CFD Online Discussion Forums

CFD Online Discussion Forums (http://www.cfd-online.com/Forums/)
-   FLUENT (http://www.cfd-online.com/Forums/fluent/)
-   -   maximum number of iterations (http://www.cfd-online.com/Forums/fluent/31340-maximum-number-iterations.html)

solomon April 22, 2003 15:44

maximum number of iterations
 
Hello!

What is the maximum number of iterations one should expect to reach convergency. I tried up to 5000 but not reached to convergency. Is there any guide line one can follow to determine the number of iterations one can tray to see if convergency can be achieved.

Alex Munoz April 22, 2003 16:37

Re: maximum number of iterations
 
Hi

Your question is too broad, keep in mind that the itertion number is different for each case and also it depends of the turbulence model selected, relaxation factors, discretization and grid type.

You should provide more infromation about your case!!

Regards

Alex Munoz

solomon April 22, 2003 19:03

Re: maximum number of iterations
 
Dear Munoz

I agree with you. But I just wonder rougly under extreme cenario what the case could be. I am enquiring not problem specific answer. I am traying to get a guide line so that I can get a feeling to say well this is too much iteration attempt let me do some adjustemnt.

You can even tell me from your previous experience the highest number of iteration which leads to convergence in CFD analysis you encountered.

ap April 22, 2003 20:16

Re: maximum number of iterations
 
It's difficult to give general guidelines because the number of iteration you need to obtain a converged solution depends on a lot of factors (models, boundary and initial conditions). FLUENT considers a solution converged if all scaled residuals reduce of three order of magnitude (six for energy). But if you set a good initial solution for your problem, you couldn't notice such a reduction because you're already close to the converged solution.

However, for steady calculations I think you should stay around 1000-2000 iterations. While for unsteady calculation, the manual suggests 20 iteration per time step, but sometimes I had to increase to 30-40 at the beginning of the calculation.

5000 iterations seem too much, maybe your problem is ill posed. Check boundary conditions. Or maybe you're initial solution is extremly good, so you can't have a big reduction of residuals. Check their behaviour to understand that (are they stable/reducing or they vary a lot?).

Hi and good work

ap

Alex Munoz April 22, 2003 23:33

Re: maximum number of iterations
 
Hi

I have follow ap's answers not only for this case but also in other cases. He seems to me like a senior CFD user. Therefore, I am completely agree to his comments.

I only want to added some ideas like

Too many iteration is sign of poor mesh or wrong mesh selection (structured, unstructured, hexahedral tgrid, etc)

I have run cases where only 600 iterations are enough, However, I have read that some researchers use 5000 iterations for the same case. In my opinion the diferences could be the goals of your project.

In my case I just want the velocity field, in the researchers case, they want the velocity, turbulent kinetic energy (TKE)and its dissipation rate(TDR). Now, you can understand that the iterations number depends of your objectives.

Boundary conditions are fundamental in whatever case, after all, what a CFD code does is projection of the boundary condition inside the domain. For instance, right boundary condition for x-velocity, TKE and TDR leads to reduce the number of iteration to half or less.

Now, If you are running an unsteady case, a few iteration per time step could tell you that the spatial resolution is not enough to capture the periodicity of the flow. Too many iteration per time step could tell you that the time resolution is too high an you need to reduce the time step size instead of increasing the number of iteration per time step.

I hope I help you! I know that you try to keep the confidenciality of your proyect. However, sooner or later you have to provide some information in order to have progress.

Anyway, If you need more help you can send me an e-mail. Do not forget that "ap" knows a lot about CFD.

Best regards

Alex Munoz


ap April 23, 2003 07:42

Re: maximum number of iterations
 
Thanks for compliments Alex.

Hi

ap

shewaferaw solomon April 24, 2003 15:42

Re: maximum number of iterations
 
Alex Munoz and ap thank you for your valuable explanation on the subject.

Solomon


All times are GMT -4. The time now is 13:49.