# Multiphase problem

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 February 10, 2006, 00:13 Multiphase problem #1 Joe Guest   Posts: n/a I am trying to generate a grid to model a white blood cell under Poiseuille flow. The geometry is very basic - a 128 X 128 X 64 grid with the cell (modelled as a sphere) located in the middle. I want to use the VOF method to then determine the shape and the passage of the blood cell. However I am not sure how to specify the sphere correctly. I have tried meshing the box and then introducing the sphere but the unmeshed sphere is not exported when I create a .msh file. I have also tried specifying the box and the sphere and meshing both. However I dont want the mesh to be dependent on the sphere - I just want a simple rectangular mesh because the sphere will be advected as the simulation progresses. How can I create a volume in GAMBIT without meshing it that can be used to determine the intial position of a particular phase in FLUENT?

 February 10, 2006, 04:46 Re: Multiphase problem #2 edi Guest   Posts: n/a You can build your basic quad mesh and specify the sphere in the adapt>region panel in fluent (after initializing the solution). Then in the solve>initialize>patch panel you can "attach" the volume fraction of blood (must be defined as a phase) to the marked sphere. Of course, the finer the grid, the better the shape of the sphere... Hope this helps Edi.

 February 10, 2006, 05:43 Re: Multiphase problem #3 Joe Guest   Posts: n/a Thanks for your help Shouldn't it be possible to accurately specify the sphere with the VOF model though? It is my understanding that if the concentration in a particular cell is mixed between the two phases (ie the fluid inside the sphere and the fluid outside the sphere) then FLUENT can specify the boundary between the phases quite accurately using interpolation. I would prefer that the shape of the sphere be independent of the grid size. Is this in fact possible? Cheers, Joe

 February 10, 2006, 06:27 Re: Multiphase problem #4 edi Guest   Posts: n/a Fluent can effectively predict free surface shapes and boundaries between phases quite good with the VOF model (supposed that you're using the geo-reconstruct interpolation scheme, forget about the others...), but of course the interpolation is much better if the grid is finer (and quad). Of course the whole thing depends on the level of accuracy you want to achieve and on the particular problem, but I would say that in this kind of multiphase problems the shape of the boundary (that varies in space and time) is always dependent on the grid size (and quality). But of course there is a soil below which things don't change too much, and sometimes too fine grids may arise instabilities too... Hope this helps Edi.

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