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April 13, 2005, 08:22 
multiphase model gives unreal results

#1 
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Hi All! I am using VOF model for simulating flow of liquid in vessel, solver is steady, segregated. Mixture of liquid and gas flow into the vessel from top (pressure inlet) and there two exits (pressure outlets) for gas and liquid (shortly it is a separator vessel). Solution converges in approx. 170 iteration BUT the result is not real. There is no phase separation taking place, the whole volume is filled up with mixture. Gravity option is enabled.
What could be the problem? I need your help very much, since there is noone ask. 

April 13, 2005, 10:05 
Re: multiphase model gives unreal results

#2 
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Hi,
steady : are you sure ? For 90 % of the cases : unsteady / Georeconstruct / small time step. phase 1 : the lightest Discretisation : Pressure : if gravity => Body force weighted / Pressurevelocity coupling : Piso. Underrelaxation factor : 1 for volume fraction And : skewness < 0,85 steph 

April 13, 2005, 10:47 
Re: multiphase model gives unreal results

#3 
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As I remember, a VOF calculation must be done in transient.
You speak about phase separation: for VOF method, the 2 fluids must be nonmiscible. So they are already separated. I think you make a confusion between VOF method and eulereuler method. 

April 13, 2005, 23:40 
Re: multiphase model gives unreal results

#4 
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Hi, If you use VOF model, it is recommended to use unsteady solver because the interface is unsteady and the liquid phase and the gas phase should be set separatly.


April 14, 2005, 01:57 
Re: multiphase model gives unreal results

#5 
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Hi All! Thanks for the response! Well I'm not sure about the steady/unsteady, so I tried as Steph offered, but in order to keep it running I had to set very small time step (1e5 or even 5e6). If i increase time step then Fluent reports "divergence detected in AMG solver: epsilon". Is it the way it should be? On the other hand according to results i have so far in unsteady (5000 time steps) it seems like the separation works.
So again I ask for your help, how can i decrease time step value? Or may be there is some other point to be considered? Thanks All again! 

April 14, 2005, 06:46 
Re: multiphase model gives unreal results

#6 
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Well, I don't know anything in particular about the reason of the divergence detected in the Algebraic MultiGrid solver, but I can suggest you a practice to find out a good TS (not too small) for a stable run:
Start the simulation with a very small TS, allowing good convergence within 20 it. per time step. After a number of TS save the data. Specify a very large TS (at least 6 orders of magnitude larger than before). Fluent should pass an error detailing the number N of VOF subtime steps needed to cover the specified deltaT. Thus, the VOF subtime step is tau=deltaT/N. They suggest to continue the simulation with a time step within the limit of 1020 subtime steps. Hope it helps, Edi. 

April 14, 2005, 07:17 
Re: multiphase model gives unreal results

#7 
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Thanks for response, presently I am doing almost the same, start with 1e5 time step, after a few steps gradually increase it so that it takes 1020 iteration per time step as you mentioned. On the other hand I'm wondering whether it is possible to increase time step without letting it diverge, may be by changing URF or something else.
Thanks for any idea and response! 

April 14, 2005, 07:29 
Re: multiphase model gives unreal results

#8 
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And I am wondering whether it is possible this problem in steady mode. Short description is as follows: liquid with gas enter horizontal vessel and fall on an inclined shelf (sorry, not sure if this the wright word for it). Then gas separates from liq, liquid flows down the shelf as a film. Degassed liq goes to the bottom of vessel and leaves through preassure outlet, gas does the same from the top. As to me it seems like it could solved in steady mode, what you say?
Thanks again for any assistance! 

April 14, 2005, 09:01 
Re: multiphase model gives unreal results

#9 
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I think you better change your model (VOF is preferred for stratified or freesurface flows, phases immiscibles), try eulerian or mixture.
By the way, I set these URFs for my liquid sloshing problem: pressure: 0.6 momentum: 0.8 others: 1 (discretization: BFW for pressure, PISO for pressurevelocity coupling) Edi. 

April 14, 2005, 09:31 
Re: multiphase model gives unreal results

#10 
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Ok I'll try to change the model, hope it will work


April 15, 2005, 17:51 
Re: multiphase model gives unreal results

#11 
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Your system is not steady, so you have to simulate it as unsteady. When you choose the time step, remember that you need to respect both numerical conditions (none for implicit schemes) and physical contraints. So your time step has to be smaller than the characteristic time of the phenomena you want to model.
The Eulerian model seems more appropriate to your case. You'd better to use a velocity inlet however, if possible. Regards, ap 

April 15, 2005, 21:03 
Re: multiphase model gives unreal results

#12 
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Well thanks, now it is clear that it should be unsteady (at least it works). As for the time step, seems like it is also clear. And the pressure inlet seems to be not the best choise, anyway to start up i have to init with velocity, otherwise fluent reports divergence in amg: pressure correction.
Well, thanks to All for the assistance again! 

April 16, 2005, 07:17 
Re: multiphase model gives unreal results

#13 
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I'd like to add something on the choice of the boundary condition.
You have to choose pressure inlet when you have a compressible flow, while if you have an uncompressible one, velocity inlet should be the choice. Regards, ap 

April 16, 2005, 16:31 
Re: multiphase model gives unreal results

#14 
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Thanks for the assistance! Solution seems to work now, all equations converge but continuity. Its scaled residual is usually increasing as more time steps are proceeded (it goes up to 10e1 or even more). Should i worry about it? May be i can use normalaized residual instead? Any advises are welcome
Thanks in advance! 

April 17, 2005, 07:52 
Re: multiphase model gives unreal results

#15 
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Try to underrelax the momentum equation. Usually a good choice is to reduce momentum underrelaxation factor and to slightly increace the pressure URF.
Regards, ap 

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