Hi, I am simulating flow behing a bluff body.I have read in some paper the this phenomena is unsteady as one can observe constant vortex shedding behind this body. But when i simulate this in Fluent using usteady code I do not get any unsteadiness. How can I capture the unsteadiness using Fluent code. pp

if you are using a RANS turbulence model (k-epsilon or something like that) it's of little or no use computing it unsteady. Because RANS is a model that is meant to give a time averaged result. You'll only be able to observe any fluctuations if your inverse turbulent timescale (k/epsilon)^-1 is bigger than your vortex shedding frequency behind your bluff body.... you should perform LES, but it means that you'll have a hihger computational cost!!!

What is they hysical reasons behind such an statement. "any fluctuations if your inverse turbulent timescale (k/epsilon)^-1 is bigger than your vortex shedding frequency behind your bluff body" Actually I have some papers where they perform this unsteady calculation and obatin the correct vortex shedding frequency. pp

That is your turbulence time scale should be smaller than vortex shedding time scale. At least an order lower. And you mesh is dense enough!