How to evaluate Strouhal Number for a adjustable time step?
 

Hello all,

I'm working in the case of the flow around cylinder subjected to vortex-induced vibration in the transverse direction to the flow (y axis). How can I evaluate the Strouhal number (vortex shedding frequency) using FFT on the lift coefficient time series if the time-step of the simulation is adjustable? Thanks in advance.

Hi,

You can either use interpolation, i.e. take points with variable interval, build interpolation function and then calculate values at constant interval. Then use FFT.

Or you can use The Lomb–Scargle periodogram (http://en.wikipedia.org/wiki/Least-s...le_periodogram).

In case of FFT you'll get frequency, in case of Lomb-Scargle you'll get 2*pi*frequency.

Thanks for your reply, it was very enlightening.

Dear Alexym,
Assuming we build interpolation function and then calculate values at constant interval, then use fft on ,for example, lift coeff in the case of vortex shedding behind a cylinder to obtain fft vector. Now we have to plot fft vector vs frequency vector in order to ,for example, find maximum amplitude. But, what is frequency vector? How to calculate it?