[Nek5000-users] Add/subtract 1d-mean profile

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

(Same mail, new formatting...)

Hi Paul,

first of all thanks for the quick reply!


   >  I guess my first question is the following.  When you say you want to
   >
   >  "add or subtract this profile which is given on the global nodes
to each point of the field"
   >  it seems we need to be talking in the context of some software
   >  that is going to read the field in and then process the data.
   >
   >  At present, I know of only 3 codes to do that: nek, VisIt, and
postnek (though there have been custom, one-off, codes written
   >  in the past).

I want to do the whole post-processing in nek. With VisIt I only
visualize my data generated with nek (the post-processed data as well
as the raw simulation data). With postnek I haven't worked until now
and I think postnek is not what I'm looking for.



   >  Second question is --- What do you want to do with this field
   >  after you have subtracted the mean profile?   Do you want to
   >  write it to disk, visualize it, use it in a calculation, analyze
it with more statistics, or....?

I want to analyze the statistics, calculate higher order moments of
the fluctuation field (to compare it with the channel data of Robert
D. Moser, John Kim and Nagi N. Mansour,
Physics of Fluids, vol 11(4), for example the skewness or the flatness
of the field) or the gradients of the fluctuation field du'_i/dx_j to
get (fluctuation based) \omega'_k or the (fluctuation based) strain
rate tensor s'_ij = 1/2*(du'_i/dx_j + du'_j/dx_i).
These are only few examples on what I would like to do...

But for all these things I need the fluctuation field and that means
u' = u - <u>.
In this context <u> would be (for example if I don't have the temporal
avg_all() means) the spatial (y-) mean over the full channel domain.

When I do my production run with the code I can obtain the temporal
averages by calling avg_all() every timestep and then write it to
disk, thats clear to me.

But my question aims on how to get the fluctuation field without
having the temporal averages. Based on only a single velocity filed
(as described above).



   >  In any case, it seems likely that processing in nek is the
   >  way to go because it has the ability to read the data, write
   >  the data, and readily compute statistics.   In this mode, you
   >  simply are using nek as a post-processor and not for timestepping.
   >
   >  Please advise if that's the path you wish to take, or if you
   >  had something else in mind.

Yes, thats the way I want  to do my post-processing (and already do it
right now...).

Regards
Jan F.