Ae341 Lab1- Flow visualization using Hydrogen bubble chamber

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1 Submitted to: AEE-METU AEE 341-AERODYNAMICS I LAB. REPORT #1 Submitted by: Ayşenur Bıçakçı 1881952 Ece Öztürk 1882513 Mohamed Abdulaziz 1681865 Sait Can Güven 2110757 Şahin Murat Karacaoğlu 1882141 Özge Sinem Özçakmak Özgür Harputlu 31 Oct. 2014

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Flow Visualization using Hydrogen Bubble chamber Test over symmetric airfoil, cylinder and flat plate

Transcript of Ae341 Lab1- Flow visualization using Hydrogen bubble chamber

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    Submitted to:

    AEE-METU

    AEE 341-AERODYNAMICS I

    LAB. REPORT #1

    Submitted by:

    Ayenur Bak 1881952 Ece ztrk 1882513

    Mohamed Abdulaziz 1681865

    Sait Can Gven 2110757

    ahin Murat Karacaolu 1882141

    zge Sinem zakmak

    zgr Harputlu

    31 Oct. 2014

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    ABSTRACT

    The purpose of this experiment is to visualize some elementary flow patterns

    using the hydrogen bubble technique. Flow patterns around the symmetric airfoil,

    cylinder and a rectangular prism are studied qualitatively. Theoretical ideal flow patterns

    are reviewed and some discrepancies are observed for the real flow patterns around the

    symmetric airfoil, cylinder and rectangular prism. Furthermore, a number of phenomena

    are investigated; namely, separation points, flow separation, pressure and velocity

    distribution, stagnation points, bound and wake vortex, Karman vortices for these

    different shapes and their effect on the flow pattern and lift. The investigations were

    conducted for the different geometries at changing Reynolds number, characteristic

    length (cylinder) and angles of attack and analyzed using the data given in the laboratory

    manual and reasonable assumptions were made to further facilitate understanding of the

    flow patterns. Javafoil was used to find the velocity and pressure distributions as well as

    the aerodynamic coefficients of the symmetric airfoil and rectangular prism.

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    CONTENTS LIST Page

    Abstract 2

    Contents List 3

    Objective 4

    Introduction 4

    Theory 4

    Experimental Set-up 9

    Experimental Procedure 9

    Results & Discussion 10

    Conclusion 22

    References 22

    Appendix 22

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    OBJECTIVE

    The lab work is aimed at improving our understanding of the flow patterns and

    their different phenomena as well as how the Reynolds number and angle of attack

    affects the flow pattern. Thanks to the hydrogen bubble technique, some of the

    observable properties of flows are visualized by using several shaped objects such as

    cylinders, rectangular prism and symmetric airfoil.

    INTRODUCTION In this experiment, we observed alterations in the flow behavior around different

    samples including cylinders with two different diameters, a rectangular prism and a

    symmetric airfoil. Moreover, the impact of changing the angle of attack was also viewed.

    Thus, in terms of aerodynamics, this experiment is essential in gaining an intuitive grasp

    of how the flow acts around these common shapes.

    In the following parts of this report, the theory of this experiment and results are

    verbalized and discussed. Then, the techniques and equipment used in the experiment are

    listed under the equipment section. Finally, references and appendix sections are located.

    THEORY

    CROSS FLOW OVER NON-ROTATING CYLINDER

    This external flow is normal to the axis of the non-rotating cylinder. It combines a

    number of phenomena such as flow separation, turbulence transition, reattachment and

    turbulence separation of the boundary layer. If the flow is inviscid the velocity

    distribution is given by:

    Where Vr is the radial velocity, V is the tangential velocity, R is the radius of the

    cylinder, r is the radial coordinate and the angle is measured from the forward stagnation point.

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    It can be seen that there are two stagnation points and that they are at the points (r,

    )= (R,0), (R, ). There is a streamline which connects the two points (there is no flow normal to the surface at r=R so the surface is by default a streamline) and therefore the

    flow past a non-rotating cylinder can be made up of some elementary potential flows;

    namely a uniform flow and a doublet. The uniform flow outside the circular streamline

    (on the surface) does not interact with the doublet flow within.

    Applying Bernoullis equation for an inviscid fluid we can find another important characteristic of the flow. That is the pressure distribution is symmetric around the axis.

    This is intuitive since the flow velocity components and stream function are also

    symmetric around the axis.

