vortex shedding frequency
Hi all
Will somebody help me that what is the most simple way to calculate the vortex shedding frequency or the Strouhal number for flow past a circular cylinder?(2D) 
Compute the lift, store the time history of the lift, and then do an FFT of the lift history. Or, simply measure the period of the cycle from the lift history and invert.

CFD is the most comperhensie and simple way to solve these problems.

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That's absolutely correct, just make sure that you take enough cycles into account before doing FFT, excluding any initial transients (e.g. when starting from potential or constant flow) 
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Just record the time history of the vertical velocity on a fix point in the wake, it's enoughand perform an FFT as Agd said. 
While the timeaveraged lift will be zero, the instantaneous lift force should oscillate at the shedding frequency. Computing the lift is extra effort, but it can provide additional information on the quality of the solver  for example by checking that the timeaveraged lift actually does equal zero.

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The lift of a symmetric body is null. You can not lift a sphere or cylinder with a circular base. For the drag force your are right 
Um, no. See for example
http://www.dtic.mil/dtic/tr/fulltext/u2/a273243.pdf The instantaneous lift force on the cylinder is not zero. Th average lift force is zero. The average drag will not be zero, and the instantaneous drag will oscillate at twice the Strouhal frequency. 
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