# free surface modelling using VOF

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 August 27, 2012, 02:43 free surface modelling using VOF #1 New Member   Join Date: Aug 2012 Posts: 9 Rep Power: 6 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.

August 28, 2012, 06:21
#2
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Quote:
 Originally Posted by sci 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?

August 28, 2012, 08:17
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I mean the continuity equation that is used for the free surface:

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August 28, 2012, 10:24
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 Originally Posted by sci 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.

 August 28, 2012, 12:10 #5 New Member   Join Date: Aug 2012 Posts: 22 Rep Power: 6 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. Mike

 August 28, 2012, 15:44 #6 Senior Member   Join Date: Aug 2011 Posts: 251 Rep Power: 8 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

 August 29, 2012, 01:50 #7 New Member   Join Date: Aug 2012 Posts: 9 Rep Power: 6 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.

 August 29, 2012, 01:55 #8 New Member   Join Date: Aug 2012 Posts: 9 Rep Power: 6 Mike I checked it out, but it wasn't related to my problem, thanks anyway.

 August 29, 2012, 01:58 #9 New Member   Join Date: Aug 2012 Posts: 22 Rep Power: 6 Okay, no worries, sorry for sending you down a blind alley! Mike

 August 29, 2012, 02:56 #10 Member   Ren/Xingyue Join Date: Jan 2010 Location: Nagoya , Japan Posts: 44 Rep Power: 8 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

August 29, 2012, 07:43
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Quote:
 Originally Posted by sci 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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