[Nek5000-users] Tolerances

[nek5000-users at lists.mcs.anl.gov]

Dear Paul and Stefan,

We are currently studying a 3D flat-plate boundary layer which is highly 
unstable to crossflow modes. When computing the base flow we realize 
that numerical noise already triggers unsteady crossflow waves with 
amplitudes of the order of 10^{-6}. We plan to study receptivity to 
external perturbations with amplitudes of 10^{-5} (to avoid early 
nonlinearity) so it would be desirable to suppress the numerically 
triggered disturbance waves.

We use the PN-PN-2 formulation, dealiasing and filtering (0.05). We also 
noticed that about 15 pressure iterations per timestep (gmres) were 
needed which seems to be high.

We further use the following tolerances:
       1.0E-08         DIVERGENCE
       1.0E-08         HELMHOLTZ

       1.0E-06         TOLREL
       1.0E-10         TOLABS

We wonder whether using stricter values may lead to smoother results. 
What is actually the meaning of TOLREL and TOLABS?

Thank you very much for your advice and some clarification.

Greetings from KTH Stockholm,

Lars-Uve and David