Reversed flow at pressure outlet

Seeker Phil

Hey all... I was wondering if some of the experts here had any idea about the physical implications of reversed flow at pressure outlets. I am running a problem with an oscillating flow and during half the cycle my flow is reversed at the pressure outlet. I have specified a guage pressure = 0 Pa at this boundary. However, due to this backflow the pressure after iterating is about -1.5 Pa. The documentation does not do a great job of explaining what happens to the flow when a reversed flow at the pressure outlet occurs. So I would appreciate any ideas and thoughts on this. Also, any tips to reduce/avoid this backflow would be appreciated. Thanks, Phil