Courant number and timestep query

siw · August 30, 2012, 07:08

Hi,

I understand from ANSYS tech support that for a transient (SAS-SST) CFX simulation (for an external transonic aerodynamics simulation) that the maximum CFL number should be <= 5.

So in order to estimate an appropriate timestep (dt) using:

CFL = (u * dt) / (dx) <= CFLmax

what values should be used for u and dx? Can u be the fresstream velocity? Since the mesh has a wide range of cell sizes should dx be the cell size where the most interesting physics occurs - in my case the wake behind a bluff body? Once I get a timestep I can then use that as a start for assessing timestep sensitivity on the simulation.

Thanks

Lance · August 30, 2012, 08:11

Please correct me if Im wrong, but I think that u is the velocity in each cell and dx would be the cube root of the cell volume. Dont know if you need a CFL number < 5 in the entire flow domain, but certainly where all the action is.

August 30, 2012, 07:08	Courant number and timestep query	#1
siw Senior Member Stuart Join Date: Jul 2009 Location: Portsmouth, England Posts: 733 Rep Power: 25	Hi, I understand from ANSYS tech support that for a transient (SAS-SST) CFX simulation (for an external transonic aerodynamics simulation) that the maximum CFL number should be <= 5. So in order to estimate an appropriate timestep (dt) using: CFL = (u * dt) / (dx) <= CFLmax what values should be used for u and dx? Can u be the fresstream velocity? Since the mesh has a wide range of cell sizes should dx be the cell size where the most interesting physics occurs - in my case the wake behind a bluff body? Once I get a timestep I can then use that as a start for assessing timestep sensitivity on the simulation. Thanks

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August 30, 2012, 08:11		#2
Lance Senior Member Lance Join Date: Mar 2009 Posts: 669 Rep Power: 22	Please correct me if Im wrong, but I think that u is the velocity in each cell and dx would be the cube root of the cell volume. Dont know if you need a CFL number < 5 in the entire flow domain, but certainly where all the action is.