[Nek5000-users] Tolerances

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

Just to give you some idea of the potential for looking at
these stabilities, resolution reqmts, etc., you might look at:

www.mcs.anl.gov/~fischer/users.pdf

which scopes out several problems, including some stability ones.
(Though I've yet to get the OS problem typed up and put in this
ever-growing document...)

Paul


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

> Dear Stefan,
>
> Thank you for your quick reply!
>
> Yes, we have already tried to refine the computational domain by rising the 
> polynomial order from 7 to 9 (lx1=8 -> 10) which does not improve the quality 
> of the results much (numerical artefacts in the freestream are damped while 
> the numerically excited crossflow waves remain unchanged). 
> In the case of higher resolution one disturbance wavelength would be resolved 
> by 37 points on average which sounds enough to us. We will try to further 
> refine the domain and to "play" with the filtering at highest order. Maybe we 
> can suppress the numerical noise triggering the physical instability in this 
> way...
>
> Cheers,
>
> David
> Lars-Uve
>
>
>
> nek5000-users at lists.mcs.anl.gov wrote:
>> 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
>> 
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