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

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

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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