Unsteady loads due to cavitation in flow systems: Using 1D...
Transcript of Unsteady loads due to cavitation in flow systems: Using 1D...
Unsteady loads due to cavitation in flow systems:
Using 1D and 3D CFD to shed light on the
phenomenon
Dansis meeting, May 22. 2019, Bjerringbro
Morten Kjeldsen (FDB) & Love Håkansson (EDR & Medeso)
Flow Design Bureau
Outline
• About
• Other Cavitation works by Kjeldsen and at FDB
• Cavitation dynamics
– Experimental cavitation mapping
– Dynamics of flow systems- Cavitation as the excitation (1D CFD)
– 3D CFD as the cavitation laboratory
• Summary
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Main objectives:
• Establish that cavitation can be an excitation source for flow
system dynamics.
• If so, conclude if this affects a cavitation test facility
EDR & Medeso
• Provide leading tools for product simulation and BIM.
• Eight offices in Sweden, Norway, Denmark, Finland and the UK
• Customers include some of Europe’s leading companies in
industry, engineering and construction, as well as consultants and
specialists at smaller engineering organizations.
– EDR & Medeso provide services to FDB on CFD related topics
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Morten Kjeldsen- A cavitation focused CV
• MSc (1991) PhD (1996) Mech. Eng. @ NTNU Trondheim, Norway
– Both cavitation related.
– 3 months of PhD spent at DTU (Fysisk Institut, K. A. Mørch)
• Military service. Cavitation on Frigate propellers
• 1997-1998. Research Associate @ University of Minnesota
w/Prof. Roger Arndt. Basic cavitation research funded by ONR.
• 2001. Associate Prof at NTNU
• 2002 Flow Design Bureau AS
• 2008, 2009 Held Professorship in Fluid Power @ NTNU
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FDB
• Started 2001
• 6 employees (5 phd’s and 1 BSc). All having hydropower
background.
• Development projects, hardware products and software/
programming services.
• Alliance member w/
– National Instruments (LabVIEW)
– Flownex (1D simulation software)
– OSIsoft (Start-up date TBD)
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FDB- Partner in H2020 project
• Start up 1. June, 2019. Total budget: 4.5 mill EUR over 4 years.
• Flow control technologies/ techniques in hydroturbines.
• FDB together with: UPC- Barcelonatech (coordinator), Luleå Tech
U, Vattenfall and Statkraft.
• AFC4Hydro
– www.afc4hydro.eu (3. June)
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Under construction
Cavitation at nanometer scale!
• Together w/ K. A. Mørch
(DTU) and A. Keller
(TUM, Obernach)
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• Physics of Fluids 15, 545 (2003)
• Cav2001: http://resolver.caltech.edu/CAV2001:sessionA1.002
Field study Flow Design Bureau AS
P=25MW, H=250m
Cavitation Intensity Instrument
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ASME- J. of Fuids Eng, January 2015.
St. Anthony Falls LabUniversity of Minnesota
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From Wikipedia
Cavitation dynamics- Mapping of characteristics
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Cavitation regimes
• Mapping of cavitation regimes
on the NACA 0015 hydrofoil.
Visual inspection under strobe
and continuous light
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0 2 4 6 8 10
Angle of Attack
0.0
0.4
0.8
1.2
1.6
2.0
Cavitation N
um
ber -Cp,min
Measurements taken for: c =2.8mg/l. 9.8<Uref<10.7 m/s
i
Bubble cavitation
Patchcavitation
Sheet Cavitation
Supercavitation
O2
Shiftingdynamics.
l/c=1/3
l/c=2/3
l/c=1
l/c=0
J. Fluids Eng 122(3), 481-487 (Mar 09, 2000)
Cavity length
• Mapping of (maximum) cavity length
over the NACA0015 hydrofoil. Visual
inspection under strobe and
continuous light.
•𝑙
𝑐= 𝐶1 − 𝐶1
𝜎
2𝛼
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2 4 6 8 10 12
0.0
0.4
0.8
1.2
l/c
3D Meas.
