# flow behind a geometry

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 March 11, 2005, 07:08 flow behind a geometry #1 pp Guest   Posts: n/a 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

 March 11, 2005, 12:41 Re: flow behind a geometry #2 diet Guest   Posts: n/a 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!!!

 March 11, 2005, 13:41 Re: flow behind a geometry #3 pp Guest   Posts: n/a 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

 March 12, 2005, 23:05 Re: flow behind a geometry #4 wr Guest   Posts: n/a 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!

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