[Nek5000-users] Question about tolerance

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

The units of pressure and velocity are not the same so there is no expectation

that the tolerances would be the same.


The velocity generally converges much more rapidly so there is no cost in making

that tolerance tighter and doing so generally helps to keep things relatively smooth.


Thus, the key is - what tolerance for the pressure.  In non dimensional units based

on convective timescales 1.e-6 is generally found to be adequate for PnPn-2; a bit

looser is ok for PnPn.


In some cases (e.g., if you are measuring very small lift values for flow past a cylinder,

say) you might need tighter tolerances, but these work for most applications.


For transition or for "exact" solution comparisons, you would also want tighter

tolerances, e.g., 1.e-10/1.e-12 or even 1.e-12/1.e-14.


You have to be careful to not get too close to machine precision because the condition

numbers of the governing system matrices will prevent you from converging to

tolerances that are too tight.


hth,


Paul


________________________________
From: Nek5000-users <nek5000-users-bounces at lists.mcs.anl.gov> on behalf of nek5000-users at lists.mcs.anl.gov <nek5000-users at lists.mcs.anl.gov>
Sent: Wednesday, September 26, 2018 11:42:34 AM
To: nek5000-users at lists.mcs.anl.gov
Subject: Re: [Nek5000-users] Question about tolerance

No, the answer is in why they are different, not why there are two tolerances. Because, it seems that tolerance 10^-4 for the pressure means the tolerance 10^-4 for the velocity, but in .par file it is 10^-6 for the velocity.


среда, 26 сентября 2018 г., 17:24 +0700 от nek5000-users at lists.mcs.anl.gov <nek5000-users at lists.mcs.anl.gov>:
There are two solvers -> two tolerances.

-----Original message-----
> From:nek5000-users at lists.mcs.anl.gov <nek5000-users at lists.mcs.anl.gov<mailto:nek5000-users at lists.mcs.anl.gov>>
> Sent: Wednesday 26th September 2018 11:27
> To: nek5000-users <nek5000-users at lists.mcs.anl.gov<mailto:nek5000-users at lists.mcs.anl.gov>>
> Subject: [Nek5000-users] Question about tolerance
>
> Hi, Neks!
>
> I'm wonder why in .par file, for example in the turbChannel case, there are different tolerances for pressure and for velocity? I mean that velocity seems to be dependent on pressure field, and if it is 10^-4 tolerance for the pressure, it will automatically the same for the velocity?
>
> Best regards,
> Vlad
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