# iterations in transient flows

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 May 22, 2003, 09:16 iterations in transient flows #1 ben Guest   Posts: n/a Hi Fluent users, I am doing a transient flow calculations and would like to set the minimum number of iterations, but don't know how to do this. Does someone know how to do this? Thank you!

 May 22, 2003, 09:35 Re: iterations in transient flows #2 prasanth Guest   Posts: n/a no of iterations will be depended oon the time upto which the problem has to be analysed..i.e. if u decide the time step size..then the no. of time steps will be obtained by dividing the total time by the time step size. now. no of iterations per time step has be arbitrarily as high as possible..such that within that..it will converge in each time step... ok. all the best prasanth

 May 22, 2003, 14:51 Re: iterations in transient flows #3 ben Guest   Posts: n/a Thanks Prasanth for your response. Sometimes Fluent does very few iterations and declare convergence. I was wondering if there is a way to fix the minimum number of iterations without changing the residual. I'd like Fluent to do a minimum of 8 iterations even if it thinks that it converged!

 May 23, 2003, 01:05 Re: iterations in transient flows #4 Philipp Beierer Guest   Posts: n/a I guess one way to enforce x iteration steps is to reduce some of the limits in the convergence criteria list. Hope this helps!

 May 25, 2003, 16:06 REducing the Empirism in Unsteady cases #5 Alex Munoz Guest   Posts: n/a Hi Ben I apologize for do not answer you before, but I have been disappointed with the quality of the forum recently. To determine the time step you have to estimate the Strouhal number of your case. Search for a case study under the name unsteady laminar flow behind a circular cylinder posted on fluent documentation (validation 6.0). Just to give you one idea Strouhal=(Obstacle diameter)/(period*velocity) Then you should get your experimental strouhal number for your case and then estimate the period that you need. The number of iteration per time step should be at least 20 iterations per time step. if you case request more number of iteration step reduce the time steps. The number of time steps should be determine as follow: Locate a dot in the point that you are expecting fluctuation of one of the velocity components. then, register the above information at every time step using the surface monitor ( see surface monitor instructions) Finally, when the history of this velocity points is stable (same wave amplitude and frequency) you will know that you have obtain the right solution for a periodic flow. Best regards Alex Munoz

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