Being new at CFX I have a simple question (I want to believe). In an internal unsteady pipe flow with multiple exits which are the best working boundary conditions at each one of the exits? In an in-house code I would implement a zero surface traction bc at every exit (in a finite element setting at least). What can I do with CFX? Your help is greatly appreciated.

D.

Hi,

What does zero surface traction mean? Are you doing a free surface simulation?

The boundaries must match the physics. Sounds basic but it is where some people slip up. The CFX documentation (in obtaining convergence) also describes acceptable combinations of boundary conditions.

Glenn Horrocks

Thanks for the tip... Zero surface traction at an exit roughly means that the force at the exit is zero (sigma_ij * n_j = 0 with sigma_ij the stress tensor and n_j the unit normal at the exit boundary surface). This is not about a free surface simulation. It is just an unsteady flow in a pipe network with multiple exit points where we do not know the pressure to prescribe as bc.

Thanks

Hi,

This sounds close enough to a pressure boundary set to zero relative pressure to me. Is there any reason why a zero pressure boundary is not applicable?

Glenn Horrocks

Glenn, I think Dimitirs is asking about the boundary through with no force can be exchanged. I think , He should go for Avg. static pressure as outlet, without any number to be specified on boundary.

Rgds

MP