# Vanishing volume fraction

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 January 9, 2003, 18:58 Vanishing volume fraction #1 Astrid Guest   Posts: n/a Hi all, I am working on a CFD simulation with two phases (two gases), one inlet, and two outlets. The volume fraction of the first is around 0.99, the second is around 0.01. You can argue about if one should perform multiphase calculations with such a low fraction but that is not what I want to discuss here. The problem is that at the inlet the fluids enters the domain with the ratio as specified above but that the smallest fraction never ever reaches the outlets. Due to some strange kind of reason, the volume fraction vanishes (<1e-10). I have changed the scheme, performed double precision, switched the multiphase model between two continuous phases and a continous/dispersed couple (I do not want to use the homogeneous option because I need two velocity fields). No satisfying result at all. What is hapening here? Any suggestion? Thanks for reading, Astrid.

 January 14, 2003, 09:47 Re: Vanishing volume fraction #2 Bob Guest   Posts: n/a Hey Astrid, I'd call the guys at CFX as we had a similar problem. We were runnning CFX551 and it was a homogeneous free surface problems were we had a loss of fluid with a moving mesh. It may be the same problem I'm not sure. I spoke to a guy called Chris Staples. He was very helpful. Bob

 January 14, 2003, 11:25 Re: Vanishing volume fraction #3 Jon Guest   Posts: n/a Is this well converged? How are you measuring the VF in the domain? Which numerical schemes are you using for Vf and other varaibles?

 January 15, 2003, 05:53 Re: Vanishing volume fraction #4 Astrid Guest   Posts: n/a We started from a converged singlephase solution. From there it is does not converge very well for the small volume fraction. The large fraction converges properly. As the smallest fraction at the outlet is approximately zero, the balance is always at 100%. We performed upwind and high resolution scheme. THe latter was slightly better. Astrid

 January 16, 2003, 04:43 Re: Vanishing volume fraction #5 Jon Guest   Posts: n/a If none of the small volume fraction have reached the outlets, I would advise running it on further. Have you had a look where the smaller phase has deposited?

 January 19, 2003, 17:33 Re: Vanishing volume fraction #6 Astrid Guest   Posts: n/a - Running it any further had no result. - The smaller fraction hasn't deposited anywhere. It has just disappeared. The folks from CFX are currently working on it....... Astrid

 January 20, 2003, 11:01 Re: Vanishing volume fraction #7 Bob Guest   Posts: n/a Astrid, keep us posted if you geta result !! bob

 February 5, 2003, 17:23 Re: Vanishing volume fraction #8 Astrid Guest   Posts: n/a Dear Bob, It is all about drag. If the drag is too low or even zero, then the solver tries to reduce one volume fraction. Astrid

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