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Old   March 11, 2005, 07:08
Default flow behind a geometry
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pp
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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
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Old   March 11, 2005, 12:41
Default Re: flow behind a geometry
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diet
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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!!!

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Old   March 11, 2005, 13:41
Default Re: flow behind a geometry
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pp
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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
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Old   March 12, 2005, 23:05
Default Re: flow behind a geometry
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wr
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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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