Solution diverging !!!

- February 12, 2020 at 5:21 am
  
  sherwin684
  Subscriber
  
  While doing steady state simulation, everything is alright and the solution converges properly. But for the same problem, when I do the transient analysis ( 30 time steps, 100 terations/time step, time step length is 10 second and is fixed). How do I solve this? Should I use variable time stepping or should I reduce the iterations ? ( My solution converges at 163th iteration in steady state).
- February 12, 2020 at 6:23 am
  
  DrAmine
  Ansys Employee
  
  Your solution converged in steady state in terms of residuals I am sure is not really deep converged anyway.
  
  For practicals: In steady state switch on Coupled Solver and Pseudo Transient increase the verbosity under the Run panel to 1. Now Fluent prints the time scale used. You have now a time scale which you can start with it in transient analysis.
- February 12, 2020 at 7:09 am
  
  sherwin684
  Subscriber
  
  Thanks for your reply Amine. Could you please tell m what does it actually mean?
  
  And where do I find the coupled solver and pseudo transient?
  
  P.S. I am new to fluent!!
- February 12, 2020 at 9:38 am
  
  DrAmine
  Ansys Employee
  
  If you are new Fluent please carry out tutorial and you will understand all the keywords I am using.
  
  It means you are diverging: the stiffness of the matrix is increasing and you are not diagonal dominant anymore. 10 s as time step size is even for diffusive processes like heat equation a quite large time step!
- February 12, 2020 at 10:03 am
  
  sherwin684
  Subscriber
  
  Will reducing the time step size work? And do you have any sources for the tutorials?
- February 12, 2020 at 10:58 am
  
  DrAmine
  Ansys Employee
  
  Yeah just hit in Fluent on F1 or click on Help button and it directs you to the HElp landing page. From there navigate back to Starting Page for Fluent and you will see the tutorials.
  
  Yes decreasing time step size might help.