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Astrid January 9, 2003 18:58

Vanishing volume fraction
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.

Bob January 14, 2003 09:47

Re: Vanishing volume fraction
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

Jon January 14, 2003 11:25

Re: Vanishing volume fraction
Is this well converged?

How are you measuring the VF in the domain?

Which numerical schemes are you using for Vf and other varaibles?

Astrid January 15, 2003 05:53

Re: Vanishing volume fraction
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.


Jon January 16, 2003 04:43

Re: Vanishing volume fraction
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?

Astrid January 19, 2003 17:33

Re: Vanishing volume fraction
- 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.......


Bob January 20, 2003 11:01

Re: Vanishing volume fraction
Astrid, keep us posted if you geta result !! bob

Astrid February 5, 2003 17:23

Re: Vanishing volume fraction
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.


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