    This leads to another important result; since the pressure distribution of the

    inviscid cross flow over a circular cylinder is symmetric there should not be any lifting

    force or drag. This is one example of the dAlembert paradox.

    http://www.thermopedia.com/content/5637/TUBES_CROSSFLOW_OVER_FIG1.gif

    However the viscous effect within fluids is not negligible and the relation between

    both the viscous and inertial forces were found to be an important indicator of flow

    pattern. This ratio of viscous to inertial forces is a dimensionless parameter known as the

    Reynolds Number (Re).

    Consider now the flow past the non-rotating cylinder with viscous effects. As the

    fluid moves of the cross flow over a non-rotating cylinder the fluid pressure increases

    from the freestream value to the stagnation point value. Then the high pressure causes the

    fluid to separate and travel along the surface of the cylinder. However, in this case, a

    boundary layer starts to build up and the pressure force is counteracted with the shear

    forces and this prevents the fluid from travelling across the surface of the cylinder and

    leaving from the rear stagnation point. This forms two shear layers those have a velocity

    distribution that is a function of the displacement from the surface. This difference in

    velocity means that the innermost part moves at a slower rate than the uppermost part and

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    this causes the shear layers to roll up. Vortices appear in the wake. How exactly the

    vortex street develops depends on the Reynolds number.

    FLOW OVER A SYMMETRIC AIRFOIL

    In the thin airfoil theory, the inviscid flow over a symmetric airfoil has a velocity

    and pressure distributions that are also symmetric. There is no net lift or drag at a 0

    angle of attack. With increasing angle of attack the coefficient of lift increases linearly

    with increasing angle of attack. This theory has its advantages such as allowing us to

    determine the center of pressure and the aerodynamic center but it does not account for

    stall. This is an important criterion for the design of airfoils and planes as it can have

    adverse effects on balance and control.

    With positive angles of attack the stagnation points which were at leading and

    trailing edge start to move. The flow meets the airfoil on the underside of the airfoil and

    separates into two flows. One flow path must flow to the leading edge and around the

    topside and the other flows along the underside. Due to the Kutta condition they should

    meet at the trailing edge and form one flow (velocities must be equal and non-zero). In

    short, the flow over the top must be faster than the one at the bottom. As a result, vortex

    flow occurs at the trailing edge and the fast flows result in strong viscous forces that build

    up near the edge. This vortex is known as the starting vortex. The fluid tends to move

    from the bottom to the trailing edge as opposed to going around to the leading edge and

    this means that the vorticity of the flow behind the airfoil is positive for positive angles of

    attack. However, due to Helmholtzs theorem which states that if the circulation of a flow was initially zero then it should still be the same regardless then there must be a vortex

    that turns the airfoil. This is known as the bound vortex.

    As seen the bound and shed vortices contribute the same but have opposite sense.

    Therefore, the total circulation is zero as the circulation of a uniform freestream velocity

    is zero. As the airfoil moves along the vortices at the T.E. form a vortex sheet.

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    FLOW OVER A FLAT PLATE

    When the viscous flow meets the flat plate the fluid sticks to the surface (no slip

    condition). That is the velocity of the fluid on the surface equals the velocity of the

    surface. This results in a boundary layer forming which is zero at the wall to the value of

    the freestream velocity some distance away. This displacement is known as the boundary

    layer thickness and is a function of distance x from the leading edge of the flat plate.

    As the flow travels along the plate the boundary layer starts to develop and this is

    seen as an increase in the boundary layer thickness. The extent to which it increases as

    well as the existence of separation depends on the Reynolds number (which is a function

    of x as well).

    http://cdn.comsol.com/wordpress/2013/09/Flow-of-a-fluid-over-a-flat-plate.png

    The transition range between laminar and turbulent flow occurs as 3x10

    5

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    both a laminar and turbulent state. Further on, in the turbulent state vortices form which

    spread as the boundary layer gets larger.

    At a zero angle of attack, the only friction that exists is the skin friction and this is

    the only contribution to the drag. The pressure distribution does not provide any

    contribution as it is symmetric and normal to the surface. With increasing angle of attack,

    the separated region becomes larger and moves further towards the leading edge. Also the

    pressure distribution starts to contribute to the drag due to the asymmetric build-up of

    pressure due to the appearance of steady spirals and later wake vortices and Karman

    vortices. Until the friction force due to the skin has a small effect compared with the drag

    due to pressure when the angle of attack reaches 90. At this angle of attack the wake is

    fully turbulent behind the plate.

    http://img.photobucket.com/albums/v145/johnfarley/CH10F7.jpg

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    EXPERIMENTAL SET-UP

    From the lab manual

    The names of the parts:

