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step by step for VOF modeling

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Old   December 16, 2003, 07:47
Default step by step for VOF modeling
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Hi I modeled a VOF but my results is unphysical.we help you to say a good reference about step by step for VOF modeling (not fluent manual please).

Thank alot.
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Old   December 17, 2003, 05:09
Default Re: step by step for VOF modeling
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Hi AK!

I think it's going to be hard to get a step by step reference about VOF (or any other special problem). The thing is (at least that's my expirience) nobody will tell you exactly what you have to do. When you look at it it's logical. If someone spends a long time developing a model (technikque, procedure) it's not likely that he will give it away just like that. For example if you read papers it looks like everithing is explained, but the fact is that the important things remain hidden and the only way to get to them is a LOT of work.

But I'm glad to say that I think this forum is in most cases an exeption to the "rule" described above.

Maybe you can give us a morfe specific description about your work and you will get an answer.

Hope I didn't demoralize you

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Old   December 17, 2003, 05:56
Default Re: step by step for VOF modeling
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I agree with Mateus. I am quite new in this forum but I also very glad to see the kind help from people here and the effort & time they invest on the others' problems.

Here are some important points about VOF modelling (from Fluent Users Service Center)I hope to give you some guidance:

* Although the VOF model will work with any mesh type, quad cells in 2D and hex cells in 3D are recommended for free surface flows. Also, try to make the mesh as uniform as possible.

* Use the PRESTO! pressure interpolation scheme for Pressure under "Discretization" in the Solution Controls GUI panel (Solve - Controls - Solution...). Make sure that the "Implicit Body Force" formulation (in Define - Models - Multiphase) is NOT activated. Note that the PRESTO scheme is only available for Quad/Hex meshes. If your model contains Tri/Tet cells, use "Body Force Weighted" for Pressure discretization in the Solution Controls GUI panel and activate the "Implicit Body Force" formulation in the Multiphase Model GUI panel (Define - Models - Multiphase).

* Use "PISO" for Pressure-Velocity Coupling in the Solution Controls GUI panel (Solve - Controls - Solution...).

* Set all the Under-Relaxation Factors in the Solution Controls GUI panel to 1 and run the calculation for a few time steps to observe convergence behavior. If the convergence behavior is poor, reduce the under-relaxation factor for Momentum to 0.7 and run the calculation again for a few time steps. If convergence is still hard to achieve, lower the under-relaxation factors for Pressure to 0.5 and Volume Fraction to 0.5.

* Select a time step size (when you start iterating) such that the solution converges within 20 to 30 iterations at each time step. To determine this, set the maximum iterations per time step to a large number initially (say, 100) and experiment with the time step size to determine how many iterations the solution converges in.

* As for convergence criteria (Solve - Monitors - Residual), use 1.0e-04 for all variables if your model has inlet/outlet boundary conditions. Otherwise, use the default 1.0e-03 value. * You may also want to use the double precision version of FLUENT if your model involves very small length scales.

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Old   December 17, 2003, 08:58
Default Re: step by step for VOF modeling
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Hi my friends

I thank you very much,because of your guidance. you believe that I do not nowise to be discomfort. I only wanted an adderres of reference about VOF modeling in fluent (for example: site 'so and so' in INTERNET),and dont expect that someone completely explain it for me in response box.also i read VOF in fluent manual completely and have understood its structures in this package but have some problems in allocating initial values for volume fraction for tow phases. but now I modeled a bridge pier in a regtangular open is an hydaralic problem. in 2D model without free suface changing,results for pattern flow is good but because of turbulence of water surface,force value is not i incorporate VOF in B.C are : pressure inlet for top of domain. velocity inlet for entrance. outflow for end of domain. wall for solid sides.

again i thank you for your friendly guidance.


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Old   December 21, 2003, 08:37
Default Re: step by step for VOF modeling
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hi Ozgur, I hope our tips will apply to mixture model too. my problem is an axisymmetric model of a cylindrical tank containing liq upto 80% and remaining 205 by vapor. now, on heating by constant heat flux on cyl. walls, natural convection has to occur and when interface reaches sat temp, evaporation has to occur. for this i have done patching and given static head distribution through custom defined function and UDF for mass & energy sources to include evaporation. can u model it without UDF and check the pressure distribution after patching and after first iteration. custom field function is rho*g*(h-coordinate); h-total height.

y i am asking is, pressure distribution, even showing correct profile...values are abruptly changing to -ve values

hope u understand regards prasanth
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