Opening Vs outlet BCs and temporary wall issue

Sam

Dear CFX Users,

I am simulating flow in a cyclone. In order to see the difference in between the 'opening' and 'outlet' types of boundary conditions, I had run two simulations with 'opening' and 'outlet' type of BCs; respectively. I got the same velocity profiles, split ratio etc. So I concluded that in my case flow does not come in to the domain at its outlets. Neither I saw any 'temporary wall raising' message in output file from 'outlet' boundary conditions.

But my curiosity led me to use the results from 'opening' boundary conditions case as the initial condition for 'outlet' boundary condition case. And now I get message that the fluid is trying to enter the domain at outlets and hence temporary walls have been raised. I am not understanding why does it happen while results described in the first paragraph were the same?

Could any one please throw some light on this 'temporary wall' thing....I am not comfortable with this issue. Please help.

Regards, Sam