces experience significantly lower force coefficients, possibly due
to flow separation on these faces of the cube. This is a force coefficient pattern
which looks very similar, as before, to the pattern of force coefficients created on
a similar model In a boundary layer-type flow, only now the tangential component
of the flow seems to be causing the boundary layer-type flow pattern of force
coefficients rather than the radial component of the flow. This Is due to the fact
that the flow regime rotating around the center of the simulator moves faster the
closer to the center of the simulator it gets due to angular acceleration, so the
tangential flow component contacting the model as the model gets closer to the
center of the simulator is greater than the radial flow component contacting the
model. This causes the model to experience greater force coefficients on the
east and west sides depending on which side of the vortex it is on rather than the
north and south sides.
This phenomenon of force coefficient patterns which look like patterns on
similar models in boundary layer-type flow experienced by the east and west
faces of the cube Is seen in a very small region of the simulator. For the low swiri
cases (s=2.23 and s=1.51), it is seen only when the cubical model is
60
approximately 0.125*ro from the center of the simulator, and for the high swiri
cases (s=8.03 and s=6.72), it Is seen only when the model is approximately
0.25*ro to 0.125*ro from the center of the simulator. At these points, the model is
just outside of the inner core vortex formation for each respective case, a point at
which the tangential velocity component Is maximum.
xlO^
y"'S^ / ^ ^ ^
3r
2.5
2
1.5
1
<
Figure 4-36: Force Coefficients on Cube at 0.25*ro (Point 12) in Simulator (a=0.5, s=8.03)
61
\, ''
Vortex
- O . A ' ^ ' " '
•• T A
\ \ °- ^ r / / W
^^ '-'^yi \ ^ ^
x10'
Figure 4-37: Force Coefficients on Cube at 0.125*ro (Point 18) in Simulator (a=0.5, s=2.23)
Figure 4-38 and Figure 4-39 show the contour plots of the force
coefficients on the cube at 0.0625*ro (point 15) In the simulator at a=0.5 and
s=2.23 and at the center of the simulator at a=1, s=6.72, respectively. At this
points, the cube Is very close to if not partially consumed by the actual tornado
vortex in the low swiri (s=2.23 and s=1.51) or single-celled vortex case and
partially consumed by the tornado vortex in the high swiri (s=8.03 and s=6.72) or
two-celled vortex case. Due to its proximity to the actual vortex, the cube
undergoes forces exerted by a very complex flow field. The side faces of the
cube all have positive force coefficients that are of comparable magnitude in
each respective case. The top face of the cube in each of the low swiri (s=2.23
and s=1.51) cases have negative force coefficients acting on them when the
cube is 0.0625*ro away from the center of the simulator. In the high swiri cases
(s=8.03 and s=6.72) when the cube is 0.0625*ro away from the center of the
simulator and In all cases when the cube Is in the center of the simulator, the top
62
faces have positive coefficients acting on them. This Is Indicative of the fact that
the local surface pressure is lower than the local pressures exerted on the
cubical model at this point In the simulator, so the cube is creating a blockage
effect in the flow field. The flow field at these points In the simulator is very
complex and is very disrupted by the model's presence. At the present time, the
flow field at these points can not be quantified due to lack of experimental
equipment and data.
3
2.5
2
1.5
1
yy^y ^,^0.75
-0.55
x10^
~~ 0.75X
\ ^ >
o o of.
/ <ra p
^^y^ - ' ^ ^ ' ' ' ^
/ / •
<o o'
-y
Figure 4-38: Force Coefficients on Cube at 0.0625*ro (Point 15) in Simulator (a=0.5, s=2.23)
63
3
2.5
2
1.5
1
^ .<^
/
.y ^tc^
' 3 . 5 ~ ^ ^
3 — ^
\
\ ) ^ / •cn /
M
" \ \ \ j ?
x10'
Figure 4-39: Force Coefficients on Cube at Center (Point 16) of Simulator (a=1, s=6.72)
64
4.3.2 Cvlinder Model
Contour plots of force coefficients as a function of position on the
cylindrical model have also been generated for this study. Each figure will
contain a set of three different contour plots that correspond to the top face of the
cylinder and the body of the cylinder. Figure 4-40 shows a map of the exploded
faces of the cylinder. This is held constant throughout all figures. For the top of
the cylinder, plots are of the force coefficient as a function of position on the
cylinder connected with a smooth line. The top left plot will be of the three taps
on the cylinder top face along the west-to-east center-line and the top right plot
will be of the five taps along the north-to-south center line.
South
East West
North
East North West South
Figure 4-40: Schematic of Exploded Cylinder
Points 1 (2.0*ro), 6 (I.OVo), 10 (0.5*ro) and 12 (0.25%) are shown in
Figures 4-41, 4-42, 4-43, and 4-44, respectively. The aspect swiri ratios are a=1
and s=6.72, a=0.5 and s=8.03, a=0.5 and s=2.23, and a=1 and s=1.51,
respectively. Due to the cylinder's geometry, each case of swiri and aspect ratio
65
behaves much the same when the cylinder Is positioned between 2.0*ro and
0.25*ro from the center of the simulator. The major difference In each case Is the
magnitude of the force coefficients. All cases show that there are positive force
coefficients on the cylinder. The greatest force coefficients occur on the north
east portion of the cylinder for all cases. The roof of the cylinder experiences
very small force coefficients on the entire face for each case shown. The force
coefficients decrease In a uniform manner as they progress around the cylinder
starting at the northeastern portion of the cylinder. Also, the flow around the
cylinder is approximately uniform in the northwest and southeast portions of the
cylinder Indicating a flow pattern similar to the pattern on a similar model in a
boundary layer flow. The coefficients are all positive due to the fact that the local
surface pressure measured without any model present is lower than the local
pressure recorded on the cylinder, and possibly that the local velocities are
disrupted In such a way that they are lower when the model is in the simulator,
thus increasing the local pressure.
As found by Holroyd using oil storage tanks [37], the force coefficient
pattern Induced on each of these figures Is strongly Indicative of the helical air
flow induced by a horseshoe vortex. This horseshoe vortex consists of a very
strong circumferential component and a weaker downward flow [37], The
stagnation point for each figure lies on the northeastern portion of the cylinder.
The region of minimum pressure lies between the southwest and the northeast
portions of the cylinder. This minimum pressure point could Indicate a separation
of flow from the cylinder.
