[Nek5000-users] calculate gradients of fluctuation

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

It depends, do you want it on the fly (while running nek5000) or in the
post-processing.  If you want it in the post-processing, first call
avg_all() routine in usrchk, this way you can calculate the temporal
average \bar{vx}, \bar{vy}, \bar{vz}, which will be dumped as separate
field files in the running directory.

Run nek5000 in the post-processing mode, load the snapshot as well as the
average field files in usrchk() routine and subtract avg from the
snapshots, vx - \bar{vx}. In this way you will get the fluctuations. Then
you can use gradm1() to calculate the gradients of du'/dx. du'/dy and
du'/dz. You can refer to the Nek5000 user guide to see how nek5000 is run
in post-processing mode.

If you want to calculate it on the fly, it is a little tricky, You cannot
resort to time-average, but you can use some other meaningful average that
would to some extent approximate the effects of temporal averaging. For
example, in channel flow, you can use horizontal average, where
streamwise-spanwise direction is periodic or in some other flows you can
choose to use a volume average. The idea is to subtract, an average of (vx)
from vx to get the fluctuations and then call the gradm1() functions. It
really depends on the geometry and the type of fluid-problem you are
solving, but the idea is the same.

On Wed, Jun 28, 2017 at 3:12 PM, <nek5000-users at lists.mcs.anl.gov> wrote:

> Hi nek users,
>
> For u=U + u’ where  U= mean streamwise velocity and u’= fluctuation of
> streamwise velocity
>
> I know how to calculate du/dx , du/dy and du/dz. But how can I calculate
> gradients of the *fluctuation* of streamwise veloctity, i.e. du’/dx ,
> du’/dy , du’/dz ?
>
> Thank you.
>
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