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

[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
>
> ----------------------------------------------------------------
> This message was sent using IMP, the Internet Messaging Program.
> _______________________________________________
> Nek5000-users mailing list
> Nek5000-users at lists.mcs.anl.gov
> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users