Nan after 1 timestep with 2D airfoil problem based on inc_2D_cylinder

Just Starting
1

Hello everyone,
I am new to pyfr, and i need to use it in order to generate some high fidelty data on a 2D incompressible airfoil problem. I tried replicating the inc_2d_cylinder case by changing the mesh, generated using gmsh. However, when i launch the run, I obtain all NaN in all the domain after the first timestep, and i don’t understand why. I upload the geo, msh and ini files.

Thank you in advance,
Giulio

ls417.msh (535 KB)

ls417.ini (1.4 KB)

ls417.geo (6.58 KB)

2

Hi Giulio,

Thanks for your interest.

The pseudo-time step size (pseudo-dt) is restricted by the explicit CFL limit. You seem to be running the case with higher Re and your mesh probably contains smaller elements than the inc_2d_cyl case, so you need decrease pseudo-dt for the simulation to remain stable.

Cheers,

Niki

3

Hi Niki,
This solved the NaN problem, thank you.

So for stability related to the CFL number i can only refer to the pseudo dt, keeping it it low enough, and keep dt arbitrary? Or what other restriction I have to impose?

Thank you,
Giulio

4

Yes, exactly. Physical dt is implicit so it can be arbitrary. Pseudo dt is explicit so it’s limited by CFL. This allows effective utilisation of modern massively parallel hardware architectures. I tend to keep the dt/pseudo-dt ratio in the range of 50x.

The other parameters that you can fine tune to achieve better convergence rates are

pseudo-dt-max-mult = 2.5
pseudo-dt-fact = 1.75

pseudo-dt-max-mult sets the factor how much pseudo time steps can be increased locally by the local-pi controller. Pseudo-dt-fact is the factor how much pseudo-dt is increased between polynomial multigrid levels. If you increase these too much the simulation tends to blow up.

Cheers,
Niki