66
XlO''
5 0
Vortex
Figure 4-41: Force Coefficients on Cylinder at 2.0% (Point 1) in Simulator (a=1, s=6.72)
5 0
Vortex
Figure 4-42: Force Coefficients on Cylinder at 1.0*ro (Point 6) in Simulator (a=0.5, s=8.03)
67
XlO' 3
2
1
C 0
-1
-2
-3 1
W
1.5 2.5
3.5 •
3 •
2.5 •
2 •
1.5 •
- \
_
1
% 1 \ /
v
IN O
~ i 1
c
M \ I
I \ CO I \ tJl I
-0.3
-
I / / / /
/ /
0.4-
/ P
' -y ,
* =)
1 1 1
CO I d 1 0.35
1
o > O. /
1 / ^
y ^ ^
,
\ y
/ , /
W Vortex
Figure 4-43: Force Coefficients on Cylinder at 0.5*ro (Point 10) in Simulator (a=0.5, s=2.23)
3
2
1
5 0
-1
-2
-3 1 W
1.5 2.5
Vortex
Figure 4-44: Force Coefficients on Cylinder at 0.25% (Point 12) in Simulator (a=1, s=1.51)
68
Figure 4-45 and Figure 4-46 show the force coefficients on the cylinder at
a=0.5 and s=2.23 and at a=1 and s=6.72, respectively, at 0.125*ro (point 14) In
the tornado simulator, but they are representative of all cases of aspect ratio and
swiri ratio when the cylinder is at this location in the tornado simulator. The
greatest force coefficients recorded on the cylinder continue to be in the north
eastern portion of the cylinder. Although the horseshoe vortex continues to be
indicated by these plots, the force coefficients on the southeast portion of the
cylinder are now greater than the force coefficients on the northwest portion of
the cylinder. This indicates that the vortex formed In the tornado simulator is
having significant impact on the flow field around the cylinder. The flow that Is
converging toward the vortex Is causing greater forces on the southeast portion
of the cylinder because the tangential flow is moving from east to west, and the
cylinder Is north of the vortex. This causes a possible flow separation from the
cylinder on the northwest side while not separating from the south-east side of
the cylinder but separating more towards the east side of the cylinder. Also, the
top face has an increased force coefficient on the west side Indicating a possible
flow separation from the top of the cylinder on the northeast side and
reattachment or slight reattachment to the cylinder on the west portion of the top
face. These contour plots also show the force coefficients starting to distribute
themselves more around the cylinder rather than vertically on the cylinder. At
these points in the simulator, the cylinder Is very close to the visualized vortex If
not touching the visualized inner vortex.
69
4
35
3
25
2
1.5
o 'p-
\ \ A \ \
^
\
1 T
/
— 0.4
'[] /
i \
-
/ 6 W
Vortex
Figure 4-45: Force Coefficients on Cylinder at 0.125*ro (Point 14) in Simulator (a=0.5, s=2.23)
x10'
O 0
Vortex
Figure 4-46: Force Coefficients on Cylinder at 0.125% (Point 14) in Simulator (a=1, s=6.72)
70
Force distributions on the cylinder at the center of the tornado vortex
simulator (point 16) at a=0.5, s=2.23 and a=1, s=6.72 and at 0.0625% (point 17)
at a=0.5, s=8.03 and a=1, s=1.51 are shown in Figures 4-47, 4-48, 4-49, and 4-
50, respectively. The greatest force coefficients acting on the cylinder at the
center of the simulator are In the north-east region of the cylinder for all cases,
but they are toward the west portion of the cylinder for 0.0625*ro (point 17). The
greatest forces are toward the west portion of the simulator at point 17 due to the
fact that at this point, the cylinder Is located on the south side of the vortex.
At the center of the simulator, in the low swiri cases (s=2.23 and s=1.51),
the force coefficient distribution propagates around the cylinder from greatest to
least force. The lowest force coefficients are found at the base of the cylinder
while the greatest force coefficients are found near the top of the cylinder. The
top In the low swiri cases have great suction forces which are fairiy evenly
distributed acting on them. For the case of high swiri (s=8.03 and s=6.72), the
force coefficients on the cylinder propagate around the cylinder from greatest to
least starting at the north-east region of the cylinder. Where the force
coefficients In the low swiri case were increasing from base to top of cylinder, the
forces In the high swiri case were much more evenly distributed from top to
bottom, and went from greatest to least force as these vertical asymptotes
propagated around the cylinder.
Force coefficients on the cylinder at 0.0625*ro (point 17) show the greatest
force coefficient on the western portion of the cylinder for the s=2.23 and s=1.51
cases, and the greatest force coefficients on the south-western portion of the
cylinder for the s=8.03 and s=6.72 cases. At this point, the cylinder Is positioned
very close to the center of the simulator, and the force coefficients on the body of
the cylinder are close to equal at different heights all the way around the cylinder.
The force coefficients also are lowest at the bottom portion of the cylinder and
Increase as the cylinder Increases vertically. The force coefficients on the roof of
71
each cylinder are negative except for the s=6.72 case, where the force
coefficients are positive on the roof of the model.
Figure 4-47: Force Coefficients on Cylinder at Center (Point 16) in Simulator (a=0.5, s=2.23)
72
Figure 4-48: Force Coefficients on Cylinder at Center (Point 16) in Simulator (a=1, s=6.72)
x10'
5 0
Figure 4-49: Force Coefficients on Cylinder at 0.0625*ro (Point 17) in Simulator (a=0.5, s=8.03)
73
xlC
Figure 4-50: Force Coefficients on Cylinder at 0.0625% (Point 17) in Simulator (a=1, s=1.51)
The remainder of the positions tested in the simulator with the cylindrical
model behaved similariy to the positions shown above. They are different only in
the fact that all of the figures shown, with the exception of the 0.0625*ro (point
17) figures, were shown with the cylinder on the north side of the vortex. If the
cylinder is located on the south side of the vortex, the greatest and least force
coefficient values will be on the opposite side of the cylinder at each respective
position tested. These figures are included in Appendix A.
74
4.4 Transient Analvsis Since tornadoes do not remain stationary In nature, a transient analysis
was also performed on the cubical and cylindrical models In the tornado
simulator to compare the force distribution from the stationary tests to determine
if the added complexity of a dynamic test is needed. According to Wang [32], the
rotational speed of a typical Fujita scale F2 tornado Is estimated to range
anywhere from 50-70m/s. Using single-channel hot film anemometry, the
rotating speed of the vortices at eaves height in the TTU TVS II ranges between
3-17m/s. Using these results and applying kinematic similarity, a scale factor
was calculated and is presented in Table 4-2.
Table 4-2: Velocity Scale Factor Range in TTU TVS 11
Rotating Speed in TTU TVS II (m/s)
3-17
Rotating Speed F2 tomado (m/s)
50-70
Scale Factor
2.9-23
Also, according to Wang [32], the translational speed of a typical F2
tornado is estimated to be between 0 and 13m/s. Using a mid value for velocity
scale factor of 13m/s, the moving tests scale to a 9.75m/s translation speed of a
natural tornado when a translation speed of 0.75m/s Is used in the TTU TVS II.
In order to take these measurements, the ZOCEIM scanning module was
employed. The ZOCEIM was connected to a switch that could be tripped to
record a predetermined voltage. Using this data combined with the fact that the
Scanlvalve system was set up to record data at 300Hz and the knowledge that
the model would move a predetermined distance, the model was pulled through
the tornado simulator, tripping switch so that the ZOCEIM would record a voltage
at the predetermined starting and stopping points. This gave the number of
frames of data sampled at 300Hz that the model traversed In the given distance.
Using this information, the speed of the model could be obtained. Equation 4.1
was used to obtain the moving force coefficient, and a series of the runs are
plotted traversing the simulator. One representative run of the series has a 6
point moving average trend line added in order to better understand and smooth
75
the data. A 6 point moving average trend line was chosen because it was the
greatest number of points that provided representative data.
4.4.1 Transient Cube
The same cubical model used in the stationary tests In the TTU TVS II
was used for the moving tests performed. Twenty total runs were performed at
each aspect ratio and each swiri ratio with the cube. The average speed of the
twenty runs was calculated and the twelve runs that had the least amount of
deviation from this average were selected for detailed analysis. The greatest
percent error of deviation from the average velocity for each case is shown In
Table 4-3.
