[Nek5000-users] Guidelines for Tolerances
nek5000-users at lists.mcs.anl.gov
nek5000-users at lists.mcs.anl.gov
Sat Mar 31 13:44:49 CDT 2018
In the PnPn formulation you don’t project into a divergence free space so you don’t prescribe the divergence. It’s a simulation result.
> On 31 Mar 2018, at 19:09, "nek5000-users at lists.mcs.anl.gov" <nek5000-users at lists.mcs.anl.gov> wrote:
>
> I guess in the end the chosen/requested tolerances depend on what one is
> interested in, and how sensitive a specific flow case is. Also, the
> spatial distribution of the divergence error might be an interesting
> quantity to look at.
>
> However, I have a question to Stefan: I am not sure I fully understand
> your comment regarding spatial resolution. Do you mean that the spatial
> errors are so large that a specific divergence tolerance cannot be
> fulfilled?
>
> Philipp
>
>> On 2018-03-31 11:20, nek5000-users at lists.mcs.anl.gov wrote:
>> Your tolerances look good to me. Typically 1e-5 for pressure is accurate enough. Note, in the Pn/Pn-2 formulation the tolerance is equivalent to the desired divergence error. This is in contract to Pn/Pn where such a relation does not exists. In most cases the divergence error is dominated by the spatial resolution. That's something you can try - just lower the tol to say 1e-5. If you'll get a similiar divergence error (compared to 1e-8) try to lower it even further.
>>
>> --
>> Stefan
>>
>>
>> -----Original message-----
>>> From:nek5000-users at lists.mcs.anl.gov <nek5000-users at lists.mcs.anl.gov>
>>> Sent: Saturday 31st March 2018 11:07
>>> To: nek5000-users at lists.mcs.anl.gov
>>> Subject: [Nek5000-users] Guidelines for Tolerances
>>>
>>> Dear Nek experts,
>>>
>>> sorry, my first mail was incomplete. I will start again.
>>>
>>> I am simulating turbulent pipe flow with 6 thermal fields treated as passive scalars.
>>> The first three thermal fields use Pr=0.71 and the remaining three use Pr=0.025.
>>>
>>> In the par file, I set residualTol = 1e-08 for pressure velocity and all thermal fields. Having read the recent discussion about another logfile, I was wondering whether these settings might be too tight.
>>>
>>> The log for one timestep is given below (sorry about the formatting).
>>> As you can see, I need 16 - 21 iterations for the scalar fields of Pr=0.025.
>>> Do you think I could reduce the residualTol for the low Pr fields to speedup the simulation?
>>>
>>> Best regards,
>>> Steffen
>>>
>>>
>>> Step 99999, t= 8.4999900E+02, DT= 1.0000000E-03, C= 0.291 3.3389E+04 3.2523E-01
>>> Solving for Hmholtz scalars
>>> 99999 Project TEMP 9.7160E-08 5.5444E+00 5.7064E+07 8 8
>>> 99999 Hmholtz TEMP 6 8.0673E-09 3.1410E-05 1.0000E-08
>>> 99999 Project PS 1 1.9038E-07 6.6127E+00 3.4734E+07 8 8
>>> 99999 Hmholtz PS 1 7 6.2818E-09 6.1081E-05 1.0000E-08
>>> 99999 Project PS 2 5.7194E-06 1.1100E+01 1.9408E+06 7 8
>>> 99999 Hmholtz PS 2 8 4.7345E-09 1.5839E-03 1.0000E-08
>>> 99999 Project PS 3 1.6110E-09 2.0874E-01 1.2958E+08 8 8
>>> 99999 Hmholtz PS 3 16 8.1941E-09 6.8549E-07 1.0000E-08
>>> 99999 Project PS 4 1.8743E-09 2.5528E-01 1.3620E+08 8 8
