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April 9, 2013, 02:02 
VOF necessary mesh and timestep

#1 
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Hello, dear community of cfdonline!
Since VOF model is of much computational cost, there are 2 important issues: How fine domain should be meshed and what timestep should be used. If we have a 2D case of straight channel and to describe crosssection we use say 20 cells in the core and 6 cells per boundary layer to solve homogeneous flow. We want to solve a flow with gas and liquid phases, which volume fraction is 10%. Does it mean that for same accuracy we have to use 10x more cels to describe cross section? If we have a comparatively large timestep which results in Courant number equal to 30 and each timestep converges, is it still a good solution? What timesteps and Courant numbers could be applied to get a relevant solution with not much time spent on calculation? Thank you in advance for your answers! 

April 9, 2013, 10:02 

#2  
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are you trying to simulate a stratified flow in a channel?
or the gas phase is diluted in the liquid one? if you are trying to simulate small bubbles or droplets, i guess you should use other multiphase model mesh resolution of VOF calculations should be fine enough to capture the interface between the fluids, when the length scale of the interface is much larger than mesh size (as in stratified flow) in a 2D domain, i would use a variable timestep with courant number of 2, as fluent manual suggests with courant number near 30, you may get qualitatively good results as well, but lower courant numbers will provide more meaningful results Quote:


April 10, 2013, 00:45 

#3 
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Are there any practical advices for mesh? Any criteria.
If I get solution but surface of the liquid in gas is not as clear as it should be, are there any approaches to estimate how far my solution from reliable one? 

April 10, 2013, 09:41 

#4 
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it's hard to tell a specific criteria for VOF calculations
along with the usual ones (skewness and ortogonality), one has to considerer the mesh size which obviously will dictate how thin the interface will be captured but it will also have an impact on solution time... since you are running 2D calculations, it will be easier to evaluate the influence of mesh size I would proceed this way: estipulate three mesh sizes and run transient simulations with variable time stepping during some time, keeping courant number equal to 2 then compare results from them: instantaneous holdup, mean holdup, position of liquid or gas front take a look at this presentation for additional information http://pt.scribd.com/doc/62308472/Be...otiveIndustry regards 

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