2 Degrees
4 Degrees
6 Degrees
8 Degrees
10 Degrees
2D Meas
Theory Acosta (2D)
Measured lift dynamics
• Measurement of lift, average and
unsteady, characteristics for the
cavitating NACA 00015 Hydrofoil.
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0.0 0.5 1.0 1.5 2.0 2.5
Cavitation Number
0.0
1.0
2.0
Norm
aliz
ed P
ress
ure
Diffe
rence
and L
ift
Comparison at 7 degs. Angle of Attack
-Power Spectra, DP and Lift, compared.
0.0 0.5 1.0
0
2
4
0
2
4
0.0 0.5 1.0
0
2
4
0
2
4
0.0 0.5 1.0
0
2
4
0
2
4
0.0 0.5 1.0
0
4
8
0
20
40
-2
0
2
Lift,
Norm
aliz
ed R
MS
Measured Lift
Measured Pressure Difference
Lift RMS
Shedding dynamics
• Joint Time Frequency Analysis of
pressure dynamics at the base of the
foil while ramping cavitation facility
absolute pressure; cavitation number
is here a function of time.
• Identified types of shedding
– 𝐼𝐼𝐼𝑓𝑙
𝑈= 𝑐𝑜𝑛𝑠𝑡
– 𝐼𝐼𝐼𝑓𝑐
𝑈= 𝑐𝑜𝑛𝑠𝑡
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0.8 0.9 1 1.1 1.2Cavitation Number
20
40
60
80
100
Fre
qu
en
cy
[Hz],
Df=
1.2
2H
z
(II)
(III)
Cavitation as
system
excitation?
Flow System Dynamics- Intro (1/2)
• System
• Description:
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q xqp
L,Ap
As
h
𝑑𝑥
𝑑𝑡=
𝑞𝑝−𝑞
𝐴𝑠, continuity of surge shaft «T»
𝑑 𝑞𝑝∙𝜌𝐿𝑖𝑞∙𝐿∙𝐴𝑝
𝑑𝑡= 𝜌𝐿𝑖𝑞 ∙ 𝑔 𝐻 − 𝑥 − 𝑓
𝐿
𝐷
1
2𝜌𝐿𝑖𝑞 𝑢𝑝 𝑢𝑝, «F=ma»
Flow System Dynamics- Intro (2/2)
• System model (LaPlace transformed):
• Frequency analysis, s=jw
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𝑞𝑃 𝑠 =1
𝐴𝑠 ∙ 𝐿𝐴𝑝 ∙ 𝑔
𝑠2 +𝐴𝑠𝐴𝑝
𝑓𝐿𝑑12𝑔 𝑢𝑝,0 ∙ 𝑠 + 1
∙ 𝑞 𝑠
Excitation(here q)
System Response(here qp)
q xqp
L,Ap
As
h
SAFL Tunnel as the system
• A smaller free surface exists
in the extension of the gas
collector dome (8)
• P1, p2 and p3 are locations of
pressure transducers used
during mapping of transient
characteristics.
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1D model of SAFL system- FloMASTER
• An elastic formulation for the
piping is applied to enable
the calculation of standing
elastic or acoustic waves.
• FloMASTER solves these
transients by applying the
methods of characteristics.
• The cavitation dynamics is
modelled as an oscillating
mass source. Justified with
3D(2D) CFD.
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Upstream stilling tank Test section (1200mm, 190x190mm2)
System excitation: Cavitation shedding modelled as oscillating flow source
p1
p2p3
10m
5mp0
ID 753mm
ID 628mm
Excitation(oscillating mass-injection or actual cavitation)
System Response(pressure at var location)
(transient FloMASTERmodel or experiment)
Calculated Frequency response
• Frequency response, i.e.
pressure variations at location
“0” and based on mass-source
excitation at the same location.
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Measured coupling ofexcitation and response
• Crosspower analysis between
accelerometers detecting cavitation
shedding (excitation) and pressure
transducer at location #3 capturing
the response.
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Foil Leading Edge Mount Pos Acc X-dir
Mount Pos Acc Y-dir
Mount Pos
Acc Z-dir@p3
Measured frequency responseExplanation for deviation
• It can be that at location #1 the
swirling structures shed off the
hydrofoil is still present.
