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David November 7, 2002 15:29

Suggestions for convergence
I am simulating gas-liquid two-phase fow in a pipe. The liquid seems to be converging but gas phase doesn't. Could anybody please give me some suggestions? Thanks


Astrid November 10, 2002 18:32

Re: Suggestions for convergence

I have read something on this subject in the CFX-4 manual. If I remember it correctly it said that if you have convergence problems you have to use a (much) smaller timescale for the gasphase than for the liquid phase. Then, after you get a pseudo-steady state, you should switch to larger timescales.


David November 11, 2002 01:43

Re: Suggestions for convergence
Thanks, Astrid. Do you mean that I can switch from a smaller timescale to larger timescale during the solver is running or after this run finished? How to implement it?

Regards David

Astrid November 11, 2002 18:02

Re: Suggestions for convergence
If you have little experience you should use physical timestepping and increase the timescale after you get the pseudo steady state. If you are more confident of the case you're running, you could increase the timescale automatically during the run. Use 'Auto Timescale' and all the options that are discussed in detail in the help.

Good luck, Astrid

David November 12, 2002 02:50

To Astrid
My simulation can converge to a critea between 0.001 and 0.0001 after the 20th step and then almost keep a constant residual but still decreasing very very slowly. It seems need a huge number of timesteps to meet a 0.0001 residual. I tried almost all sort of timestep, the results are same. The velocity balance is small while for other variables such as mass fraction the balance is very big. Could you please tell me how to do it? Thanks


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