- **CFX**
(*http://www.cfd-online.com/Forums/cfx/*)

- - **Convergence Criteria for first Timestep in transient run**
(*http://www.cfd-online.com/Forums/cfx/113212-convergence-criteria-first-timestep-transient-run.html*)

Convergence Criteria for first Timestep in transient runHello,
Does anyone know if it is possible to have a different convergence criteria for the first timestep of a transient simulation? I would like to converge the first timestep for a minimum of ~500 iterations, then allow each subsequent timestep to end at a maximum of say ~50 iterations. Is this possible? THANKS! |

You could try making the max number of iterations a CEL expression.
But 500 coeff loops for an iteration is crazy. This is FAR too much. Even 50 per iteration is too much. A far better way is to use adaptive time stepping homing in on 3-5 coeff loops per iteration. Much faster and more accurate. |

Thank you. I think you are right after a bit of research. These transient sims are pretty time intensive so it's not the time to be learning by trial and error.
My specific case is a turbomachinery case and I have found that the solution takes many timesteps to converge towards a realistic and periodic one. As a result I end up throwing out many of the first few timesteps. That works, but I'd prefer to find a way to converge to a useful solution in the first couple of timesteps to save time and space. At any rate... I am trying an approach with more timesteps and a lower number of iterations to see if the results come out acceptably... thanks again. |

If you do not care about the initial transient then do not bother accurately simulating it. Just do it approximately and quickly and use a more accurate setup when the flow has matured.
But the adaptive timesteps with 3-5 coeff loops per time steps is almost always the best option available. You are wasting your time by doing anything else. |

All times are GMT -4. The time now is 01:29. |