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Old   May 22, 2011, 03:38
Default calculate Strouhal number
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Hi all
I am solving a turbulence flow around a bluff body, using DES model.
How can i calculate strouhal number for it?

Best regards
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Old   May 24, 2011, 00:08
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Mayank Ojha
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Find the values of coefficient of lift for the body at different time values and do a FFT (fast fourier transform). Excel or even Matlab have built in functions to perform FFT. That will give you the frequency of vortex shedding (f).

For a cylinder St=f*D/U, for any bluff body I do not have any idea. You may look up literature.

Hope that helps.

-Mayank
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Old   May 24, 2011, 02:04
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Thanks ojha.mayank485
can time step size effect the frequency?
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Old   May 24, 2011, 07:39
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Yes. Time step size is one of many parameters you will need to check to get an accurate simulation http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F
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Old   May 24, 2011, 13:52
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Your timestep size should be smal enough to predict the flow properties accurately. For example if you monitoring the vortex shedding, then use time step atleast 1 to 2 % of vortex shedding period.

BTW what bluff body are you using ???
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Old   August 14, 2016, 17:06
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Quote:
Originally Posted by ojha.mayank485 View Post
Your timestep size should be smal enough to predict the flow properties accurately. For example if you monitoring the vortex shedding, then use time step atleast 1 to 2 % of vortex shedding period.

BTW what bluff body are you using ???


I would like to add clarity. In general, the vortex shedding period is very large compared to the simulation timestep (considering time-explicit schemes).But here's the actual constraint: The timestep must be smaller than the vortex shedding period, mainly to avoid Aliasing effect. Read: https://en.wikipedia.org/wiki/Aliasing

To avoid aliasing, the sampling frequency must be greater than f/2, where f is the measured signal frequency. Therefore, maintaining atleast 3 measuring points per shedding period captures frequency information. However, more the data points better the shedding frequency captured.
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