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
On 13/01/20 14:48, Giulio Ortali wrote:
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
Il giorno lunedì 13 gennaio 2020 15:34:19 UTC+1, Niki Loppi ha scritto:
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
On 13/01/20 08:25, Giulio Ortali wrote:
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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
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