# calculate Strouhal number

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 May 22, 2011, 03:38 calculate Strouhal number #1 Member   omid Join Date: Apr 2011 Posts: 98 Rep Power: 7 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

 May 24, 2011, 00:08 #2 New Member     Mayank Ojha Join Date: May 2011 Posts: 22 Rep Power: 7 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

 May 24, 2011, 02:04 #3 Member   omid Join Date: Apr 2011 Posts: 98 Rep Power: 7 Thanks ojha.mayank485 can time step size effect the frequency?

 May 24, 2011, 07:39 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 12,638 Rep Power: 98 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

 May 24, 2011, 13:52 @Omiduit #5 New Member     Mayank Ojha Join Date: May 2011 Posts: 22 Rep Power: 7 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 ???

August 14, 2016, 17:06
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Sathish
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Quote:
 Originally Posted by ojha.mayank485 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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