Table4-3:
Greatest Percent Error fi-om
Average Velocity
Cube Model Percent Error from Average Velocity
a=0.5, s=low
11.7
a=0.5, s=high
6.3
a=1, s=low
7.2
a=1, s=high
9.1
Several distinctive locations on the cube were selected to be analyzed for
the transient analysis. On the top face of the cube, the leading edge and trailing
edges were picked due to their significance. The leading edge is defined as the
edge that gives the positive dot product of the velocity and the outward normal of
the cube. The trailing edge is the edge that gives the negative dot product of the
velocity and the outward normal of the cube. Since the cubical model does not
have a centeriine of pressure taps, the two center pressure taps on the leading
edge of the top face and the two center taps on the trailing edge of the top face
were averaged to obtain data that would correspond to the center of the leading
and trailing edges. The leading edge of the cube will be defined as the edge
adjacent to the south face of the cube. This edge Is adjacent to the leading face
of the cube as It traverses through the TTU TVS II. The trailing edge Is just the
opposite edge of the leading edge, and Is adjacent to the north face of the cube.
The trailing edge is adjacent to the trailing face of the cube as it traverses
through the TTU TVS II. The two other points to be studied on the cubical model 76
are the center of the south face of the cube and the center of the north face of
the cube. These faces represent the leading and trailing faces of the cube as It
traverses the TTU TVS II.
At an aspect ratio of 0.5 and s=2.23, figures are shown for the leading
edge, the trailing edge, the center of the leading face, and the center of the
trailing face on the cube (Figure 4-51, Figure 4-52, Figure 4-53, and Figure 4-54).
Each figure shows raw data from several of the moving tests, 6-polnt moving
average for a representative moving test series, and the values of the force
coefficients on the cube for the stationary cases as the cube traverses the
simulator. The leading and trailing edges of the roof show that at around
+0.25*ro(a negative ro value Indicates that the model has not passed through the
vortex core, positions 1-15 in Figure 4-27), the force coefficients are lowest,
although the leading edge has a larger force coefficient in magnitude. These
figures show that the values for the moving data are very representative of the
values for the stationary data. The leading, or south, face of the cube shows a
negative force coefficient at approximately -0.25*ro. From this point, the force
coefficient gets larger until the maximum force coefficient Is reached at the center
of the simulator. The force coefficient then decreases until it levels off when the
cube reaches approximately 0.25*ro In the simulator. After leveling off, the force
coefficient is slightly larger than It was at the points before -0.25*ro. The trailing,
or north, face of the cube, as expected, has force coefficients just opposite of the
south face. The force coefficient is fairiy constant until the cube reaches a point
approximately -0.25*ro, when the force coefficient Increases, until it Is at its
maximum at the center of the simulator. At this point, the force coefficient
decreases until the cube Is at approximately 0.25*ro, where the force coefficient
is lowest. The force coefficient then Increases again until the cube Is at
approximately 0.5*ro where the force coefficients level off to an approximate
constant level, just lower in magnitude than the force coefficients on the trailing
face before the cube Is at -0.25*ro. These figures show that although the data for
the moving tests have slightly different magnitudes than the data for the
77
stationary tests, the stationary tests show the same trends as the moving tests
for all cases. However, the transient data can be larger than either the moving
average or the stationary data by as much as 50 percent (Figures 4-53 and 4-
54).
Stationary
-Moving Average
Figure 4-51: Force Coefficients at Leading Edge on Roof of Cube (a=0.5, s=2.23)
78
•Moving Average
Stationary
Figure 4-52: Force Coefficients on Trailing Edge on Roof of Cube (a=0.5, s=2.23)
•Moving Average
Stationary
Figure 4-53' Force Coefficients on Center of Leading Face of Cube (a=0.5, s=2.23) 79
•Moving Average
Stationary
Figure 4-54: Force Coefficients on Center of Trailing Face of Cube (a=0.5, s=2.23)
Figure 4-55, Figure 4-56, Figure 4-57, and Figure 4-58 show the leading
edge, trailing edge, leading face, and trailing face force coefficients, respectively,
of the cube at a=0.5 and s=8.03. The leading and trailing edges of the roof show
very similar patterns for this case as they did in the s=2.23 case, but the
magnitudes are much greater. They both show that there Is a large force
coefficient at the center of the simulator. Also, both figures show that the lowest
force coefficients occur at approximately +0.25*ro, but rather than abruptly
Increasing to positive values, the force coefficients Increase rather slowly,
indicating that the vortex in the s=8.03 case Is larger in diameter than In the
s=2.23 case. The force coefficients on the leading, south, and trailing, north,
faces also are very similar in the s=8.03 case to the s=2.23 case, but they are
much greater In magnitude In the s=8.03 case. These faces also show the
slower decrease and Increase around the +0.25ro regions of the simulator
80
Indicating once again a larger vortex diameter. These figures also show, as the
figures before did, that the data from the moving tests closely corresponds with
the data from the stationary tests.
Stationary
•Moving Average
Figure 4-55: Force Coefficients on Leading Edge of Roof of Cube (a=0.5, s=8.03)
81
Figure 4-56: Force Coefficients on Trailing Edge of Roof of Cube (a=0.5, s=8.03)
• Stationary
^ ^ M o v i n g Average
r/ro
Figure 4-57: Force Coefficients on Center of Leading Face of Cube (a=0.5, s=8.03)
82
-25
• Stationary
•"••Moving Average
Figure 4-58: Force Coefficients on Center of Trailing Face of Cube (a=0.5, s=8.03)
Force coefficients on the leading edge, trailing edge, leading face, and
trailing face of the cube are shown in Figures 4-59 through 4-62, respectively, for
a=1, s=1.51. The leading and trailing edges on the roof of the cube show a
different trend for a=1 case than they did for the a=0.5 case. They each still have
the largest force coefficient at the center of the simulator, but the leading edge
has an abrupt decrease in force coefficient when the cube reaches approximately
-0.25*ro and the trailing edge has an abrupt decrease in force coefficient when
the cube reaches approximately 0.25*ro where the largest In magnitude force
coefficient Is reported. These trends are similar to the trends seen on the cube
faces rather than edges in the a=0.5 cases. The leading and trailing faces
continue to behave as they did In the previous cases. The windward face, as It
approaches the vortex, has force coefficients that decrease, then Increase, and
then level off just as In previous cases, and the leeward face, as it approaches
the vortex, has force coefficients that Increase, then decrease, then level off just
83
as In previous cases. The moving test data Is representative of the stationary
test data in all cases except for the trailing edge of the cube where the stationary
data Is very different from the moving data. This could be due to a flaw in the
stationary data since the moving data was repeated several times and these
repetitions showed a general trend.
- + * •
" stationary
^ ^ M o v i n g Average
Figure 4-59: Force Coefficients on Leading Edge of Roof of Cube (a=1, s=1.51)
84
- 4 T 5 -
Stationary
•Movmg Average
Figure 4-60: Force Coefficients on Trailing Edge of Roof of Cube (a=1, s=1.51)
Figure 4-61: Force Coefficients on Center of Leading Face of Cube (a=1, s=1.51) 85
stationary
•Moving Average
Figure 4-62: Force Coefficients on Center of Trailing Face of Cube (a=1, s=1.51)
Figure 4-63, Figure 4-64, Figure 4-65, and Figure 4-66 show force
coefficients acting on the cube at leading edge, trailing edge, leading face, and
trailing face, respectively, at a=1 and s=6.72. These figures are much like those
from the a=0.5 and s=2.23 case, only slightly different in magnitude. The
patterns that the force coefficients produced for the a=0.5 and s=2.23 case are
the same as the patterns that the force coefficients produced for the a=1, s=6.72
case. Also, the moving test data is very representative of the stationary test data
at a=1 and s=6.72 for all of the cases shown.
