# PyFR setup and error **URL:** \ **Category:** Just Starting\ **Created:** [19 November 2020 12:39 UTC](https://pyfr.discourse.group/t/pyfr-setup-and-error/311 "2020-11-19T12:39:15Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![Amir.Matin](https://avatars.discourse-cdn.com/v4/letter/a/4bbf92/32.png) [@Amir.Matin](https://pyfr.discourse.group/u/Amir.Matin)\ **Post date:** [19 November 2020 12:39 UTC](https://pyfr.discourse.group/t/pyfr-setup-and-error/311/1 "2020-11-19T12:39:15Z")
Dear PyFR developers, I am writing my master thesis using pyfr to simulate the flow over a turbine cascade similar to T106c test case used in “F. D. Witherden and A. Jameson Impact of Number Representation for High-Order Implicit Large-Eddy Simulations” paper. However I have some questions about the setup and an error I am facing at: 1. The Error “Minimum sized time step rejected”, does it mean, that the simulation diverges? Sometimes the simulation goes on, when I just restart the simulation from the last solution, however sometimes I get the error at one specific time. I also tried to limit the time step using dt-max, but it did not work. If I turn off the PI controller, I get NaN values instead. I should mention that I have relatively fine mesh (comparing with your T106c test case). Do you have any suggestions, how I can prevent this error? 2. Do I need to use anti-ailiasing just for Polynomial orders higher than 2? Did you use it for the T106c case at P2 and Re=80000? 3. In my understanding the reference Reynolds number is not defined in pyfr in case of non-dimensionalaizing, therefore we have to eliminate it by choosing a specific reference dynamic viscosity, namely Mu\_t0 = rho\_t0 \* a\_t0 \* Lc (Mu\_t0 : Reference dynamic viscosity, rho\_t0 : Reference density, Lc : Reference Lenght). So that the reference Reynolds-number would be 1. Is that correct? Regards, Amir Matin ---
**Author:** ![fdw](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@fdw](https://pyfr.discourse.group/u/fdw)\ **Post date:** [19 November 2020 14:48 UTC](https://pyfr.discourse.group/t/pyfr-setup-and-error/311/2 "2020-11-19T14:48:10Z")
Hi Amir, > 1. The Error “Minimum sized time step rejected”, does it mean, that the simulation diverges? Sometimes the simulation goes on, when I just restart the simulation from the last solution, however sometimes I get the error at one specific time. I also tried to limit the time step using dt-max, but it did not work. If I turn off the PI controller, I get NaN values instead. I should mention that I have relatively fine mesh (comparing with your T106c test case). Do you have any suggestions, how I can prevent this error? This typically means that the simulation is diverging. There are many possible reasons for this; including your mesh and start-up procedure. > 2. Do I need to use anti-ailiasing just for Polynomial orders higher than 2? Did you use it for the T106c case at P2 and Re=80000? Here it very much depends on the mesh, polynomial order, and Reynolds number. For this particular case I did not run with anti-aliasing, however I did need to be careful with how I started the simulation (running at p = 1 for a period of time before restarting at p = 2). > 3. In my understanding the reference Reynolds number is not defined in pyfr in case of non-dimensionalaizing, therefore we have to eliminate it by choosing a specific reference dynamic viscosity, namely Mu\_t0 = rho\_t0 \* a\_t0 \* Lc (Mu\_t0 : Reference dynamic viscosity, rho\_t0 : Reference density, Lc : Reference Lenght). So that the reference Reynolds-number would be 1. Is that correct? The Reynolds number is typically fixed by changing the viscosity, yes. Regards, Freddie. ---
**Author:** ![Amir.Matin](https://avatars.discourse-cdn.com/v4/letter/a/4bbf92/32.png) [@Amir.Matin](https://pyfr.discourse.group/u/Amir.Matin)\ **Post date:** [19 November 2020 20:55 UTC](https://pyfr.discourse.group/t/pyfr-setup-and-error/311/3 "2020-11-19T20:55:29Z")
Hi Freddie, thanks for your kind reply. Regards Amir