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Old   August 27, 2012, 03:43
Default free surface modelling using VOF
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sci
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I'm trying to write a code in Frotran to simulate free water surface in 2DV using Volume Of Fluid method. My code isn't giving the right answer.
I assume that cells with free surface have known pressures. For the rest of the cells using the momentum equations and free surface continuity equation, I find the new pressure terms. Is it right to use free surface equation for cells that aren't free surface? if not what should I do?
Thanks for helping.
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Old   August 28, 2012, 07:21
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Quote:
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Is it right to use free surface equation for cells that aren't free surface? if not what should I do?
Thanks for helping.
What is what you call free surface equation?
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Old   August 28, 2012, 09:17
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I mean the continuity equation that is used for the free surface:


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Old   August 28, 2012, 11:24
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Quote:
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I mean the continuity equation that is used for the free surface:


Sorry I don't know what you are talking about and what your equation corresponds to.
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Old   August 28, 2012, 13:10
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If I remember correctly you will always get some spurious currents when using the VoF equation in 2D. I'm not sure if that is what you are experiencing or not, if it is checkout the openFOAM tutorials, I think there is one there which demonstrates what I mean.
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Old   August 28, 2012, 16:44
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VOF method is based on the solving of this equation:
dF/dt +udF/dx+vdF/dy=0
here d is the partial derivative, F is the volume fraction.
When F(i,j)=0 cell (i,j) is fully occupied bythe first fluid, when F(i,j)=1 cell (i,j) is fully occupied by the second fluid. when 0<F(i,j)<1then cell (i,j) is occupied by the two fluids and then the interface cross the cell.
Now I don't know what you are talking about with your "free surface continuity equation"
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Old   August 29, 2012, 02:50
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I've written a code solving this equation you mentioned using young method which works accurately.
what I'm trying to do now is to solve the continuity and momentum equations in order to find the new pressures and velocities in the new time step and then solve the VOF equation with the new velocities.
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Old   August 29, 2012, 02:55
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Mike I checked it out, but it wasn't related to my problem, thanks anyway.
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Old   August 29, 2012, 02:58
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Okay, no worries, sorry for sending you down a blind alley!
Mike
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Old   August 29, 2012, 03:56
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nik,
The surface tension is not very important in single phase model.
You don't consider the air phase in your model. Pressure and velocity were calculated by boundary condition at the free surface, so the sharp change across the free surface won't affect your calculation. See SOLA-VOF and NASA-VOF as a reference.

If you want to use free surface tension, the surface force should be changed into body force and only exist in the smooth area based on CSF model, using a delta function.

Ren
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Old   August 29, 2012, 08:43
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Quote:
Originally Posted by sci View Post
I've written a code solving this equation you mentioned using young method which works accurately.
ok fine ! It's the first step

Quote:
what I'm trying to do now is to solve the continuity and momentum equations in order to find the new pressures and velocities in the new time step and then solve the VOF equation with the new velocities.
yes ! so you can use either projection method for variable density flows
so you should solve a poisson equation for pressure which looks like DIV(1/rho GRAD P)= DIV(U*)
Many papers exist on this topic.

or you can use SIMPLE algorithm alos for variable density flow.
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