86
- 4 - 1 -
-2.5
Stationary
•Moving Average
Figure 4-63: Force Coefficients on Leading Edge of Roof of Cube (a=1, s=6.72)
stationary
"Moving Average
Figure 4-64: Force Coefficients on Trailing Edge of Roof of Cube (a=1, s=6.72)
87
^^Mov ing Average
• Stationary
Figure 4-65: Force Coefficients on Center of Leading Face of Cube (a=1, s=6.72)
• stationary
^ ^ M o v i n g Average
Figure 4-66* Force Coefficients on Center of Trailing Face of Cube (a=1, s=6.72) 88
The flow regime around the cube behaves in much the same way at each
aspect ratio and each swiri ratio considered with the exception of the roof of the
cube In the a=1 and s=1.51 case. The major difference in the resulting data
when the aspect ratio and swiri ratios are changed Is the magnitude of the data.
These tests also showed that overall, the moving tests data was very
representative of the stationary tests data with the exception being in the a=1 and
s=1.51 case. This indicates that future testing on cubical models can be done In
the stationary case rather than the moving case and yield approximately the
same results, although in some specific cases the results could be off by up to 50
percent, and the same trends. Performing only stationary tests on the cubical
model would greatly simplify the experimental process.
89
4.4.2 Transient Cvlinder
As with the cubical model, the same cylinder model that was used In the
stationary tests In the TTU TVS II was used for the moving tests performed.
Twenty total runs were performed at each aspect ratio and each swiri ratio with
the cylinder. The average speed of the 20 runs was calculated and the 12 runs
that had the least amount of deviation from this average were selected to be
used to analyze force coefficients on the cylinder as it traversed the simulator.
The greatest percent error of deviation from the average velocity for each case Is
shown in Table 4-4.
Table 4-4: Cylinder Model Percent Error from Average Velocity
a=0.5, s=2.23
Greatest Percent Error from
Average Velocity 6.03
a=0.5, s=8.03
3.7
a=1, s=1.51
3.9
a=1, s=6.72
15
Several distinctive locations on the cylinder model were selected to be
analyzed for the transient analysis. On the top face of the cylinder, the leading
edge and trailing edges were picked due to their significance. The leading edge
of the cylinder will be defined as the edge adjacent to the south side of the
cylinder and results In the positive dot product of the velocity and outward normal
direction of the cylinder. This edge is adjacent to the leading side of the cylinder
as It traverses through the TTU TVS II. The trailing edge is just the opposite
edge of the leading edge, and Is adjacent to the north side of the cylinder. The
trailing edge is adjacent to the trailing side of the cylinder as It traverses through
the TTU TVS II. The two other points to be studied on the cylindrical model are
the center of the south side of the cylinder and the center of the north side of the
cylinder. These sides represent the leading and trailing sides of the cylinder as It
traverses the TTU TVS 11.
Figures 4-67 through 4-70 show the force coefficients on the cylinder at
the leading edge, the trailing edge, the leading side, and the trailing side, 90
respectively, for a=0.5 and s=2.23. This case for the cylinder looks much like the
same case for the cube. The leading and trailing edges are fairiy constant, and
at about +0.25*ro the force coefficient decreases to a negative value. At the
center of the simulator, the force coefficient is at its maximum value. The force
coefficient on the leading side of the cylinder decreases slightly at approximately
-0.25*ro and increases to Its maximum value at the center of the simulator. It
then decreases again until the cylinder Is at approximately 0.25*ro where It
seems to level off at a magnitude slightly larger than It was before the cylinder
reached -0.25*ro. The trailing side of the cylinder behaves slightly differently. At
approximately -0.25*ro the force coefficient Increase until It is at a maximum at
the center of the simulator. It then decreases to a minimum value at
approximately 0.25*ro. The force coefficient on the trailing side of the cylinder
then increases slightly until it levels off at a magnitude slightly lower than the
magnitude of the force coefficient on the cylinder before it reached -0.25*ro in the
simulator. The moving test data corresponds to the stationary test data on the
leading edge and leading side of the cylinder, but the stationary test data has a
much lower force coefficient value at 0.25*ro for the trailing edge and trailing side
of the cylinder. Although the magnitudes of the force coefficients are different,
the moving test results continue to follow the general trends of the stationary test
results.
91
stationary
•Moving Average
Figure 4-67: Force Coefficients on Leading Edge of Roof of Cylinder (a=0.5, s=2.23)
Figure 4-68: Force Coefficients on Trailing Edge of Roof of Cylinder (a=0.5, s=2.23) 92
Stationary
•Moving Average
Figure 4-69: Force Coefficients on Leading Side of Cylinder {a=0.5, s=2.23)
stationary
•Moving Average
Figure 4-70" Force Coefficients on Trailing Side of Cylinder (a=0.5, s=2.23)
93
Figure 4-71, Figure 4-72, Figure 4-73, and Figure 4-74 show force
coefficients on the cylinder at the leading edge, the trailing edge, the leading
side, and the trailing side, respectively, for a=0.5 and s=8.03. The leading edge
behaves in the same manner as It did for the s=8.03 case, but the trailing edge
behaves differently. The trailing edge of the cylinder behaves In a manner similar
to the leading side of the cylinder in the a=0.5 and s=2.23 case. The force
coefficient decreases to a minimum level when the cylinder reaches
approximately -0.25*ro and then increases to a maximum at the center of the
simulator. The force coefficient on the trailing edge then decreases as the
cylinder progresses through the simulator until It reaches a point approximately
0.25*ro in the simulator where it levels off. Both the leading and trailing sides
have force coefficients that are fairiy stable until the cylinder reaches
approximately -0.5*ro In the simulator. At this point, both sides experience a
decrease in force coefficient until it reaches a minimum value when the cylinder
Is at approximately -0.25*ro in the simulator. The force coefficient then Increases
to a maximum value when the cylinder is at 0.25*ro, where the force coefficient
then decreases and levels out at a magnitude approximately equal to the
magnitude of the force coefficient before the cylinder reached -0.5*ro. The
leading and trailing sides both experience the same force coefficients due to the
fact that the swiri ratio Is very high, meaning that the tangential component of
flow provides a greater contribution to the flow field than the radial component of
flow. The moving test data Is very close to the stationary test data In the leading
edge and leading side cases, but not in the trailing edge or trailing side of the
cylinder. In the trailing edge and trailing side cases, the stationary tests show a
force coefficient that is much lower than the force coefficient for the moving tests
at approximately 0.25*ro.
