Unsteady loads due to cavitation in flow systems: Using 1D ...s3-eu-west-1.amazonaws.com/foreninglet-wordpress... · Morten Kjeldsen- A cavitation focused CV • MSc (1991) PhD (1996)

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

Flow Design Bureau AS

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


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


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)


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)


Under construction

http://www.afc4hydro.eu/

Cavitation at nanometer scale!

• Together w/ K. A. Mørch

(DTU) and A. Keller

(TUM, Obernach)


• 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


ASME- J. of Fuids Eng, January 2015.

St. Anthony Falls LabUniversity of Minnesota


From Wikipedia

Cavitation dynamics- Mapping of characteristics


Cavitation regimes

• Mapping of cavitation regimes

on the NACA 0015 hydrofoil.

Visual inspection under strobe

and continuous light


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


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.


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


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:


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


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.


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.


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.


Measured coupling ofexcitation and response

• Crosspower analysis between

accelerometers detecting cavitation

shedding (excitation) and pressure

transducer at location #3 capturing

the response.


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.


Cavitation shedding- Steady oscillatory?

• Joint Time

Frequency

Analysis of lift

dynamics of a

cavitating

NACA0015

Hydrofoil at two

different test


Cavitation tunnel at Tech U

of Munich, Obernach

Cavitation tunnel at SAFL

http://resolver.caltech.edu/CAV2001:sessionA9.001

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?


Excitation System Response

Using 3D (here 2D) CFD to understand more of cavitationshedding


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


To be resolved: Sheet, clouds and (hairpin) vortices.


Parametric study- Postprocessing


dt

Parametric study- Results


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.


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?


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


=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]

[email protected] (for CFD related inquiries)


«Whitepaper» to be published subsequent meeting

Documents

Unsteady loads due to cavitation in flow systems: Using 1D ...s3-eu-west-1.amazonaws.com/foreninglet-wordpress... · Morten Kjeldsen- A cavitation focused CV • MSc (1991) PhD (1996)