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Old   October 30, 2012, 08:48
Default strange wave profile.
  #1
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Hello all,

I use waveFoam to generate 1st order regular wave and plot the wave profile at t=50sec
The wave properties is :
w=3.1415926, H=0.1, T=2s, wavelength=3.6949, c=1.8474 and k=1.7003

Freesurface function is set to be:
functions
(
freeSurface
{
type surfaces;
functionObjectLibs
(
"libsampling.so"
);
outputControl outputTime;
outputInterval 1;
surfaceFormat vtk;
fields
(
alpha1
);
surfaces
(
freeSurface
{
type isoSurfaceCell;
isoField alpha1;
isoValue 0.5;
interpolate false;
regularise false;
}
);
interpolationScheme cell;

the result is very strange

is there any suggestion to get a good wave profile?

Many thanks,
Cloudy
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Old   October 30, 2012, 08:49
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here is the wave profile (wave elevation against length)
wave tank length is 18m and relaxation zone is added at the inlet and outlet of the tank
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Old   November 2, 2012, 06:19
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Niels Gjoel Jacobsen
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Hi Cloudy,

What you experience is due to the use of first of stokes theory. You should use a higher order wave theory to obliterate this strange, but physically correct, surface elevation.

If you want to understand it better, please search the web for spurious waves.

/ Niels
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Old   November 5, 2012, 05:50
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Hi Niels,

there is another question I would like to ask you. Since I would like to plot the wave profile in a 3D wave tank at different Y position, how do setup the freesurface Sampling function and is there any instruction about how to use this and all other utilities?

I have checked their c++ files but there is no information which tell you how to setup

Many thanks,
Cloudy
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Old   November 5, 2012, 07:47
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Niels Gjoel Jacobsen
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Hi Cloudy,

Please read the Wiki (http://openfoamwiki.net/index.php/Contrib/waves2Foam).

/ Niels
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Old   November 12, 2012, 13:13
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HI Niels,

I would like now to try to use turbulence model for my case, how should I set the BCs of k and epsilon for inlet and outlet?

Many thanks,
Cloudy
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Old   November 13, 2012, 05:19
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Hi

It depends entirely on your problem. If you have pure waves, I typically disregard turbulence on the boundary (setting small values), but if you are modelling waves and current, then you need to think a lot about the correct boundary condition - I do not have the answer for that.

Please note what I write in my paper describing waves2Foam about the turbulence models in OF. These are not ready for modelling free surface flows! There is no density in the formulation of the turbulence models, which leads to a huge diffusion of turbulence across the interface. This excess diffusion of turbulence (typically from air to water) dampens the wave very, very fast. In my case of studying wave breaking on a sloping beach, it lead to a dissipation of the wave energy without wave breaking - the wave simply disappeared.

Till now, I have not included the turbulence models in the waves2Foam library, but there might be an extension at some point, but until now, I recommend that you make your own modifications to your favourite turbulence model. These modifications basically goes along the line of specifying the 2-equation framework in a way, which is NOT normalised by density.

With respect to outlet conditions, I always have a beach, so I do not have an outlet.

Good luck,

Niels
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