94
-2.5 -2
Stationary
•Moving Average
Figure 4-71: Force Coefficients on Leading Edge of Roof of Cylinder (a=0.5, s=8.03)
stationary
•Moving Average
Figure 4-72' Force Coefficients on Trailing Edge of Roof of Cylinder (a=0.5, s=8.03) 95
stationary
•Moving Average
Figure 4-73: Force Coefficients on Leading Side of Cylinder (a=0.5, s=8.03)
Stabonary
•Moving Average
Figure 4-74: Force Coefficients on Trailing Side of Cylinder (a=0.5, s=8.03) 96
Figures 4-75 through 4-78 show force coefficients on the cylinder at the
leading edge, the trailing edge, the leading side, and the trailing side,
respectively, for a=1 and s=1.51. The force coefficients In the leading edge case
follow the trend of the force coefficient decreasing when the cylinder approaches
the point -0.25*ro and then Increasing until the cylinder reaches the center of the
simulator. The force coefficient then decreases until the cylinder reaches
approximately 0.25*ro where the force coefficients start to increase and continue
to Increase until the cylinder reaches a point approximately 0.5*ro where the force
coefficient levels off to a level at approximately the same magnitudes recorded
before the cylinder reaches the -0.25*ro point In the simulator. The leading side
of the cylinder behaves much like the leading side of the cylinder In the a=0.5
and s=8.03 case. The trailing edge and trailing side of the cylinder have force
coefficients that start to decrease slightly at -0.25*ro and then Increases until the
cylinder Is at the center of the simulator. At this point, the force coefficient
decreases until the cylinder reaches approximately 0.25*ro where the force
coefficients Increase and then level out when the cylinder reaches approximately
0.5*ro In the simulator. The moving test data is very different In magnitude from
the stationary test data for each of these cases except for the trailing side case
where the moving test data corresponds to the stationary test data. The overall
trend of the stationary test data loosely matches the trends for the moving test
data.
97
-2:fr-
-4 :5 -
r/ro
Stationary
•Moving Average
Figure 4-75: Force Coefficients on Leading Edge of Roof of Cylinder (a=1, s=1.51)
stationary
•Moving Average
Figure 4-76: Force Coefficients on Trailing Edge of Roof of Cylinder (a=1, s=1.51)
98
2.5 L-
stationary
•Moving Average
Figure 4-77: Force Coefficients on Leading Side of Cylinder (a=1, s=1.51)
-a*-
-4T5-
Z -2.5 -2 -1.5 -1 -0.5
• stationary
^ ^ M o v i n g Average
Figure 4-78: Force Coefficients on Trailing Side of Cylinder (a=1, s=1.51) 99
Figure 4-79, Figure 4-80, Figure 4-81, and Figure 4-82 show the force
coefficients on the cylinder at the leading edge, the trailing edge, the leading
side, and the trailing side, respectively, for a=1 and s=6.72. The leading edge
and leading side of the cylinder show approximately the same trend as the
cylinder traverses the simulator. The force coefficient starts to decrease when
the cylinder reaches approximately -O.SVQ in the simulator until It reaches a
minimum value when the cylinder is approximately -0.25*ro from the center of the
simulator. The force coefficient then Increases until the cylinder reaches the
center of the simulator where the force coefficient starts to decrease. When the
cylinder reaches approximately 0.25*ro In the simulator the magnitude of the
force coefficient levels off to be approximately constant as the cylinder continues
to traverse the simulator. The trailing edge and trailing side of the cylinder
experience force coefficients just opposite of what the leading edge and leading
side of cylinder experienced. The stationary test results show force coefficients
similar to those experienced by the cylinder In the leading and trailing side cases
for a=0.5 and s=8.03. The force coefficients decrease to a minimum when the
cylinder is at approximately -0.25*ro In the simulator, then increase until the
cylinder reaches the center of the simulator, then decrease to another minimum
when the cylinder reaches approximately 0.25*ro In the simulator, and then level
off as the cylinder continues to traverse the simulator. The moving test data
does not follow the stationary test data very well except for the trailing side case.
100
Figure 4-79: Force Coefficients on Leading Edge of Roof of Cylinder (a=1, s=6.72)
r/ro
Figure 4-80- Force Coefficients on Trailing Edge of Roof of Cylinder (a=1, s=6.72) 101
" stationary
^ ^ M o v i n g Average
Figure 4-81: Force Coefficients on Leading Side of Cylinder (a=1, s=6.72)
stationary
"Moving Average
Figure 4-82: Force Coefficients on Trailing Side of Cylinder (a=1, s=6 72)
102
The flow regime around the cylinder does not behave In the same way at
each aspect ratio and each swiri ratio considered as the flow around the cube.
Some differences In the resulting data occurred when the aspect ratio and swiri
ratios were changed. These tests also showed that overall the stationary tests
data was not very representative of the moving tests data In magnitude, but in
most cases, the trends were somewhat close to one another. This Indicates that
future testing should consider whether stationary or transient tests should be
performed on the cylinder depending on the experimental requirements.
103
4.5 Statistical Analvsis Originally, the pressure measurement data was collected by means of a
Scanlvalve DSM 3000 system connected to a ZOC 33/64Px electronic pressure
scanning module at 300Hz for six seconds. This provided 1800 data points for
each data set taken. For the models that were tested, five different data sets
were obtained for each of the 31 positions In the tornado simulator. This means
that each pressure tap on the model would have five separate data sets of 1800
points each for each of the 31 positions In the simulator. Statistical data is
presented for the north and south faces and the center of the roof on the cube
and cylinder when each Is positioned at the center of the tornado simulator and
at 2.0*ro In the simulator.
The first step was to obtain the mean value of each data set for each
pressure tap for each of the five runs. The data that was reported was obtained
by taking the mean of the five runs at each position. This is. In fact, the mean of
the means of the data which gives a representative value of what the pressure
difference would be for each pressure tap at any given time. The mean values of
the five different runs when the cubical and cylindrical models were placed at the
center (point 16) and at 2.0*ro in the tornado simulator are shown in Tables 4-5,
4-6, 4-7, 4-8, 4-9, and 4-10. These mean values are the values which were used
to calculate the force coefficients on the cube and cylinder In the previous two
sections.
The standard deviation values were also calculated using the five mean
values found and are shown below for the cube and cylinder at the center of the
tornado simulator and at 2.0*ro In the simulator on the north and south faces and
at the center of the roof. The standard deviation for all cases Is very small, so
the distribution should be concentrated towards the center of the normal
distribution. The skewness was calculated using the raw data obtained from the
five different data sets described above. Most of the skewness values are
negative indicating the normal distribution is skewed to the right of the centeriine.
These negative skewness values also indicate slow. Infrequent variations In
104
pressure below the mean [37]. The positive values for skewness indicate that
the normal distribution is skewed to the left of the centeriine. The kurtosis was
found using the same raw data used to find the skewness. To have the same
peak height as the normal distribution, the kurtosis should equal 3. Each of the
cases where the kurtosis Is negative has a distribution curve that is more flat than
the normal distribution curve, and each of the cases where the kurtosis Is positive
has a distribution curve that Is more peaked than the normal distribution curve.
Very high kurtosis values like the ones shown for the center of the roof of the
cylinder at the center of the simulator indicate an increase in the high-frequency
content of the fluctuating pressure signals read by the Scanlvalve system [37].
Table 4-5
Mean St. Dev.
Skewness Kurtosis
: Statistical Values at Center of Roof on Cube and Cylinder at Center of Simulator
Cube Roof Center a=0.5, 8=2.23 -0.403 0.030 -0.608 4.444
a=0.5, s=8.03 -0.682 0.038 -0.011 -0.237
a=1, 8=1.51 -0.303 0.030 -1.072 4.048
a=1, 8=6.72 -0.628 0.008 0.035 0.085
Cylinder Roof Center a=0.5, 8=2.23 -0.586 0.033 -3.212 25.957
a=0.5, s=8.03 1.387 0.031 -0.367 0.369
a=1, 8=1.51 -0.150 0.006 -2.908 29.739
a=1, 8=6.72 -0.611 0.030 -0.405 0.432
Table 4-6: Statistical Values at Center of Roof on Cube and Cylinder at 2.0*ro in Simulator
Mean St. Dev.
