Learning Forum

[Milne Ando]

G'day,

I want to use Coupled scheme to calculate this case and therefore need to setup a suitable CFL number (Courant number). I've read the theory guide and the user guide, but its statement isn't clear to me. The CFL number in this scheme isn't used to calculate adptive time step size, a relaxation factor instead. As the pic shows its reciprocal is used as a relaxation factor, and the statement suggests that 

If ANSYS Fluent immediately diverges in the AMG solver, then the CFL number is too high and should be reduced. Reducing the CFL number below 10 is not recommended since......In most transient cases, the CFL number should be set to a large value such as 10^7 and explicit relaxation factors to 1.0.

Well, 10≤ CFL ≤10^7 is such a wide range, so what value is good to start with?

Hope someone can help me. Thanks so much!

[Rob]

Default is 200, which can be a little optimistic. 2,000 is not going to go well in most cases. For transient solutions it's more common to use PISO, the benefits of PBCS are seen in steady cases.