Hi Manuel,
I am not sure if I understand your question, but please keep in mind the following. If you want to produce a standard energy spectrum typical of turbulent flows, there is no direct functionality in Nektar for this. You would have to probe the solution in space and perform the required Fourier transforms with an auxiliary software, e.g. Matlab. Alternatively you could probe a single point in space over a given time span and rely on Taylor's frozen flow hypothesis to produce a spectrum. The latter is of course the only alternative in case you don't have a homogeneous direction in your flow.
I suggest you take a look on a 2005 study by Richard Pasquetti, named "Spectral vanishing viscosity method for LES: sensitivity to the SVV control parameters", which seems to address the questions you are interested in.
Kind regards,
Rodrigo
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