Vanishing volume fraction

January 9, 2003, 17:58

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, 08:47

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, 10:25

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, 04:53

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, 03:43

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, 16:33

- 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, 10:01

Astrid, keep us posted if you geta result !! bob

February 5, 2003, 16:23

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

January 9, 2003, 17:58	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, 08:47	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, 10:25	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, 04:53	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, 03:43	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, 16:33	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, 10:01	Re: Vanishing volume fraction	#7
Bob Guest Posts: n/a	Astrid, keep us posted if you geta result !! bob

February 5, 2003, 16:23	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

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
how to set periodic boundary conditions	Ganesh	FLUENT	15	November 18, 2020 06:09
alphaEqn.H in twoPhaseEulerFoam	cheng1988sjtu	OpenFOAM Bugs	15	May 1, 2016 16:12
On the damBreak4phaseFine cases	paean	OpenFOAM Running, Solving & CFD	0	November 14, 2008 21:14
fluent add additional zones for the mesh file	SSL	FLUENT	2	January 26, 2008 11:55
[blockMesh] Axisymmetrical mesh	Rasmus Gjesing (Gjesing)	OpenFOAM Meshing & Mesh Conversion	10	April 2, 2007 14:00