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

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

Hi George,

That would seem unnecessary... It could simply be the case
that you are facing lack of resolution.

Also, have you already experienced transition to a fully
turbulent state?   The transition phase normally takes more
resolution to get through because of the intense vortex
stretching.   Additional filtering usually cures this, however.

Given that you've already tried this, perhaps this is something
we need to look at.  If you want me to look in detail at the
case that blows up, contact me off list and we can arrange a 
file transfer.

Regards,

Paul




On Wed, 7 Dec 2011, nek5000-users at lists.mcs.anl.gov wrote:

>
> 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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