>>> 99999 Hmholtz PS 4 16 9.3461E-09 7.7774E-07 1.0000E-08
>>> 99999 Project PS 5 4.9138E-08 4.4469E-01 9.0499E+06 7 8
>>> 99999 Hmholtz PS 5 21 8.2110E-09 1.3142E-05 1.0000E-08
>>> 99999 Scalars done 8.5000E+02 1.7601E-01
>>> Solving for fluid
>>> 99999 Project VELX 5.5774E-09 6.9874E-03 1.2528E+06 8 8
>>> 99999 Hmholtz VELX 4 2.7784E-09 1.7303E-06 1.0000E-08
>>> 99999 Project VELY 5.4643E-09 6.9118E-03 1.2649E+06 8 8
>>> 99999 Hmholtz VELY 4 2.7477E-09 1.6969E-06 1.0000E-08
>>> 99999 Project VELZ 5.0326E-09 6.9213E-03 1.3753E+06 8 8
>>> 99999 Hmholtz VELZ 4 6.0439E-09 2.0347E-06 1.0000E-08
>>> 99999 Project PRES 1.5397E-04 9.8607E-02 6.4043E+02 14 20
>>> 99999 U-PRES gmres 13 9.9860E-09 8.3983E-08 1.0000E-08 4.3257E-02 7.0065E-02
>>> 99999 volflow Z 1.8796E-02 8.0416E-06 7.8539E-01 7.8540E-01
>>> 99999 Fluid done 8.5000E+02 1.2875E-01
>>> ________________________________________
>>> Von: Straub, Steffen (ISTM)
>>> Gesendet: Samstag, 31. März 2018 10:45
>>> An: nek5000-users at lists.mcs.anl.gov
>>> Betreff: AW: Nek5000-users Digest, Vol 109, Issue 32
>>>
>>> Dear Nek experts,
>>>
>>> I am simulating turbulent pipe flow with 6 thermal fields treated as passive scalars.
>>> The first three thermal fields use Pr=0.71 and the remaining three use Pr=0.025.
>>>
>>>
>>> ________________________________________
>>> Von: Nek5000-users <nek5000-users-bounces at lists.mcs.anl.gov> im Auftrag von nek5000-users-request at lists.mcs.anl.gov <nek5000-users-request at lists.mcs.anl.gov>
>>> Gesendet: Freitag, 30. März 2018 17:02
>>> An: nek5000-users at lists.mcs.anl.gov
>>> Betreff: Nek5000-users Digest, Vol 109, Issue 32
>>>
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>>> Today's Topics:
>>>
>>> 1. Re: logfile (nek5000-users at lists.mcs.anl.gov)
>>> 2. Re: logfile (nek5000-users at lists.mcs.anl.gov)
>>> 3. Re: logfile (nek5000-users at lists.mcs.anl.gov)
>>> 4. Re: logfile (nek5000-users at lists.mcs.anl.gov)
>>>
>>>
>>> ----------------------------------------------------------------------
>>>
>>> Message: 1
>>> Date: Fri, 30 Mar 2018 15:34:49 +0200
>>> From: nek5000-users at lists.mcs.anl.gov
>>> To: nek5000-users at lists.mcs.anl.gov <nek5000-users at lists.mcs.anl.gov>
>>> Subject: Re: [Nek5000-users] logfile
>>> Message-ID:
>>> <mailman.12194.1522416912.19102.nek5000-users at lists.mcs.anl.gov>
>>> Content-Type: text/plain; charset=utf-8
>>>
>>> That par file doesn't currently support a relative and absolute tolerance for the Helmholtz solver. I think there are two options to tackle your tolerance problem:
>>>
>>> * Specify the parameters below in usrdat3 of your usr file.
>>> * Solve a non-dimensional problem and use a residual tolerance of e.g. 1e-8 (for velocity and temp) and 1e-5 for pressure
>>>
>>> --
>>> Stefan
>>>
>>> -----Original message-----
>>>> From:nek5000-users at lists.mcs.anl.gov <nek5000-users at lists.mcs.anl.gov>
>>>> Sent: Friday 30th March 2018 14:19
>>>> To: nek5000-users at lists.mcs.anl.gov
>>>> Subject: Re: [Nek5000-users] logfile
>>>>
>>>> Thanks, Paul,
>>>> Could you please tell me the parameters in terms of ###.par file.
>>>> I am using the latest version.