Skewness Kurtosis
Cube Roof Center a=0.5, 8=2.23 -0.024 0.001 -0.007 0.035
a=0.5, 8=8.03 -0.030 0.001 -0.054 0.033
a=1, 8=1.51 -0.004 0.001 0.021 -0.028
a=1, 8=6.72 -0.035 0.002 -0.089 0.027
Cylinder Roof Center a=0.5, s=2.23 -0.021 0.001 -0.020 0.062
a=0.5, 8=8.03 -0.030 0.001 0.036 -0.062
a=1, s=1.51 -0.010 0.001 0.005 0.077
a=1, 8=6.72 -0.024 0.001 -0.056 0.120
Table 4-7: Statistical Values on North Face of Cube and Cylinder at Center of Simulator
Mean St. Dev.
Skewness Kurtosis
Cube North Face a=0.5, 8=2.23 -0.185 0.018 -0.713 0.662
a=0.5, s=8.03 -0.606 0.021 -1.134 1.500
3=1, s=1.51 -0.135 0.024 -0.991 1.869
3=1, 8=6.72 -0.582 0.014 -0.426 0.237
Cylinder North Side 3=0.5, 8=2.23 -0.205 0.011 -0.596 0.575
3=0.5, 8=8.03 -0.761 0.053 -0.294 0.022
3=1, 8=1.51 -0.444 0.028 -0.860 1.086
a=1, s=6.72 -0.614 0.029 -0.309 0.164
105
Table 4-8: Statistical Values on North Face of Cube and Cylinder at 2.0*ro in Simulator
Mean St. Dev.
Skewness Kurtosis
Cube North Face 3=0.5, 8=2.23 -0.0140 0.0007 -0.0067 -0.0178
a=0.5, 8=8.03 0.0050 0.0007 0.0349 -0.0397
3=1, 8=1.51 0.0152 0.0008 0.0004 -0.0669
3=1, 8=6.72 -0.0033 0.0017 -0.0778 0.2493
Cylinder North Side 3=0.5, 8=2.23 0.0001 0.0005 -0.0286 0.0355
3=0.5, 8=8.03 -0.0144 0.0013 0.0231 -0.0199
3=1, 8=1.51 -0.0131 0.0012 -0.0041 0.0186
3=1. 8=6.72 -0.0142 0.0027 -0.0290 -0.0343
T3ble 4-9: Statistical Values on South Face of Cube and Cylinder at Center of Simulator
Mean St. Dev.
Skewness Kurtosis
Cube South Face 3=0.5, s=2.23 -0.127 0.007 -0.296
3,490.000
3=0.5, 8=8.03 -0.683 0.044 -0.364 0.109
3=1, 8=1.51 0.007 0.005 -0.743 1.496
3=1, s=6.72 -0.618 0.014 -0.254 0.052
Cylinder South Side 3=0.5, s=2.23 -0.339 0.013 -0.509 0.326
3=0.5, 8=8.03 -0.967 0.058 -0.244 0.039
3=1, s=1.51 -0.183 0.010 -0.280 -0.055
3=1, 8=6.72 -0.652 0.038 -0.708 1.011
Table 4-10: Statistical Values on South Face of Cube and Cylinder at 2.0% in Simulator
Mean St. Dev.
Skewness Kurtosis
Cube South Face a=0.5, 8=2.23 -0.015 0.001 0.001 -0.031
3=0.5, s=8.03 -0.023 0.001 -0.045 -0.033
3=1, s=1.51 -0.733 0.001 0.004 0.000
a=1, 8=6.72 -0.015 0.002 -0.050 0.069
Cylinder South Side 3=0.5, s=2.23 -0.022 0.001 -0.052 -0.033
3=0.5, 8=8.03 -0.028 0.000 -0.031 -0.056
a=1, 8=1.51 0.002 0.001 0.007 0.011
3=1, s=6.72 -0.021 0.002 0.021 -0.095
106
CHAPTER 5
CONCLUSIONS AND RECOMMENDATIONS
5.1 Conclusions
A Ward-type tornado simulator has been built using a configuration of 16
slotted jets instead of a rotating screen to create the required far field circulation
needed to produce a tornado-like vortex. This configuration provided an aspect
ratio In the same range as the aspect ratio reported for natural tornadoes [8], but
produced a swiri ratio that was unique to the Texas Tech University Tornado
Vortex Simulator II. Flow visualization data, velocity data and pressure data
were all obtained using the simulator. The observed vortices that were produced
ranged from a rope-like, single-celled vortex to a much larger diameter, two-
celled vortex.
Helium bubbles were used to visualize the vortices In the convergent
region of the tornado simulator. As the swiri ratio, the ratio of the tangential flow
rate to the flow rate at the updraft hole, was increased from zero, a small
protrusion of a vortex was observed in the center of the updraft hole. Continuing
to increase the swiri ratio, the vortex of bubbles descended to until it reached the
surface of the TTU TVS II. At this point, the vortex was a rope-like, single-celled
vortex. Velocity and flow rate measurements were obtained with the TTU TVS II
in the same configuration as It was when the single-celled vortex was observed
in order to calculate the swiri ratio acting in the TTU TVS II at this configuration.
This would be the swiri ratio used for the low swiri cases in the pressure tests. At
an aspect ratio of a=0.5, this swiri ratio was s=2.23 and at a=1, this swiri ratio
was 1.51. As the swiri ratio continued to Increase, an adverse pressure gradient
starts to occur spawning the 'breakdown bubble' at the top of the vortex. This
•bubble' forms the boundary between the supercritical flow upstream and the
subcritical flow downstream. The supercritical flow upstream of the breakdown
107
bubble Is very similar, In appearance, to the single-celled vortex described
eariier. However, the subcritical flow downstream is tripped by the bubble and is
turbulent in nature. As the tangential flow rate Is Increased, the bubble moves
down the vortex toward the surface of the simulator. This causes a deceleration
In the axial direction of the vortex inner core and eventually, an actual downdraft
in the very center of the vortex. This central down flow region Is surrounded by
the vertical vorticity of upflow. When the breakdown bubble reaches the surface
of the simulator, the core of the vortex expands radially and the down flow in the
central core penetrates to the surface. When the combination of the updraft and
downdraft are each present when the breakdown bubble has penetrated the
vortex core to the surface of the simulator, the vortex Is defined as two-celled.
Again, velocity and flow rate measurements were obtained with the TTU TVS II In
the two-celled vortex configuration. This would be the swiri ratio used for the
high swiri cases in the pressure tests. At a=0.5, this swiri ratio was s=8.03 and at
a=1, this swiri ratio was 6.72. The swiri ratios calculated are unique to the TTU
TVS II.
In order to study low-rise structure Interaction In tornadic winds, pressure
data was obtained on a cubical and cylindrical model that were positioned at
various radial locations within the simulator. The models were also subjected to
moving tests through the TTU TVS II in order to compare the stationary data to
the moving data. Using the pressure data, non-dimensional force coefficients
were calculated and contour plots of the force coefficients on the cube and
cylinder were generated for the stationary tests while, for the moving tests,
specific points on the models were chosen, and the force coefficients at these
points were plotted as a function of position in the TTU TVS II. These plots
indicate that the cubical model severely disrupts the flow at the center of the
simulator.
