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<p style="margin-top:0;margin-bottom:0">The units of pressure and velocity are not the same so there is no expectation</p>
<p style="margin-top:0;margin-bottom:0">that the tolerances would be the same.</p>
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<p style="margin-top:0;margin-bottom:0">The velocity generally converges much more rapidly so there is no cost in making</p>
<p style="margin-top:0;margin-bottom:0">that tolerance tighter and doing so generally helps to keep things relatively smooth.</p>
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<p style="margin-top:0;margin-bottom:0">Thus, the key is - what tolerance for the pressure. In non dimensional units based</p>
<p style="margin-top:0;margin-bottom:0">on convective timescales 1.e-6 is generally found to be adequate for PnPn-2; a bit</p>
<p style="margin-top:0;margin-bottom:0">looser is ok for PnPn.</p>
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<p style="margin-top:0;margin-bottom:0">In some cases (e.g., if you are measuring very small lift values for flow past a cylinder,</p>
<p style="margin-top:0;margin-bottom:0">say) you might need tighter tolerances, but these work for most applications.</p>
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<p style="margin-top:0;margin-bottom:0">For transition or for "exact" solution comparisons, you would also want tighter </p>
<p style="margin-top:0;margin-bottom:0">tolerances, e.g., 1.e-10/1.e-12 or even 1.e-12/1.e-14.</p>
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<p style="margin-top:0;margin-bottom:0">You have to be careful to not get too close to machine precision because the condition</p>
<p style="margin-top:0;margin-bottom:0">numbers of the governing system matrices will prevent you from converging to </p>
<p style="margin-top:0;margin-bottom:0">tolerances that are too tight.</p>
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<p style="margin-top:0;margin-bottom:0">hth,</p>
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<p style="margin-top:0;margin-bottom:0">Paul</p>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> Nek5000-users <nek5000-users-bounces@lists.mcs.anl.gov> on behalf of nek5000-users@lists.mcs.anl.gov <nek5000-users@lists.mcs.anl.gov><br>
<b>Sent:</b> Wednesday, September 26, 2018 11:42:34 AM<br>
<b>To:</b> nek5000-users@lists.mcs.anl.gov<br>
<b>Subject:</b> Re: [Nek5000-users] Question about tolerance</font>
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<div>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. </div>
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среда, 26 сентября 2018 г., 17:24 +0700 от nek5000-users@lists.mcs.anl.gov <nek5000-users@lists.mcs.anl.gov>:<br>
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<div id="x_style_15379574560000000785_BODY">There are two solvers -> two tolerances.
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-----Original message-----<br>
> From:nek5000-users@lists.mcs.anl.gov <<a href="mailto:nek5000-users@lists.mcs.anl.gov">nek5000-users@lists.mcs.anl.gov</a>><br>
> Sent: Wednesday 26th September 2018 11:27<br>
> To: nek5000-users <<a href="mailto:nek5000-users@lists.mcs.anl.gov">nek5000-users@lists.mcs.anl.gov</a>><br>
> Subject: [Nek5000-users] Question about tolerance<br>
> <br>
> Hi, Neks!<br>
> <br>
> 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?<br>
> <br>
> Best regards,<br>
> Vlad<br>
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