I have a problem about pressure boundary. I use the pressure boundary applied to the outflow of the exhaust of an engine. The shape of the duct is divergent; if I apply the pressure boundary directly on exhaust surface I obtain unreliable results. Instead if I exstrude about ten layers of cells everything is ok. Could anybody tell me why, please? Thank you in advance,

Massimo

have you checked for "inflow" through your pressure boundary?

I have a similar problem. In my case there is severe back flow from pressure BC which is outlet. How to reduce backflow which is not nigligible.

Have you used the Environmental pressure option? If not, this could solve your problem.