The stationary tests show that both the cube and the cylinder models
experience flow patterns, at different points In the TTU TVS II, similar to that of
models In a boundary layer-type flow. At s=2.23 and s=1.51 (low swiri ratios)
108
when the cube Is positioned between O.SVo and 2.0*ro In the tornado simulator,
the pattern of force coefficients seen on the cube indicates a pattern of flow like
that of the pattern associated with a boundary layer-type flow. When the cylinder
is between 0.25*ro and 2.0*ro in the TTU TVS II at s=2.23, s=1.51, s=8.03 and
s=6.72, the force coefficient contour plots also indicates a pattern of flow which Is
associated with that of a boundary layer type flow. Also for these cases for the
cylinder, the contour plots indicate a horseshoe vortex forms around the cylinder.
In a very small region of the simulator, 0.125*ro for s=2.23 and s=1.51 and
0.125*ro to 0.25*ro in the simulator for s=8.03 and s=6.72, the cubical model
experiences the pattern of flow similar to that of a boundary layer-type flow, but
with the flow Impinging on the east or west (side) faces of the cube rather than
the north or south faces. This Indicates that the model is very close to the
converging vortex, and the tangential flow component of the flow field Is the
major contributor to the flow on the model. At the center of the simulator, both
the cylinder and the cube disrupt the flow field significantly. Also, at this point,
the flow field Is very complex and at the present time the experimental equipment
and data are not sufficient to quantify the flow In this region. At all other points in
the simulator, the flow field comprised of the radial and tangential components of
flow are acting on the cubical and cylindrical models In such a way that the force
coefficients indicate a combination of each of the components of flow.
For the moving tests performed on the cube and cylinder in the TTU TVS
II, several interesting points on each model were chosen to study. The leading
and trailing edges of the roof as well as the leading and trailing sides of each
model were chosen and force coefficients were calculated and plotted as a
function of radial position in the TTU TVS II. Each of these moving tests had
approximately the same trends for the leading edge and side and the trailing
edge and side with a few exceptions. The force coefficient on the leading edge
and side of each model was fairiy constant until the model reached a point
approximately -0.25*ro away from the center of the simulator. At this point In the
simulator, the force coefficients on the leading edge and side of each model
109
decreased to a minimum value when the model reached a point between
-0.25*ro and the center of the simulator before starting to Increase again. The
force coefficients Increase until the model reaches the center of the simulator
where the force coefficient starts to decrease again. The force coefficient
decreases until the model reaches approximately 0.25*ro In the simulator where
the force coefficient levels off to approximately the same as the level when the
model was between -2.0*ro and -0.25*ro In the simulator. The trailing edge and
side of each model behaves In a manner similar to the leading edge and side,
only on the opposite side of the simulator. This Is the way that the force
coefficients behave in most of the moving tests.
An important note is that the stationary test data followed the trends of the
moving test data In most cases tested. This would mean that less significance
could be placed on the much more complicated moving tests and more
significance on the less complicated stationary tests In future testing. The
stationary tests are much easier to perform and control than the moving tests, so
a greater quantity of experimental data could be collected easier with the
stationary tests.
Limited statistical analysis was also performed on the data. This showed
that standard deviation for all cases is very small, so the distribution should be
concentrated towards the center of the normal distribution. Most of the skewness
values are negative Indicating the normal distribution is skewed to the right of the
centeriine. These negative skewness values also Indicate slow, Infrequent
variations In pressure below the mean [37]. Very high kurtosis values like the
ones shown for the center of the roof of the cylinder at the center of the simulator
indicate an increase In the high-frequency content of the fluctuating pressure
signals read by the Scanlvalve system [37].
110
5.2 Recommendations
Several recommendations can be made for future work using the TTU
TVS II or other Ward-type tornado simulators utilizing the slotted jet design.
• Increasing the scale of the simulator and Increasing the swiri ratio should
produce greater vortex breakdown and ultimately, multiple vortices.
• A less intrusive velocity measurement system would be very beneficial In
collecting data due to the fact that It would not Interrupt the flow regime.
Ideally, PIV or LDV systems should be used to collect data in such a
complex flow field.
• Longer sampling times and more repeatable data would also be beneficial
In quantifying the flow regime.
• A greater range of swiri ratios should be studied.
• Different building model configurations should also be explored in the
tornado simulator.
I l l
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structure Interaction: investigation of structural loading on cubic building.
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Proceedings of 1V^ International Conference on Wind Engineering, pp 837-844.
34. Chi, S. W. & JIh, J. (1974). Numerical modeling of the three-dimensional flows in the ground boundary layer of a maintained axisymmetrical vortex. Tellus, 26, pp. 444-455.
35. Howells, P. & Smith, R. K. (1983). Numerical simulations of tornado4lke vortices. Geophys. Astrophys. Fluid Dynamics, 27, 253-284.
36. Smith, D. R. (1986). Effect of boundary conditions on numerically simulated tornado-like vortices. J. Atmo. Sci., 44, 648-656.
37. Holroyd, R. J. (1983). On the behaviour of open-topped oil storage tanks in high winds. Part I. Aerodynamic aspects. J. Wind Eng. Ind. Aero., 12, 329-352.
114
APPENDIX A
LIST OF CONTOUR PLOTS OF FORCE COEFFICIENTS ON CUBE AND
CYLINDER INCLUDED ON CD
115
The following figures are on the CD Included.
