a rans based prediction method of ship roll damping moment kumar bappaditya salui supervisors of...
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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
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•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
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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
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Grid size independency studies
1l
1r
2l 3l
3r2r
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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)
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3-d Calculations without forward speed
Boundary conditions
Ship hull- No-slip boundary
Sliding condition
Domain boundary: No-slip condition
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3-d Calculations without forward speed
Grid generation
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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)
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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
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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
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• 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
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Boundary conditions
Hydrostatic pressure
Inlet
No-slip b. c. on the hull surfaceSliding Condition
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Example grid
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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
University Research Presentation Day, 16th January 2004___________________________________________________________________________________________________________________________________________________________________________
•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:
University Research Presentation Day, 16th January 2004___________________________________________________________________________________________________________________________________________________________________________
Thank you for your attention