    1. Feet 2. Electrode Holders 3.Platinum Wire

    4. Mounting Pillar 5. Pump Body 6.Baffle Plate

    7. Weir 9, 10. Rod 11.Adjustable Connectors

    13. Casing 14. Flash Gun 15.Flow Straightener

    16. Block 17. Support 18.Guide

    EXPERIMENTAL PROCEDURE

    The observation of the external flow behavior over submerged bodies, being the

    primary objective of the experiment, was realized using the Hydrogen Bubble Flow

    Visualization technique. Generally, when the fluid is a liquid, the flow is visualized using

    bubbles or dyes as they are a cheap yet powerful tool. In this case, hydrogen bubbles are

    used and are generated by electrolysis of the purified water conditioned with salt filled in

    the flow table. The bubbles are generated at the cathode and carried with the flow

    supplied by a water pump. The water is circulated during the operation and the

    circulation process is sufficiently proper so as not to create unwanted vibration induced

    turbulence in the flow. The bubble generation may set to be intermittent or continuous.

    The intermittent bubble generation is conducted by simply turning on and off the

    electrical current through the electrodes. The flow is evened by means of a honeycomb

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    flow staightener so as to increase the discernibility of the flow pattern by letting the

    bubbles swept away in an accurate manner. A build-in light and an acrylic light guide are

    also contained within the system in order to accentuate the flow.

    Different acrylic objects are placed in the flow by mounting them on the pin fixed

    in the flow table. Initially, flow over a cylinder is studied. The change of the flow pattern,

    location of the separation point, wake region, the laminar to turbulence transition and

    Karman vortices are the phenomena to be observed. The same procedure is repeated for

    different speeds and for a cylinder of a larger diameter, which is the characteristic length

    for a cylinder. Thus, by changing speed and characteristic length, the changes in the flow

    pattern with respect to Reynolds number is studied. After the flow over the cylinder is

    studied, to see the effect of the shape of the body on the flow, a flat plate is placed onto

    the pin both parallel and perpendicular to the flow direction and again the separation and

    wake regions are observed at different flow speeds. Finally, a symmetrical airfoil is

    pinned into the flow to emphasis the difference between blunt and streamlined bodies.

    The effects of flow speed at different values of angle of attack (AOA) are aimed to be studied whilst the formation of starting vortex and the stall condition for the airplanes are

    discussed.

    RESULTS & DISCUSSION

    FLOW PAST CYLINDER

    At Reynolds numbers below 1, separation does not occur. However the

    streamlines shape is different from that in an inviscid fluid. From 5 Re 45, the flow separates from the rear side of the tube and a symmetric pair of vortices is formed in the

    near wake. The streamwise length of the vortices increases linearly with Reynolds

    number as shown in the figure below.

    |

    Streamwise length of vortices. From Taneda S. (1956) J Phys

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    With increasing Reynolds number wake becomes unstable and vortex shedding

    begins. At first, one of the two vortices breaks away and then the second is shed because

    of the non-symmetric pressure in the wake. This oscillating shed vortices form a laminar

    periodic wake of staggered vortices of opposite sign. A phenomenon called the Karman

    vortex street. (See picture below).

    .

    http://fmeabj.lecturer.eng.chula.ac.th/FMRL/public_html/Flow%20Visualization/Flow%20Images/Karman

    %20Vortex%20Street/Karman%20Vortex%20Street%202.jpg

    In the Reynolds number range 150 < Re < 300, unpredictable disturbances are

    found in the wake. The flow is transitional and gradually becomes turbulent as the

    Reynolds number is increased.

    The Reynolds number range 300 < Re < 1.5105 is called subcritical (the upper

    limit is sometimes given as 2105). The laminar boundary layer separates around 80

    degrees downstream of the front stagnation point and the vortex shedding is periodic and

    strong.

    With a further increase of Re, the flow enters the critical regime. The laminar

    boundary layer separates on the front side of the tube, forms a separation bubble and

    later reattaches on the tube surface. Reattachment is followed by a turbulent boundary

    layer and the separation point is moved to the rear side, to about 140 degrees downstream

    the front stagnation point. As an effect, the drag coefficient is decreased sharply.

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    Figure of pressure coefficiet vs forward stagnation point http://www.thermopedia.com/content/5637/TUBES_CROSSFLOW_OVER_FIG2.gif

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    Reynolds number vs flow speed for 3mm and 25mm radius cylinder

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    FLOW PAST SYMMETRIC AIRFOIL

    In real flows there is a viscous effect and as the flow travels along the surface of

    the airfoil a boundary layer develops. This causes two shear flows that separate near the

    rear of the airfoil. The flow pattern changes significantly with changing angles of attack.

