[Nek5000-users] Failure with TORDER = 3 (P027)

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

Hi Paul,

Thanks for the quick answer. I have been using a fixed time-step.  
After a suggestion from Johan, I will try to overintegrate with lxd  
\approx 2*lx1, since I use a curved geometry. In addition, it might be  
useful to try with an even polynomial order? (Since my problems start  
as I change from N=5 to N=7, maybe N=8 is better?)

George

Quoting nek5000-users at lists.mcs.anl.gov:

>
> Hi George,
>
> Is your timestep size variable? or fixed?
>
> You can fix it (highly recommended) by setting it to the value
> you want, but negative  (e.g., param 12 = -.001 in the .rea file
> would imply that DT=.001 for all timesteps).
>
>> From what you describe, I am guessing that this is the issue.
> In particular, variable timestepping + projection has some difficulties.
>
> With dealiasing and filtering, I never have problems with Torder=3,
> and I would recommend using this.  I concur, however, with Johan
> that Torder=2 is more stable.  (See the CRAS article with J. Mullen
> in 2001.)
>
> I hope this helps - please let us know if problems persist.
>
> Paul
>
>
> On Wed, 7 Dec 2011, nek5000-users at lists.mcs.anl.gov wrote:
>
>>
>> Dear All,
>>
>> I have been conducting a direct numerical simulation of turbulent pipe flow
>> using nek5000 at a moderately high Reynolds number (at present Re_tau=550).
>> For this, I have to use a very fine grid in order to capture the flow
>> physics accurately (for a length of 25R, I am using a total of 450 million
>> grid points when assuming a polynomial order 7). However, I have been having
>> problems with the stability of the code when using TORDER = 3, i.e.
>> parameter 027. The way I conducted my simulation was as follows:
>>
>> - I started with polynomial order 3, TORDER set to 3, and run for 4 flow
>> through times, starting from random noise. At this point I had the
>> projection parameters, i.e. P094 and P095 set to non-zero values.
>>
>> - Afterwards, I increased the polynomial order to 5, with TORDER kept at 3,
>> and ran for another 2 flow through times. Here, I set the projection
>> parameters P094 and P095 to zero.
>>
>> - Now, starting from the last field of the previous run, I changed to my
>> target polynomial order 7, and keeping TORDER = 3. However, the codes
>> explodes shortly after restarting. The error is: "failed in HMHOLTZ."
>> I also tried a considerably lower time-step, but the problem persisted.
>> Meanwhile, when I change TORDER to 2, the code runs fine and does not
>> explode. The "physical" results are absolutely fine, and the statistics I
>> get are in perfect agreement to our expectations.
>>
>> I did not get the same problem while running a turbulent pipe flow at a
>> lower Reynolds number (Re_tau=180) and with less grid points.
>>
>> Probably also of importance, I did test with various ways of restarting,
>> i.e. storing in fld or f files, using 4 or 8 byte accuracy, and storing
>> multiple consecutive fields for the multi-step time scheme. None of these
>> changes made significant impact. Also, I am using overintegration and
>> filtering (0.01 in parameter P103); increasing the filtering (by a factor of
>> 5) did also not help.
>>
>> I am wondering if any faced a similar issue before, and if would have an
>> insight on how to tackle it. Could my problem maybe be related to the
>> restart-issue that Adam brought up recently?
>>
>> Best regards
>> George
>>
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