Transient outlet boundary condition problem
 

Hello Eveyone,

I am doing premixed combustion in a compressible transient flow simulation using CFX. I first had a converged steady state simulation before switching to unsteady. My trouble is, after some time, i am getting this message in the output file:

"
****** Notice ****** |
| A wall has been placed at portion(s) of an OUTLET |
| boundary condition (at 100.0% of the faces, 100.0% of the area) |
| to prevent fluid from flowing into the domain. |
| The boundary condition name is: out. |
| The fluid name is: Propane Air Mixture. |
| If this situation persists, consider switching |
| to an Opening type boundary condition instead.
"

The outlet boundary i used is "pressure outlet" and the and i have "velocity inlet" for the air and "mass flow inlet" for the fuel. Because the outlet is reflecting i think pressure waves could be moving in into the flow, which is what I would expect. But do I have to specify any other boundary in addition to just a "pressure outlet"?

Thanks!

Jimmy

Whether this is a problem depends on your model. Do you need to have correct reverse flow? If so you need an opening. If it does not matter then stick with an outlet. Even better, move the boundary condition further away so it always has flow in one direction.