prediction of cavitation dynamics in marine · pdf filemethod to predict also complex...

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PREDICTION OF CAVITATION

DYNAMICS IN MARINE

APPLICATIONS

A.I. OPREA & N. BULTEN

WÄRTSILÄ GLOBAL R&D, NETHERLANDS

STAR GLOBAL CONFERENCE 2012

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Introduction & Summary

Introduction:

Cavitation = liquid changes into vapour state due to a decrease of the

pressure (bellow the vapour pressure)

Start of the cavitation modeling work using STAR-CD begins September

2007 (PhD work)

Goal: predict cavitation propulsion units by means of CFD simulations

Cavitation prediction will improve the understanding and consequently

the performance can be increased/improved

Presentation summary:

Numerical settings

Wetted and cavitating results on:

2D NACA0015 section

3D Twist -11 hydrofoil

LE Skew propeller

Conclusions

3/28/2012 2 CAVITATION MODELLING USING RANS APPROACH / I.Oprea & N.Bulten

Skew Propeller

3D:Twist 11

2D: NACA 0015

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Basis of the simulations:

– STAR-CD v4.10.

– RANS solver

– Two-equation RNG k-ε turbulence model + modified turbulent

viscosity

– Wall functions approach

– VOF method + Rayleigh model, for cavitation modeling

Solver approach:

– 2nd order MARS in space

– 1st order Euler implicit in time

– SIMPLE algorithm


General numerical settings

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2D NACA0015 section: case presentation

2D NACA0015 test case conditions:

• NACA0015 profile

• Angle of attack: 6 degrees

• O-grid approach

• Inlet velocity: 6m/s

• Outlet pressure: 20kPa => sigma=1.0

• Slip walls

• Time step: 0.00001(cavitation)

2c 4c


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2D NACA0015 section: wetted flow results

• Pressure coefficient distribution:

• Lift and drag:

CFD Experiment

Lift 0.667 0.658

Drag 0.014 0.014

Within 1% deviation

1.5% deviation from the

analytical value 1


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2D NACA0015n section: cavitating flow results

• Cavity volume variation over one period:

EXP: 16Hz (Arndt & co., 2000) In literature frequency variations from 11 to 24HZ

T~0.06s

f ~16Hz


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2D NACA0015 section: cavitating flow results


Simulated cavity shedding patterns are confirmed by experiments and simulations

../VIRTUE/WP4/2D_foil/naca0015/RNG_user/per_0.672_0.754/per_0.672_0.754.gif

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3D Twist-11 hydrofoil: case presentation

3D test case:

• NACA0009 profile

• O-grid approach

• Angle of attack: -2 (root) & 9 (mid) degrees

• Inlet velocity: 6.97m/s

• Outlet pressure: 29.0kPa=> sigma=1.07

• Time step: 0.00001(cavitation)

v


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3D Twist-11 hydrofoil: wetted flow results

• Pressure coefficient distribution:

• Lift prediction:

Mid-span low pressure due to

the foil design

Pressure and lift well predicted on the wetted flow case


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3D Twist-11 hydrofoil: cavitating flow results

• Cavity volume variation over one period:

EXP: f=31.64Hz (E.J. Foeth 2008 PhD)

T~0.033s

f ~30Hz

Cavitating Twist-11 foil frequency well in agreement with the measurements


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CFD EXPERIMENT


CFD EXPERIMENT


3D Twist 11 hydrofoil: cavitating flow results

Cavity shedding patterns are confirmed by experiments (E.J. Foeth 2008 PhD)

../VIRTUE/WP4/DelftTwistFoil/RNG_modif/cav_0.1_per_0.2125_0.244s/cav_0.1_top.gif

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• For on period of shedding lift coefficient variation is:


3D Twist 11 hydrofoil: cavitating flow results

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2D & 3D benchmark test cases conclusions

• Wetted flow results on NACA0015 and Twist-11foil have been validated

and the results are well in agreement with measurements

• Cavitating flow on NACA0015 and Twist-11 foil do predict complex

cavity shapes well in agreement with the measurement observations

• Please note that these results could not be obtained with the standard

RNG k-ε turbulence model

• Safe to apply the current CFD method on a real propeller test case

• Wetted and cavitating flow results of the model scale skew propeller are

presented in the following


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Skew propeller: case presentation

• Propeller diameter=0.2333m

• Propeller RPM=840

• 4 million cells (1 million within the

tip vortex location based on

Q-factor & Cp grid refinement for one blade)

• Open water curves


5242 Dn

TorqueKq

Dn

ThrustKt

nD

vJ ad

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.2 0.4 0.6 0.8 1 1.2

Kt,

10

Kq

, E

ffic

ien

cy

Advance Ratio, J

Kt CFD

10Kq CFD

Eta0 CFD

Kt EXP

10Kq EXP

Eta0 EXP

V

RPM

Performance is inline with the measurements

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Skew propeller: wetted flow case

• Pressure coefficient (Cp) contour

on blade at Kt=0.32:

• Cp and Q-factor downstream

of propeller:


i

j

j

i

i

i

x

u

x

u

x

uQ

2

2

1

2)(2

1nD

pCp

Tip vortex (low pressure + high Q-factor)

Q>0 => rotation is dominant and the region

is determining a vortex tube

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• Blade pressure coefficient

contour and iso-surface=-2.27

at Kt=0.32:

• Cp downstream of propeller

(cavitation influence) and

vapor content


Skew propeller: cavitating flow case

Lower pressure within the tip vortex core when cavitation is enabled

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Skew propeller: tip vortex cavitation observations

• CFD simulations

(Pressure coefficient iso-surface=-2.27)

• Experiments

(HSVA, EU Leading Edge project)

3/28/2012 CAVITATION MODELLING USING RANS APPROACH / I.Oprea & N.Bulten 19

Predicted leading edge-tip vortex cavitation in agreement with the

experiment observations

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Conclusions

• RANS simulations + modified RNG k-ε turbulence model + Rayleigh

cavitation model => cavitation simulation well agreement with the

measurement for the 2D NACA0015 and 3D Twist11 test cases

• Simulations on the skew propeller test case reveals the capability of the

method to predict also complex sheet-tip vortex cavitation on real

propeller design

• Based on the CFD cavitating simulations the design of future propellers

should benefit in terms of performance, noise and vibration



Questions?

prediction of cavitation dynamics in marine · pdf filemethod to predict also complex...

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