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

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

Thanks so much for your reply.

Is it possible for you to elaborate on the specific way to interpolate
using the nek5000 built in routine? Say I have run a simulation already and
have a number of outputs in ASCII format how would I go about interpolating
all the outputs onto this new mesh.

Would it be possible, once I have interpolated onto this new mesh to
convert back to the old one such that I can restart the simulation using it?

Once again, I appreciate your patience as I am still quite new to CFD in
general and these questions might seem too basic.

Cheers,
William

On 20 August 2018 at 17:33, <nek5000-users at lists.mcs.anl.gov> wrote:

> 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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