>>>>
>>>> Thank you
>>>>
>>>>> On Fri, Mar 30, 2018 at 5:35 PM, <nek5000-users at lists.mcs.anl.gov> wrote:
>>>>>
>>>>>
>>>>> 1520 Hmholtz TEMP 12 3.8789E-11 4.5120E+07 1.0000E-10
>>>>>
>>>>>
>>>>> On Step 1520, it took 12 iterations to reach a residual of 3.8e-11, starting
>>>>> with a residual of 4.5e7, with target residual 1.e-10.
>>>>>
>>>>>
>>>>> [ This is actually too tight of a tolerance, since you are asking for 17
>>>>> orders of magnitude reduction in residual and are working only with 16
>>>>> significant digits. It seems like you might need to change your .rea file in
>>>>> the following way:
>>>>>
>>>>>
>>>>> set p21 and p22 to 0
>>>>>
>>>>>
>>>>> set p24, tolrel , to 1.e-3
>>>>>
>>>>> set p25, tolabs, to 1.e-3
>>>>>
>>>>>
>>>>> The only time you'll need tolrel/abs tighter than this is if you're doing an
>>>>> analytic convergence study where you know the answer to an infinite number
>>>>> of digits. For engineering calculations, 1.e-2 to 1.e-3 is fine. The
>>>>> residual tolerances are set conservatively based on these input values.
>>>>>
>>>>>
>>>>> 1520 Helmh3 fluid 17 3.6594E-11 4.7524E-01 1.0000E-10
>>>>> 1520 U-PRES gmres 120 1.3494E-08 4.5675E-06
>>>>> 1.0000E-10 7.8799E-01 3.0171E+00
>>>>>
>>>>>
>>>>> You have 17 iterations for the moment equations.
>>>>>
>>>>>
>>>>> You have 120 iterations for pressure. Again, your tolerances are too tight
>>>>> for engineering calculations -- you do not need 1.e-10 for divergence (i.e.,
>>>>> for the pressure residual tolerance).
>>>>>
>>>>>
>>>>> 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: Friday, March 30, 2018 4:52:26 AM
>>>>> To: nek5000-users at lists.mcs.anl.gov
>>>>> Subject: Re: [Nek5000-users] logfile
>>>>>
>>>>> The following is my logfile entry (sample lines). The first line is
>>>>> understandable. What do the values in the third and following lines
>>>>> denote?
>>>>>
>>>>>
>>>>> ------------------------------------------------------
>>>>> Step 1520, t= 7.6000000E-02, DT= 5.0000000E-05, C= 0.072 5.3831E+03
>>>>> 3.5403E+00
>>>>> Solving for Hmholtz scalars
>>>>> 1520 Hmholtz TEMP 12 3.8789E-11 4.5120E+07 1.0000E-10
>>>>> 1520 Scalars done 7.6000E-02 8.3823E-02
>>>>> Solving for fluid
>>>>> 1520 Helmh3 fluid 17 3.6594E-11 4.7524E-01 1.0000E-10
>>>>> 1520 U-PRES gmres 120 1.3494E-08 4.5675E-06
>>>>> 1.0000E-10 7.8799E-01 3.0171E+00
>>>>> 1520 Fluid done 7.6000E-02 3.3650E+00
>>>>> --------------------------------------------------------
>>>>>
>>>>>> On Fri, Mar 30, 2018 at 3:05 PM, <nek5000-users at lists.mcs.anl.gov> wrote:
>>>>>> I am not sure what you are talking about?
>>>>>>
>>>>>> -----Original message-----
>>>>>>> From:nek5000-users at lists.mcs.anl.gov <nek5000-users at lists.mcs.anl.gov>
>>>>>>> Sent: Friday 30th March 2018 10:30
>>>>>>> To: nek5000-users at lists.mcs.anl.gov
>>>>>>> Subject: [Nek5000-users] logfile
>>>>>>>
>>>>>>> Hi Neks,
>>>>>>>
>>>>>>> What are the entries in the logfile that is output from running
>>>>>>> the code. Please let me know where the entries are determined in the
>>>>>>> code.
>>>>>>>
>>>>>>> Thanks
>>>>>>> Swarandeep
>>>>>>> _______________________________________________
>>>>>>> Nek5000-users mailing list
>>>>>>> Nek5000-users at lists.mcs.anl.gov
>>>>>>> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users
>>>>>>>
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