Page In Word Doc. On CD
A l : Cube at 2.0*ro (a=0.5, s=2.23. North of Vortex) 3
A2: Cubeat1.0*ro(a=0.5, s=2.23. North of Vortex) 4
A3: Cube at 0.5*ro (a=0.5, s=2.23, North of Vortex) 5
A4: Cube at 0.25% (a=0.5, s=2.23, North of Vortex) 6
A5: Cube at 0.125*ro (a=0.5, s=2.23. North of Vortex) 7
A6: Cube at 0.0625% (a=0.5, s=2.23. North of Vortex) 8
A7: Cube at Center (a=0.5, s=2.23) 9
A8: Cube at 0.0625*ro (a=0.5, s=2.23. South of Vortex) 10
A9: Cube at 0.125% (a=0.5, s=2.23. South of Vortex) 11
A10: Cube at 0.25% (a=0.5, s=2.23. South of Vortex) 12
A l l : Cubeat0.5%(a=0.5,s=2.23, South of Vortex) 13
A12: Cube at 1.0% (a=0.5, s=2.23, South of Vortex) 14
A13: Cubeat2.0%(a=0.5,s=2.23, South of Vortex) 15
A14: Cubeat2.0%(a=0.5, s=8.03, North of Vortex) 16
A15: Cube at 1.0% (a=0.5, s=8.03. North of Vortex) 17
A16: Cube at 0.5% (a=0.5, s=8.03, North of Vortex) 18
A17: Cube at 0.25% (a=0.5, s=8.03. North of Vortex) 19
A18: Cube at 0.125% (a=0.5, s=8.03, North of Vortex) 20
A19: Cubeat0.0625%(a=0.5,s=8.03, North of Vortex) 21
A20: Cube at Center (a=0.5, s=8.03) 22
A21: Cubeat0.0625%(a=0.5,s=8.03, South of Vortex) 23
A22: Cube at 0.125% (a=0.5, s=8.03, South of Vortex) 24
A23: Cube at 0.25% (a=0.5, s=8.03. South of Vortex) 25
A24: Cube at 0.5% (a=0.5, s=8.03, South of Vortex) 26
A25: Cube at I.OVo (a=0.5, s=8.03. South of Vortex) 27
A26: Cubeat2.0*ro(a=0.5,s=8.03, South of Vortex) 28
A27: Cube at 2.0% (a=1, s=1.51. North of Vortex) 29
116
A28: Cube at 1.0% (a=1, s=1.51. North of Vortex) 30
A29: Cube at 0.5% (a=1,s=1.51. North of Vortex) 31
A30: Cube at 0.25*ro (a=1, s=1.51. North of Vortex) 32
A31: Cube at 0.125% (a=1,s=1.51. North of Vortex) 33
A32: Cube at 0.0625% (a=1,s=1.51. North of Vortex) 34
A33: Cube at Center (a=1, s=1.51) 35
A34: Cube at 0.0625*ro(a=1,s=1.51, South of Vortex) 36
A35: Cube at 0.125% (a=1, s=1.51, South of Vortex) 37
A36: Cube at 0.25% (a=1,s=1.51, South of Vortex) 38
A37: Cube at 0.5% (a=1, s=1.51, South of Vortex) 39
A38: Cube at 1.0% (a=1, s=1.51, South of Vortex) 40
A39: Cube at 2.0% (a=1, s=1.51. South of Vortex) 41
A40: Cubeat2.0%(a=1,s=6.72, North of Vortex) 42
A41: Cube at 1.0% (a=1, s=6.72. North of Vortex) 43
A42: Cubeat0.5%(a=1,s=6.72, North of Vortex) 44
A43: Cube at 0.25% (a=1,s=6.72. North of Vortex) 45
A44: Cube at 0.125% (a=1,s=6.72, North of Vortex) 46
A45: Cube at 0.0625% (a=1, s=6.72. North of Vortex) 47
A46: Cube at Center (a=1,s=6.72, ) 48
A47: Cube at 0.0625% (a=1, s=6.72. South of Vortex) 49
A48: Cube at 0.125% (a=1, s=6.72. South of Vortex) 50
A49: Cube at 0.25% (a=1, s=6.72, South of Vortex) 51
A50: Cube at 0.5% (a=1, s=6.72, South of Vortex) 52
A5^: Cube at 1.0% (a=1,s=6.72. South of Vortex) 53
A52: Cubeat2.0%(a=1,s=6.72, South of Vortex) 54
A53: Cylinder at 2.0% (a=0.5, s=2.23, North of Vortex) 55
A54: Cylinder at 1.0% (a=0.5, s=2.23. North of Vortex) 56
A55: Cylinder at 0.5% (a=0.5, s=2.23, North of Vortex) 57
A56: Cylinder at 0.25% (a=0.5, s=2.23. North of Vortex) 58
117
A57
A58
A59
A60
A61
A62
A63
A64
A65
A66
A67
A68
A69
A70
A71
A72
A73
A74
A75
A76
A77
A78
A79
A80
A81
A82
A83
A84
A85
: Cylinde
Cyllnde
Cylinde
Cyllndei
Cylindei
Cyllndei
Cylindei
Cylindei
Cylindei
Cylindei
Cylindei
Cylindei
Cylindei
Cylindei
Cylindei
Cylindei
Cylindei
Cylindei
Cylindei
Cylindei
Cylindei
Cylindei
Cylindei
Cylindei
Cylinder
Cylindei
Cylinder
Cylinder
Cylinder
'a t
•at
'at
•at
•at
•at
•at
•at
•at
•at
•at
•at
•at
•at
•at
•at
•at
•at
•at
•at
•at
•at
•at
•at
•at
•at
•at
at
at
0.125*ro (a=0.5, s=2.23. North of Vortex) 59
0.0625*ro (a=0.5, s=2.23. North of Vortex) 60
Center (a=0.5, s=2.23) 61
0.0625% (a=0.5, s=2.23, South of Vortex) 62
0.125% (a=0.5, s=2.23. South of Vortex) 63
0.25% (a=0.5, s=2.23. South of Vortex) 64
0.5% (a=0.5, s=2.23. South of Vortex) 65
1.0%(a=0.5, s=2.23. South of Vortex) 66
2.0% (a=0.5, s=2.23. South of Vortex) 67
2.0%(a=0.5,s=8.03, North of Vortex) 68
1.0%(a=0.5,s=8.03, North of Vortex) 69
0.5*ro (a=0.5, s=8.03. North of Vortex) 70
0.25% (a=0.5, s=8.03. North of Vortex) 71
0.125% (a=0.5, s=8.03. North of Vortex) 72
0.0625*ro (a=0.5, s=8.03. North of Vortex) 73
Center (a=0.5, s=8.03) 74
0.0625% (a=0.5, s=8.03. South of Vortex) 75
0.125*ro (a=0.5, s=8.03. South of Vortex) 76
0.25% (a=0.5, s=8.03, South of Vortex) 77
0.5*ro (a=0.5, s=8.03. South of Vortex) 78
1.0%(a=0.5, s=8.03. South of Vortex) 79
2.0*ro (a=0.5, s=8.03. South of Vortex) 80
2.0%(a=1,s=1.51, North of Vortex) 81
1.0%(a=1,s=1.51, North of Vortex) 82
0.5*ro (a=1, s=1.51. North of Vortex) 83
0.25% (a=1, s=1.51. North of Vortex) 84
0.125*ro(a=1, s=1.51, North of Vortex) 85
0.0625% (a=1, s=1.51. North of Vortex) 86
Center (a=1, s=1.51) 87
118
Inder at 0.0625% (a=1,s=1.51, South of Vortex) 88
inder at 0.125% (a=1, s=1.51. South of Vortex) 89
inderat0.25%(a=1,s=1.51, South of Vortex) 90
Inder at 0.5*ro(a=1, s=1.51, South of Vortex) 91
Inderal 1.0*ro(a=1, s=1.51, South of Vortex) 92
Inder at 2.0% (a=1, s= 1.51, South of Vortex) 93
Inder at 2.0% (a=1, s=6.72. North of Vortex) 94
inder at 1.0% (a=1, s=6.72, North of Vortex) 95
Inder at 0.5*ro(a=1, s=6.72. North of Vortex) 96
Inderat0.25%(a=1,s=6.72, North of Vortex) 97
Inder at 0.125% (a=1,s=6.72. North of Vortex) 98
Inder at 0.0625% (a=1,s=6.72. North of Vortex) 99
inder at Center (a=1, s=6.72) 100
inder at 0.0625*ro (a=1, s=6.72, South of Vortex) 101
A100: Cylinder at 0.125*ro(a=1,s=6.72, South of Vortex) 102
A101: Cylinder at 0.25% (a=1, s=6.72, South of Vortex) 103
A102: Cylinder at 0.5% (a=1, s=6.72. South of Vortex) 104
A103: Cylinder at 1.0% (a=1, s=6.72. South of Vortex) 105
A104: Cylinder at 2.0% (a=1, s=6.72. South of Vortex) 106
A86
A87
A88
A89
A90
A91
A92
A93
A94
A95
A96
A97
A98
A99
Cyl
Cyl
Cyl
Cyl
Cyl
Cyl
Cyl
Cyl
Cyl
Cyl
Cyl
Cyl
Cyl
Cyl
119
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