# How to evaluate Strouhal Number for a adjustable time step?

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 December 12, 2014, 01:10 How to evaluate Strouhal Number for a adjustable time step? #1 New Member   Leonardo Antonio de Araujo Join Date: Feb 2014 Location: Porto Alegre Posts: 17 Rep Power: 8 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. amuzeshi likes this.

 December 12, 2014, 02:44 #2 Senior Member   Alexey Matveichev Join Date: Aug 2011 Location: Nancy, France Posts: 1,887 Rep Power: 34 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. araujo and amuzeshi like this.

December 14, 2014, 03:01
#3
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Leonardo Antonio de Araujo
Join Date: Feb 2014
Location: Porto Alegre
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Quote:
 Originally Posted by alexeym 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.

August 24, 2019, 13:30
#4
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Ali Shayegh
Join Date: Oct 2015
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Quote:
 Originally Posted by alexeym 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.
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?