symmetrie in unsteady flow
 

Hello, I try to simulate the vortex shedding frequency behind a symmetric bluff body in a channel. For the steady calculation I used the symmetrie boundary condition. Now I am not sure, if I also can use it when switching to the unsteady solver? Using the symmetry plane I cant find any periodic frequency. But when I use the whole geometry without the symmetry plane I find a peridic changing dynamic pressure behind the bluff body (1-5 Pascal Ampitude). Now I dont know if it is a numerical error, or if it really is the vortex shedding frequency. Has anybody experience with those problems? Thanks for any help. Kind Regards Gerd Sattler

Re: symmetrie in unsteady flow
 

Often a symmetry bc is not appropriate for vortex shedding and other types of oscillating flow. If the flow velocity across the symmetry bc is supposed to be oscillating then this type of bc is inappropriate (imagine the full field flow vectors for your case, if they are not exact mirror images of each other accross the symmetry line, then the symmetry BC's are incorrect).

Re: symmetrie in unsteady flow
 

Thanks for ur quick response. So, is it right to say, that i can not use symmetry, if the the shedding of the vortices doesnt happen similarly at the same time at both sides of the bluff body? Regards Gerd

Re: symmetrie in unsteady flow
 

That's right. If there are vortices being created on both sides of the symmetry line at the same time, and there is no movement across the symmetry line, then it would be ok to use the Symmetrical bc's, otherwise, you need to solve the full field.

axis-symmetric BC
 

hi how to create axi-symmetric bc in gambit for a pipe. regards Rashmi

Re: axis-symmetric BC
 

Hello Rashmi, You create it like any other BC. U first build ur geometry -in this case only one half of the geometry. Then u go to the boundary contitions menu. Here u define the boundaries where u cut the geometrie as "symmetry" BC. Hope I could help u!? Regards Gerd