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How to evaluate Strouhal Number for a adjustable time step?

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Old   December 12, 2014, 00:10
Default How to evaluate Strouhal Number for a adjustable time step?
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Leonardo Antonio de Araujo
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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.
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Old   December 12, 2014, 01:44
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Alexey Matveichev
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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.
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Old   December 14, 2014, 02:01
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Leonardo Antonio de Araujo
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Quote:
Originally Posted by alexeym View Post
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.
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Old   August 24, 2019, 12:30
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Ali Shayegh
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Quote:
Originally Posted by alexeym View Post
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?
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