Twenty-Fourth Symposium on Naval Hydrodynamics (2003) / Chapter Skim
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Efficient Methods to Compute Steady Ship Viscous Flow with Free Surface
Efficient Methods to Compute Steady Ship Viscous Flow with Free Surface
Pages 161-176
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From page 161... ...
The complementary use of inviscid wave pattern predictions and RANS predictions for viscous flow generally implies a separate consideration of these aspects. Often the viscous flow computations suppose symmetry boundary conditions at the undisturbed water surface, the so-called 'double-body' approximation.
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From page 162... ...
Thus the free surface is a boundary of the domain and the FSBC's can be imposed in a straightforward manner. Alternatively, in fixed-grid methods the grid is not free-surface conforming, and the boundary conditions either have to be interpolated/extrapolated towards the free surface location, or are inherently satisfied by including the flow of the air above the water surface into the computation.
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From page 163... ...
in which viscous flow was computed under a measured wave surface. Compared with a full RANS/FS solution we have to disregard one of the boundary conditions, the normal component of the dynamic boundary condition
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From page 164... ...
The boundary conditions being nonlinear and to be imposed on an unknown wave surface, the solution needs to be found by an iterative process in which the free surface and the coefficients in the FSBC's are updated and the trim and sinkage of the hull are adjusted. In practical cases, ~10)
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From page 165... ...
The meaning of a solution having some local pressure deviations may not be immediately apparent but can be understood as follows. Suppose that the viscous flow under the prescribed wave surface is found to have a pressure deviation /~r~x,y)
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From page 166... ...
~ c ~ c o~_~ ~ i'' '"\\. ~1 ~ ~ experimen ~1 prescribed wave surface from Rapid - - corrected wave surface from Parnassos, model scale ~ -- -; -- - corrected wave surface from Parnassos, full scale JO ° 0.5 0.6 0.7 0.8 ,== x/L 1 0.9 Figure 5: Stern wave of KCS, at the centreline 1.0
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From page 167... ...
Under the same wave surface we have generated an appropriately refined grid and have run the RANS computation for the full-scale Reynolds number of 2.5 x 109. Of course viscous effects decrease for increasing Reynolds number, most notably resulting in a decrease of the boundary-layer thickness and the width of the wake.
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From page 168... ...
Nevertheless, it can be seen that it is capable of capturing some relevant physical phenomena in the flow, and in the present case indicates the wish for a hull form modification to suppress this wave-induced flow separation. 4 A STEADY ITERATIVE SOLUTION METHOD FOR FREESURFACE VISCOUS FLOWS Motivation While the composite approach described in the previous section is a step forward and works efficiently, the fact that the wave pattern is determined by an inviscid approximation is a restriction.
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From page 169... ...
Nevertheless, the number of required time steps generally ranges from 3000 to 30000 for published 3D ship cases. At each step the grid needs to be adjusted in a surface-fitting method, or the free surface reconstructed in a VOF method; and the effort is ambitious to say the least.
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From page 170... ...
is imposed, together with the tangential dynamic conditions; followed by a free-surface and grid update based on the normal dynamic condition (8~. The combined FSBC in itself demands neither a zero normal velocity nor a constant pressure at the estimated wave surface; but imposes a relation between the normal velocity and the (10)
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From page 171... ...
To the solution orocess described before, an outer iteration loop was added in which the free surface and grid are adjusted using the normal dynamic condition (8) after a sufficient degree of convergence of the inner loop that solves the RANS equations.
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From page 172... ...
3D Pressure patch Next we considere the wave pattern generated by a 3D pressure patch travelling over the free surface. The parameters of the pressure distribution were taken equal to those of Wyatt (2000~.
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From page 173... ...
Of course, much more needs to be done. Specifically, stability and convergence rate of the computation with free-surface boundary conditions needs to be addressed, iteration for the free surface and grid needs to be performed, and efficiency improved.
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From page 174... ...
We believe that efficient methods to compute steady ship viscous flow with free surface can be designed by choosing the right level from this hierarchy in the right case and in the right part of the flow domain; and we hope this will bring a largescale and fruitful application of such methods in practical ship design closer. Acknowledgement We thank Mervyn Lewis, Centre for Mathematics and Computer Science (CWI)
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From page 175... ...
. cedure for ship viscous flow with free surface", 3rd Num.
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From page 176... ...
In the case of the composite approach, further grid updates based on the pressure residual at the free surface would correspond to a decoupling of the kinematic and dynamic FSBC's. This is what we have avoided using the combined conditions in our steady iterative method for free-surface flows; as motivated in our paper.
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