• For both the assumption of clearly
distinct sonic speed in pipe
segments can be at fault. For the
system the presence of
accumulated gas will affect the
dynamic response of the whole
tunnel.
• Finally, the loop system dynamics
can allow the response to affect
the excitation source.
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Cavitation shedding- Steady oscillatory?
• Joint Time
Frequency
Analysis of lift
dynamics of a
cavitating
NACA0015
Hydrofoil at two
different test
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Cavitation tunnel at Tech U
of Munich, Obernach
Cavitation tunnel at SAFL
http://resolver.caltech.edu/CAV2001:sessionA9.001
Steady oscillatory cavitation
•𝑓𝑐
𝑈= 𝑐𝑜𝑛𝑠𝑡 indicating lock-in with system dynamics? Not
conclusive
• Pressure oscillations will make oscillations in , and hence in
l/c=func() and therefore in frequency since fl/U =const?
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Excitation System Response
Using 3D (here 2D) CFD to understand more of cavitationshedding
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Pressure BC
Massflow inletu 10m/s
2DWidth 0.19m
NACA 0015
c=0.081m
AoA or = 8
• Ansys CFD (Fluent)
• Transient (dt=50µs)
• Cavitation model
• Fast turbulence model
• Parametric study
– P,exit = const
– P,exit superimposed
oscillating pressur (DP,
f)
– Focus on low values
Results
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To be resolved: Sheet, clouds and (hairpin) vortices.
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Parametric study- Postprocessing
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dt
Parametric study- Results
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D f Av Lift (N) p2p Lift f (Hz) StDev f
2.5 0 0 469 690 9.19 0.62
3 0 0 562 824 8.75 0.12
3.5 0 0 671 1022 8.91 0.0075
3 0.1 f0=8.75Hz 558 786 8.81 0.0058
3 0.2 f0=8.75Hz 578 803 8.85 0.184
3 0.1 f0-0.1f0=7.96Hz 567 827 8.73 0.32
3 0.1 f0+0.1f0=9.55Hz 581 830 9.02 0.37
Theoretical average lift cL=2p 674 (N)
Concluding 3D (2D) CFD
• Modelled cavitation shows resistance to superimposed pressure
pulsations. No apparent lock-in (enhanced oscillations)
• Note: The fact that vapor volumes can collapse rapidly the liquid
should allow for elastic transients or establish an improved best-
practise of calculation.
• Useful tool to address certain feature of the flow
• Acceptable for engineering analysis.
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Conclusions
• «Auto-oscillating» cavitation can be an excitation source.
– Careful to assume steady oscillatory flow.
– Can affect itself (feedback on frequency term)
• Is cavitation affecting test the facility at SAFL?
– The tunnel systems shows a response to excitation
– Reason to assume little feedback on shedding frequency.
• 1D and 3D CFD combined can be a useful tool for e.g.
troubleshooting in complex system, and eventually design of
same system.
• Can pressure transients actively be used to control/mitigate
cavitation shedding dynamics?
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Main objectives:
• Establish that cavitation can be an excitation source for flow
system dynamics.
• If so, conclude if this affects a cavitation test facility
Extra- Dynamics also valid for non-condensible gas
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=0.96c
C=81mm
D=3.5mm
Slith width 0.5mm, roughly 45 degrees at Zero degs AoA
0 2 4 6 8
vent/2 and /2
0
0.04
0.08
0.12
0.16
CL,r
ms/C
L,th
eo
Vent 8 degs AoA (June12)
Vent 6 degs AoA (June 12)
Cav 9 degs AoA (SAFL 98)
Cav 7 degs AoA (SAFL 98)
0 2 4 6 8
vent/2 and /2
0
0.2
0.4
0.6
0.8
1
CL
Vent 8 degs AoA(June 12)
Vent 6 degs AoA (June 12)
Cav 8 degs AoA (July13)
Cav 6 degs AoA (July13)
Thank you!
[email protected] (for CFD related inquiries)
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«Whitepaper» to be published subsequent meeting