[Nek5000-users] pressure normalisation in periodic flows

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

Dear all,
We are running turbulent pipe flow. During these runs we have observed some
interesting behaviour of the instantaneous pressure, namely its mean value
can be of order 10e7 depending on the run. We have tried various ways to
bring that mean instantaneous pressure down to around zero, but we only
succeeded partially. Of course, for computing the flow statistics (such as
<p> and prms) we do have an additional normalisation based on the mean wall
pressure, but having such a high "computational" pressure may lead to loss
of precision which we would like to avoid.

Therefore, we have a few questions which we tried to figure out during the
last weeks: 
1) from the Deville, Fischer and Mund book (and previous channel
experiences) we assume that nek5000 will in the case of periodic bc (that is
without any "O" condition) set the mean instantaneous pressure to zero. For
some of our runs this is most probably fulfilled (checked only
graphically...). Which routine would you suggest to use to do a volume
average of the pressure (pnpn-2)?
2) Some of our runs were initially ran using the older restart scheme with
only a single field/single precision. Since a few weeks we are exclusively
using the newer restart option with four previous fields/double precision.
The runs that were originally run with the old restart option are the ones
that experience the high pressure values, whereas the cases that were only
run (i.e. from the initiation with random noise) with the new restart have
"correct" pressure levels (i.e. around zero). 
We would like to reduce the high pressures from the older runs. We have
tried many things; subtracting a mean pressure in userchk, removing the wall
pressure during the run etc., but none of these options completely solved
our issue (i.e. the pressure was rising again). Would there be an option to
restart nek without specifying a pressure?
3) To learn more about how nek is really doing the normalisation, which part
of the code is doing the p-normalisation?

By the way, we are using PnPn-2, mainly because our experience is that we
are faster for our case.

Thanks a lot for any help!
Philipp