open TELEMAC-MASCARET The mathematically superior suite of solvers

Hello!
I am running a simulation with Telemac-3D but I have a problem.
I tried the simulation with timestep=4 seconds and it worked. Now, I am trying the same simulation with time-step= 2 seconds and the simulation shows NaN values after a few hours (error message in picture attached).


This error should be due to a CFL number which is too high, right? But I lowered the time-step, so in theory the CFL should be reduced.
I really don't understand what's going on.

Also, is there a way to have an automatic CFL calculation or to impose a CFL number =1 in the simulation?
Thank you a lot if you can help me

Hello Giulia,

As told many times in this forum, as soon as you get a NaN or segfault, use debug options to compile and run:
you should use a debug configuration with debug options to investigate.
See e.g. the S11.gfortran.debug configuration in the $HOMETEL/configs/systel.edf.cfg configuration file, in particular the flag fflags_debug_gfo for gfortran compiler.
fflags_debug_gfo: -g -Wall -fcheck=all -fbacktrace -fbounds-check -finit-integer=-1 -finit-real=nan -ffpe-trap=invalid,zero,overflow

It will show in which subroutine the issue/nan/segfault occurs, the first suspicious line and may help you to change something in your computation.
And in particular, the computation will stop as soon as a segfault or a NaN accurs.

In TELEMAC-3D, there is no way to have any automatic CLF calculation.

Your issue may not be a CFL issue, perhaps you were lucky with your 4 s time steps.

Anyway, with NaN, I start with my 1st advice.

Hope this helps,

Chi-Tuan