# vortex shedding not captured

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 February 25, 2008, 19:47 vortex shedding not captured #1 Deloat Guest   Posts: n/a Sponsored Links Hi everyone, I tried to model a 3d cylinder of diameter 0.58m. The flow speed is constant at 0.392m/s. This means that the Reynolds number should be around the sub-critical region, i.e vortex shedding will occur. But i run my transient simulation yesterday, and i only managed to capture the symemtrical vortices at the end of the cylinder, i.e. no vortex shedding was captured.Will this be due to a not-too-fine meshing? But, I think there might be something wrong with my timesteps. Any of you guys had managed to captured the vortex shedding before, or have any ideas, please let me know. Thanks, Deloat

 February 26, 2008, 06:05 Re: vortex shedding not captured #2 Glenn Horrocks Guest   Posts: n/a Hi, What Reynolds number is it at? Is it laminar or turbulent? To get vortex shedding you need to run second order differencing in time and space. Also a good idea to have a good boundary layer mesh transitioning to the bulk mesh very smoothly. Glenn Horrocks

 February 26, 2008, 19:23 Re: vortex shedding not captured #3 Deloat Guest   Posts: n/a Hi Glen, The Reynolds number is 2.26x10^5 (sub-critical region). I did my meshing in CFX-mesh, and use the 'point spacing and line spacing' to control the finer mesh around the cylinder. Is there a better way to control the fine mesh around the cylinder and to control the smooth transition? Thanks

 February 27, 2008, 18:04 Re: vortex shedding not captured #5 Glenn Horrocks Guest   Posts: n/a Hi, In this region the flow is turbulent so you need a turbulence model. As Andy20 suggests the SST model with a y+ around 1 would probably be required. Also mesh quality around the cylinder is important. I would use lots of inflation layers (maybe 20-40) and make sure you adjust the mesh settings to give a good transition to the main tet/tri mesh in the near-wake region. Glenn Horrocks

 February 27, 2008, 19:37 Re: vortex shedding not captured #6 Maga Guest   Posts: n/a Thanks for the reply, You said it's better to have a y+ <1. How do you check whether the y+ is less than 1?? Can you show me please? Thanks again.

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