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-   -   free surface modelling using VOF (http://www.cfd-online.com/Forums/main/106351-free-surface-modelling-using-vof.html)

sci August 27, 2012 02:43

free surface modelling using VOF
 
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.;)

leflix August 28, 2012 06:21

Quote:

Originally Posted by sci (Post 378808)
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?

sci August 28, 2012 08:17

1 Attachment(s)
I mean the continuity equation that is used for the free surface::)

http://www.cfd-online.com/Forums/dat...AASUVORK5CYII=http://www.cfd-online.com/Forums/dat...AASUVORK5CYII=
http://www.cfd-online.com/Forums/dat...AASUVORK5CYII=

leflix August 28, 2012 10:24

Quote:

Originally Posted by sci (Post 379094)

Sorry I don't know what you are talking about and what your equation corresponds to. :confused:

AeroMike August 28, 2012 12:10

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

leflix August 28, 2012 15:44

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"

sci August 29, 2012 01:50

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.

sci August 29, 2012 01:55

Mike I checked it out, but it wasn't related to my problem, thanks anyway.

AeroMike August 29, 2012 01:58

Okay, no worries, sorry for sending you down a blind alley!
Mike

hilllike August 29, 2012 02:56

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

leflix August 29, 2012 07:43

Quote:

Originally Posted by sci (Post 379216)
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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