[Nek5000-users] Channel Flow 1D/2D energy spectra

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

Hi,

how to exactly do it depends on your case. For a channel (or pipe), we 
typically map the mesh onto a normal rectilinear mesh which is 
equidistant in the wall-parallel (streamwise/spanwise or 
axial/azimuthal) directions. This is done via the nek-built-in 
interpolation routines. however, one needs to check carefully that the 
new mesh does not lead to interpolation errors close to element 
boundaries (for curved elements). Once you have everything on a regular 
mesh, then you just go ahead and to FFTs in the language of choice.

Philipp

On 2018-08-20 05:22, nek5000-users at lists.mcs.anl.gov wrote:
> Hi Philipp,
> 
> Thanks for the reply. So you are saying that I would map the output to a 
> simpler mesh then do the fourier transforms for the spectras?  Would you 
> be able to give me a basic idea behind how the spectra could be computed 
> using a simpler mesh?
> 
> Thanks,
> William
> 
> On 20 August 2018 at 02:42, <nek5000-users at lists.mcs.anl.gov 
> <mailto:nek5000-users at lists.mcs.anl.gov>> wrote:
> 
>     Hi,
>     For most cases, we typically do all these things (statistics,
>     spectra, budgets) outside of Nek5000, on simpler meshes (equidistant
>     or polar meshes for instance). For instance in our paper on pipe
>     flow, we show exactly those 1D and 2D energy spectra.
> 
>     https://link.springer.com/article/10.1007/s10494-013-9482-8
>     <https://link.springer.com/article/10.1007/s10494-013-9482-8>
> 
>     For your purpose however, I would suggest some physical space filter
>     with specific transfer function. That might be the easiest way.
> 
>     Best regards,
>     Philipp
> 
> 
>     On 2018-08-19 07:54, nek5000-users at lists.mcs.anl.gov
>     <mailto:nek5000-users at lists.mcs.anl.gov> wrote:
> 
>         Hi Nek Users,
> 
>         I am a new user to Nek5000 and I have been playing around with
>         the channel flow example. I have set up an example similar to
>         Kim and Moin. The mean velocity profile seems to agree quite
>         well so I have set up the initial case correctly.  Now, my goal
>         is to try and recreate the energy spectra as in the paper above.
>         I am confused as to the best way to go about this, would it be
>         best to obtain output files then do the post processing outside
>         of Nek or can it be done inside Nek as well?
> 
>         My ultimate goal is to be able to cut off some of the higher
>         energy modes and transform it back and feed it at the start of
>         the simulation. Any advice on this would be appreciated as I
>         have been struggling as to how I should proceed in Nek5000.
> 
>         Thanks in advance,
>         David
> 
> 
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