[Nek5000-users] Tolerances

Wed Apr 28 07:02:35 CDT 2010

Hi Lars-Uve and David,

I have been recently looking at the Rayleigh inviscid instability in 2D Navier-Stokes with Nek5000.  

A combination of higher resolution and tighter DIVERGENCE/HELMHOLTZ tolerances (1e-10/1e-11) did the trick...

Best,
Aleks

----- Original Message -----
From: nek5000-users at lists.mcs.anl.gov
To: nek5000-users at lists.mcs.anl.gov
Sent: Wednesday, April 28, 2010 5:39:36 AM GMT -06:00 US/Canada Central
Subject: Re: [Nek5000-users] Tolerances

Do you really think 15 pressure iterations are too much? Considering you strict tolerance I would say it's pretty good! I am not sure what triggers your noise but did you try to use a high resolution?

In your case TOLREL/TOLABS are not important!

The tolerance you specify in DIVERGENCE is for the pressure solver. In fact if you use the PN/PN_2 scheme you'll end up with a divergence of the velocity which is comparable to DIVERGENCE. The pressure solve projects the velocity on a divergence free space. and you can control the divergence by varying the pressure tolerance (that's why we call it DIVERGENCE). In the PN/PN scheme things are different.

The HELMHOLTZ controls the tolerance for all Helmholtz solves so if you don't have any passive scalars it's just for the velocity.

Stefan

On Apr 28, 2010, at 10:13 AM, nek5000-users at lists.mcs.anl.gov wrote:

> 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
> 
> 
> 
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