a case which is very tough to simulate.......
Hi Foamers...
I have been into this simulating problem for a few weeks.but I find there is no CFD software can simulate this case accurately. I want to simulate a stirred tank full of xanthan gum which is very viscid, and the air was injected into the tank. 1 In experiment, there are lots of fine bubbles(diameter 1mm),and some big bubbles(diameter about 100mm),the exact diameter veries from the air flow. In OpenFOAM, I know interFoam can simulate bubbles at very low fraction. twoPhaseEulerFoam can simulate fine bubbles at high fraction.. but which solver can handle that complicated situation ? 2 Actually, there are break up and coalescence,cause the xanthan was sitrred in a tank. does that mean I need a solver which has been implanted population balance model? 3 Xanthan is a kind of nonnewtonian fluid. but It seems a little bit easier to implant the pow law model.? I dont know if CFX can simulate this situation accurately, but seems its a tough job for me. If anyone can give me a hint. I would be very appreciated!!!!!!!! 
1) In theory interFoam should be able to simulate larger bubble fractions as well. The only issue with interFoam (and any standard VOF solver for that matter) is that you will get numerical coalescence, which means that if the diffuse interfaces of two bubbles touch they will coalesce. Even if this is not physically correct behaviour.
2) a solution to the problem I mention above would indeed be a method that involves a population balance + a criterion for coalescence. This can be done of course, but I think it is rather cumbersome. 3) Power law fluids are not hard to implement, this is pretty much a standard feature in OF. This tutorial might be helpful: http://http://www.foodextrusion.org/OpenFOAM_tutorial.html 
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actually, I have simulated different mesh using the same condition. and I find VOF has to be set a fine mesh.and twofluid model dont need to. am I right? 
I think you are right that VOF will need much higher grid resolution to be able to capture the small bubbles, I didn't think of that when I first answered.
So I can indeed imagine that this is unpractical in your situation. With the large separation of length scales that you have I am not sure whether a standard solver can do this with high accuracy and (reasonably) low computational cost. Maybe you will need to look into subgrid modelling?! You could use twoPhaseEulerFoam on a coarse grid and a VOFlike implementation or a population balance model within each cell to keep track of the small bubbles. 
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so I have to do some implementation into twophaseeulerfoam to visualize the interface between the big bubble and the fluid.....What bumps into my mind is this two ways: either implant VOF into twoPhaseeulerfoam, or implant PBE into twoPhaseeulerfoam. but I dont know if VOF and twofluid model can coexist in a solver. or I should implant the PBE into twophaseeulerfoam? 
I am not sure. I suggest you check the scientific literature on ways that other people have modeled situations with twophase flow and a large separation of length scales and go from there.

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