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-   -   Convergence Problem,urgent!! (http://www.cfd-online.com/Forums/fluent/29433-convergence-problem-urgent.html)

Fuyou March 1, 2002 04:46

Convergence Problem,urgent!!
 
Hello everyone, I just start my CFD study for a few weeks. Two weeks ago, I finished a prediction of low pressure transient gas jet using Fluent. And the predicted results were in good agreement with the experimental measurements. However, when I began to simulate high pressure transient gas jet, I encountered a convergence problem. I have tried many methods: lower the under-relaxation factors, reduce the time step size, refine the mesh; but they donĄŻt seem to work.. Please give some advice to me! Thanks in advance! And I have another problem here, propane turn to liquid under pressures more than 0.8MPa when its temperature is 300K. If I am to simulated propane jet with its driving pressure more than 0.8Mpa, when it is injected out, its pressure will decrease obviously and it will turn to gas phase. I donĄŻt know whether the material database include this property for propane. And if it doesnĄŻt, what should I deal with it? Thanks a lot!!

david March 1, 2002 17:19

Re: Convergence Problem,urgent!!
 
A) U can add/change the properties of materials B) DO NOT increase the pressure in one step.. for example, for the case of 0.8 MPa, start with 0.1 MPa, the output data of this should be used as initial conditions of a case of 0.2MPa and so on.. C) write script file for this purpose. DC

Fuyou March 4, 2002 07:47

Re: Convergence Problem,urgent!!
 
David, thank you very much for your suggestions. IĄŻll try them. But what I currently calculate is a unsteady state jet flow problem. Your suggestion of increasing the pressure step by step seems to work in the case of simulating steady state problem. Would you please give me some further suggestions? Many thanks in advance!!

Alain March 6, 2002 06:19

Re: Convergence Problem,urgent!!
 
Do you estimate the your mach number for the high pressure case.

With a pressure of 0.8 MPa you may be at a quite high Mach number.

In this case you should turn to coupled solver (implicite or explicite depending of your transient flow behavior).

Your second point is far more complex because you should use a multiphase model with phase change (heat and mass transfert between phase). To solve this kind of flow with compressibility effect is actually a hard task.

I hope this could help you

best regards

Alain

Fuyou March 8, 2002 03:14

Re: Convergence Problem,urgent!!
 
Alain,

Thanks a lot for your suggestions.Yes,the jet I currently investigated is a high Mach jet flow. It should be more than 1.5 Mach.I have tried to solve it using segregated solver before. Now, I am trying the coupled solver.

Thanks again!

Fuyou


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