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alfaben November 24, 2011 04:54

Problem with free surface ship hull
 
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Hi,

i have the following problem (picture) when i run a resistance calculation. Normaly i have a finer mesh (0.1m for the hull and stretching to 1m - free surface), but the result is the same. I get this behaviour only when i work with prismatic layers. When i change the under - relaxation factor from k-e turbulent viscosity from 1.0 to 0.3 or 0.1 i get a normal behaviour. With hex i must also change the under relaxtion factor from k-e turbulence and segregated vof to 0.4 or 0.3. Is there any way to avoid the change from this factors? With polymesh i only must change the under relaxation factor from k-e turbulent viscosity. With hex itīs conspicuous that the water takes al long time to flow over the bow.
With the default values the water has a behaviour like honey.
Without prismatic layers my results are very good (for pressure 141KN share force 290KN NET 431KN). With prismatic layer the pressure is to high, shear to low and net ok.

Mesh: fine-15.000.000 cells
coarse-5.000.000 cells
Top,bottom, left, front = inlet (velocity)
back = outlet (hydrostatic pressure of wave)
ship = wall
symmetrie
VOF
6 DOF
multiphase
primsatic layer thickness 0.03m
layer 10
stretching 1.2
ship lenth 141m
velocity 9.2m/s (for air and water)
Re=1.1e9
farfield (top 26m bottom 65m left 90m length 650m)
timestep 0.1 - 0.3s
iteration 5
Y+ 30-300
Turbulent viscosity ratio 10 (default) - 1000 (depends on mesh)


Thank you very much
alfaben


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