    As the angle of attack is increased the flow separates nearer the leading edge.

    Flow separation begins at small angles of attack while attached flow is dominant. With

    increasing angle of attack the separated regions become larger and after a critical angle of

    attack the separated flow is so dominant that it causes a reduction in lift with increasing

    angle of attack as the separated region continues to increase.

    http://upload.wikimedia.org/wikipedia/commons/thumb/8/8d/StallFormation.svg/350px-

    StallFormation.svg.png

    Flow past airfoil at 0 angle of attack

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    Flow past airfoil at 8 angle of attack

    Flow past airfoil at 16 angle of attack

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    Coefficient of lift at varying Reynolds number vs Coefficient of drag

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    Aircraft polars at varying angles of attack

    As you can see increasing the velocity increases the Reynolds number (at a

    positive angle of attack).This further increases the coefficient of lift value in an almost

    linear fashion. Near 15 angle of attack there is a sudden drop in the coefficient of lift.

    This is called the stall angle of attack.

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    In addition, the coefficient of drag is significantly reduced. This is due to the

    movement of separated region in the downstream direction caused by the increase in

    momentum of the flow near the surface. The starting vortex is swept away and through

    the circulation theory we can see that this causes a bound vortex on the airifoil which

    gives it a positive coefficient of lift.

    It is important to note that the coefficient of moment is zero for all angles of

    attack. This is because the center of pressure does not move and that all the forces act on

    one point (there is no moment arm).

    Pressure Coefficient distributions vs x/c at different angles of attack

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    FLOW PAST RECTANGULAR PRISM

    The transition range between laminar and turbulent flow occurs as 3105

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    Flow past rectangular prism at 30 angle of attack

    Flow past rectangular prism at 60 angle of attack

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    Flow past rectangular prism at 90 angle of attack

    Pressure coefficient distribution vs x/c at varying angles of attack

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    CONCLUSION

    To conclude, this experiment displays the power of the hydrogen bubble

    technique to visualize elementary flow patterns at varying Reynolds numbers and angles

    of attack for a rectangular prism, cylinders of varying diameter and the symmetric airfoil.

    In addition, the flow patterns characteristics for the symmetric airfoil and rectangular

    prism at different angles of attack were observed. Departure from the ideal flow pattern

    was seen. This was evidenced by the observation of phenomena such as flow separation

    and various vortex types. It was observed that when the stream velocity was increased,

    the vortex gained more length. Separation of flow was seen on the upper side of the

    symmetric airfoil for non-zero angles of attack. In the case of the airfoil and flat plate it is

    worthy of remark that the separated region increased in size with increasing angles of

    attack. This had adverse effects on the coefficients of lift when the stall angle was

    reached.

    It must be noted that although it was possible to observe the variations of the

    characteristics of the wake for 35< Red < 4.5103 it was not possible to reach the critical

    Reynolds number and therefore the transition of the laminar boundary layer to turbulent

    as well as the reestablishment of the turbulent vortex street is outside the scope of this

    experiment.

    As a final comment, although other flow visualization techniques exist such as

    helium-filled bubbles technique, colored oil applying technique, laser sheet, surface oil,

    schlieren, smoke and tufts etc. they can be prohibitively expensive and complex.

    However, the hydrogen bubble technique is cheap and sufficiently effective at analyzing

    elementary flows. Therefore with this scope in mind the hydrogen bubble technique is

    suitable for the purpose of a qualitative analysis of simple flows.

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    REFERENCES

    1. http://www.consultexim.hu/katalogus/armfield/dshtml/fseries/f14.htm 2. www.imaph.tu-bs.de/lehre/99/irro/conformi_e.html 3. Anderson, D.J., Fundamentals of Aerodynamics, McGraw-Hill 4. AER 303 F Aerospace Laboratory I Aerodynamic Forces on an Airfoil 5. Experiment 1 Lab Handout given in the course website 6. Puttkammer, P. P. (2013). Boundary layer over a flat plate. (Master's thesis,

    University of Twente).

    7. Trinh, K. T. (2007). On the critical reynolds number for transition from laminar to turbulent flow. (Master's thesis, Massey University).

    8. Yemenici, O. (2014). An experimental study on the aerodynamics of a symmetrical airfoil with influence of reynolds number and attack angle.

    9. (30, October, 2014). Retrieved from http://www.consultexim.hu/katalogus/armfield/dshtml/fseries/f14.htm

    10. 8.(30, october 2014). Retrieved from www.imaph.tubs.de/lehre/99/irro/conformi_e.html