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-   -   ATTENTION!! Validty of Fluent's VOF?? (http://www.cfd-online.com/Forums/main/7178-attention-validty-fluents-vof.html)

ozgur February 18, 2004 14:14

ATTENTION!! Validty of Fluent's VOF??
 
Hi all,

I am simulating free falling water drops (d=1-4 mm) in a surrounding media of supercritical CO2 (liquid like density, gas like viscosity). The terminal velocities I got so far are about 50 % lower than I expected. Moreover, the solution is highly dependent on the time step (an increase in the time step may result unpysical deformation, even the solution seems to converge!!) In addition, simulation is unstable for smaller drops, i.e. when the drop is 1mm, some strange velocity currents appear in the drop, which is not the case for bigger drops. (for all simulations diameter is resolved by 80 cells in 2D DP solver, time step btw. 10^-4 - 10^-5 s, geo-recon. scheme used, no turbulance model was employed)

After reading the article of C.W. Hirt (the founder of VOF) in http://www.flow3d.com/Cfd-101/whatsina.htm , I have a doubt that Fluent is able to make precise VOF calculations. He does not even call them (VOF in some commercial CFD codes, icluding Fluent) as VOF, but pseudo-VOF instead!!

What are the experience of the VOF users, regarding to validation of the simulation results?

One more thing. I think the following page gives a good summary of basic CFD concepts, which may be helpful for the CFD beginners (but it is so much filled up with the promotion of their Flow3D software):

http://www.flow3d.com/Cfd-101/cfd101.htm


John February 18, 2004 18:04

Re: ATTENTION!! Validty of Fluent's VOF??
 
With all the respect I have for Dr. Hirt (as he has developed VOF in fact), I think his comments may not be true.

His argument means that VOF only works for one-fluid formalism and this, if correct, limits application of VOF to a little number of applications where the effect of tjhe second fluid in fact is not important. But, many people have used two-fluid VOF and got reasonable results. He claims that one can get good results only with his software, let's correct this: in his opinion, you can get good results with one-fluid VOF codes (like RIPPLE, NASA-VOF etc.).

But the question is that what if we have a liq.-liq. jet like glycerine-water solution flowing into silicon oil, what you could do with one-fluid VOF and how one can impose the dynamics boundary conditions at the interface (here the effects of second liq. is no longer applicable using pressure jump condition etc.), here you should SOLVE for BOTH FLUIDS!!

Some people have used VOF for two-fluid applications and got good results. What's the difference between VOF and Level-Set (LS) in this matter? Why LS can solve it, but VOF can't.

I ask for others' comments to this ideas, but I think if your VOF method is accurate enough, it WOULD work for two-fluid Liq.-gas flows with large density ratios althuogh it's not easy to do (main problem is treating surface tension terms that is not mature even after several modifications to the original CSF method of Kothe' et al.).

John February 18, 2004 18:06

Re: ATTENTION!! Validty of Fluent's VOF??
 
I should add that I don't know how good is Fluent's VOF method and I'm not a fan of it, but I think that it's not true to talk about all other softwares as pseudo-VOF codes.

Hrvoje Jasak February 18, 2004 19:19

Re: ATTENTION!! Validty of Fluent's VOF??
 
Hi,

Firstly, the VOF-like methods will always have problems with parasitic velocities at high surface tension: the surface tension force is distributed over the interface (which is no longer a discrete surface) and this causes trouble.

For some details on surface-capturing free rising bubble simulations using FOAM, have a look at

http://monet.me.ic.ac.uk/people/henrus/

or ask the guy for a copy of the Thesis. As far as I know, he got the terminal velocities (and a bunch of other things as well) spot on, so there's nothing fundamentally wrong with the VOF as such.

Hrv


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