A RANS Based Prediction Method of Ship Roll Damping Moment Kumar Bappaditya Salui Supervisors of...

A RANS Based Prediction Method of Ship Roll Damping Moment

Kumar Bappaditya Salui

Supervisors of study:

Professor Dracos Vassalos and

Dr. Vladimir Shigunov

The Ship Stability Research Centre

Department of Naval Architecture and Marine Engineering

Universities of Glasgow and Strathclyde

Overview of Presentation•Introduction

•Model

•Two dimensional calculations

•Three dimensional calculations without forward speed

•A cfd-strip Theory

•Three dimensional calculations with forward speed

•Conclusions

University Research Presentation Day, 16th January 2004___________________________________________________________________________________________________________________________________________________________________________

Introduction

BackgroundRoll damping is strongly affected by viscosity, it is feasible to try RANS equations for its prediction


•Unstructured mesh

•SIMPLE algorithm for pressure correction equation

•HRIC algorithm for free surface simulation

•Dynamic fluid pressure on the ship hull

•Calculation of added moment of inertia and damping moment using Fourier analysis


Description of the model and test procedure•harmonic rolling oscillations:•the roll axis is fixed with respect to the hull

sin( )a t

Post-processing:•Moment due to hydrodynamic pressure is considered, hydrostatic part of pressure (with respect to the initial undisturbed free surface) is subtracted • As the moment due to shear stresses is negligible compared to the pressure part, it was neglected• linear component of the damping moment (~to the angular velocity) estimated by Fourier analysis of the time history of the hydrodynamic moment• the coefficient of this moment:

/ 2

44

/ 2

1( ) ( )sin( )

t T

wa t T

A t M t t dt

Non-dimensional

4444 2

ˆ AA

B

/ 2

44

/ 2

1( )cos( )

t T

wa t T

B M t t dt

44

44 2ˆ

2

B BB

B g

Two-dimensional calculationsBoundary conditions:

Sliding Boundary

Free surface

No-slip wall

No-slip wall

Description of section

r/B=0.00625


Grid size independency studies

1l

1r

2l 3l

3r2r


Results and discussionsa)

0

0.02

0.04

0.06

0.08

0 0.5 1 1.5 2

present calculation Vugts' experiments

44A

b)

0

0.01

0.02

0.03

0.04

0 0.5 1 1.5 2

44B

c)

0

0.02

0.04

0.06

0 0.5 1 1.5 2

44A

d)

0

0.01

0.02

0.03

0 0.5 1 1.5 2

44B

Added moment of inertia (a) and damping moment (b) coefficients for the square body with rounded corners for the rolling amplitude 11.50. Respective curves for 5.750 amplitude are shown in (c) and (d)


3-d Calculations without forward speed

Boundary conditions

Ship hull- No-slip boundary

Sliding condition

Domain boundary: No-slip condition


3-d Calculations without forward speed

Grid generation


Results and discussionsa)

0.02

0.025

0.03

0.035

0.04

0 0.5 1 1.5 2

experiments present calculations

44A

b)

0

0.025

0.05

0.075

0.1

0 0.5 1 1.5 2

44B

c)

0.02

0.03

0.04

0.05

0 0.5 1 1.5 2

44A

d)

0

0.025

0.05

0.075

0.1

0 0.5 1 1.5 2

44B

Added moment of inertia (a) and damping moment (b) for the ro-ro hull rolling at the amplitude 50; respective plots for 100 amplitude are shown in (c) and (d)


x

z

0 1 2 3 4

0

0.2

0.4

1234567891011121314

w.l.

0Total 14 sections has been taken

a)

0

0.015

0.03

0.045

0 0.5 1 1.5 2

3d calculations CFD-strip calculations Experiments

44A

b)

0

0.02

0.04

0.06

0.08

0.1

0 0.5 1 1.5 2

44B

A CFD strip theory

Result and discussions

Comparison of added moment of inertia (a) and damping moment (b) coefficients for the ro-ro hull with rolling amplitude 5 0


3-d calculations with forward speed

4321

5

APB

A

WL

FP

9.5

98.5

87

6

Body plan of a high-speed vessel

Description of the model and test procedure


• The running trim and sinkage defined in towing tests without rolling:

0.2 0.4 0.6 0.8Froude number

-20

-10

0

10

20

Sinkage,mm

0.2 0.4 0.6 0.8

Froude number

-0.5

0

0.5

1

1.5

Trimangle,degree

•In forced rolling tests trim and sinkage were fixed to respective values

•The same fixed values were used in the calculations


Boundary conditions

Hydrostatic pressure

Inlet

No-slip b. c. on the hull surfaceSliding Condition


Example grid


Damping dependency on frequency, forward velocity and amplitude

0 0.2 0.4 0.6 0.80.04

0.05

0.06

0.07

0.08

0.09B44

Fn=0.59

0 0.2 0.4 0.6 0.80

0.01

0.02

0.03B44

Fn=0.0

0 0.2 0.4 0.60

0.02

0.04

0.06

0.08B44

Fn

=0.3

0 0.2 0.4 0.60

0.02

0.04

0.06

0.08

0.1B44

Fn

=0.673

0 2.5 5 7.5 100

0.01

0.02

0.03B44

Fn =0

=0.673

a

0 2.5 5 7.5 100.03

0.04

0.05

B44

Fn =0

=0.673

a


•The method in general predicting the roll damping moment quite

accurately in the low and medium frequency range

•At high frequency, in case of simulations without forward speed

computed results have large errors

•At the high Froude numbers and high frequency, the deviation of the

computed results from the experiments may be larger

• Maximum deviation is about 14% for roll with forward speed

• Further improvement of grid quality will decrease this discrepancy

Conclusions:


Thank you for your attention

A RANS Based Prediction Method of Ship Roll Damping Moment Kumar Bappaditya Salui Supervisors of...

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