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-   -   Vortex Shedding (http://www.cfd-online.com/Forums/cfx/19389-vortex-shedding.html)

Dave December 23, 2002 07:58

Vortex Shedding
 
Hi, I've been trying to simulate 2D vortex shedding in CFX551 using the new 2D mesher and the SST turbulence model. I've been trying to match a Re number of 140. So Far I have not managed to get anywhere with the simulation. Does anyone have any advice one the modelling of vortex shedding. Also what sort of Timestep should I be using ?? Physical ? and what sort of Time step size ?? Cheers for the help, and have a great christmas and new year Dave

Neale December 24, 2002 12:44

Re: Vortex Shedding
 
Dave,

- Re=140 is probably a laminar flow, so turn off the turbulence model.

- Run it transient with a small enough timestep to resolve the shedding frequency you expect

- You might have to perturb the flow to get the shedding going. Numerical diffusion can tend to cause the flow to look stable when it shouldn't be and sometimes you just need a bit of kick to get things going.

- Watch your confinement of the cylinder. I'd say you probably want at least 4 diameters on the cross stream direction.

Neale


Jan Rusås December 28, 2002 15:57

Re: Vortex Shedding
 
Dear Neale, I have had the same kind of problem, In CFX4 I did predict the vortex shedding within 0.1% accuracy. In CFX5 I did not get any transient flow at all. What is the fastest approach to initiate some disturbance? Regards Jan

xincheng December 29, 2002 00:01

Re: Vortex Shedding
 
i have the same problem,would any one solve it,thanks!

Neale December 30, 2002 17:46

Re: Vortex Shedding
 
I have not had much time to play around with this but perhaps a small perturbation on the inlet velocity upstream of the cylinder? Something like 1% of the mean speed or so....

Neale

Bob January 2, 2003 09:08

Re: Vortex Shedding
 
HI Everyone Happy New Year and thanks for the help on the problem. One last question to Neale. You said that you should try and perterb the flow. Could this be achieved by applying a wall roughness to a single side of the cylinder. Just a curiosity really rather than a specific reason for doing it this way rather than the way you suggested. Cheers again Bob


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