# How to unserstand Correct boundary condition at outlet for incompressible flow?

I am simulating flow around a model. This model have an inlet and an outlet. Flow is incompressible. I know that value of pressure at the outlet is very much different from ambient pressure(Operating pressure). But, I don't know real value of pressure at the outlet. Now, I applied value of zero gauge pressure at the outlet (Is this OK? Why?) and this means that pressure at the outlet is equal to operating pressure while this is not. How do i find correct boundary condition for outlet?

But some people say that when flow is incopressible and boundary condition at inlet is velocity, we can use pressure-outlet as a boundary condition at the outlet because flow is not dependent on outlet pressure (so, we can use zero gauge pressure at the outlet). Why?

Thank.

• It might help if you include an image or sketch. It's a little confusing, because you say 'flow around a model', which implies you are simulating external flow; however, you say that the model also has inlets and outlets. Normally, for an external flow situation, the outlet (downstream) pressure would be the same as ambient. – Time4Tea Jul 24 '18 at 21:19