Fast Fourier Transform (FFT)
 

I am running a transient simulation of an axial compressor which has a casing treatment.

When I analyse the unsteady pressure signal I have a series of frequencies unrelated to the IGV/Rotor/Stator passing frequencies.

There exist flow separation and recirculation in some areas.

In general, does a recirculation or flow separation appear in frequency spectrum?

It depends :)

Many oscillating flows show regimes, where at some regimes they show highly periodic behaviour and in other regimes they go chaotic. The Von Karman vortex street is an example. So whether your separation shows no signal (ie steady), a single strong frequency or a range of frequencies (chaotic) depends on what regime your simulation is running at.

It would be worth your effort in doing some background reading on some benchmark cases which show this behaviour, like the Von Karman vortex street (for flow over a cylinder) or flow over a back facing step.

This operating point is thought to be close to rotating stall or in rotating stall condition.

There is a low frequency (about 140 Hz) in the spectrum.

How can I trace this frequency?