Flow boundary conditions
 

Hi guys,

Probably a simple question for most but since I'm rather new to Fluent.

Consider a simple case with a pipe with on the left side a fixed flow going in and on the right side the flow is going out, as boundary condition I would take on the left side a velocity inlet with fixed velocity and on the right side a pressure outlet (on which I define the pressure, suppose it is known.)

What if the flow direction is reversed and on the left side I know what flow is going out (and suppose on the right side there is a certain T junction with two unequal sized openings). As opposed to the previous example, is it possible to take a velocity inlet on the left side with a negative velocity, and on the two openings on the right side make two pressure outlets with a specified pressure (suppose these are known)?

Or should I use a mass-flow-outlet on the left side and pressure inlets at the right side with defined pressures?

Thanks

First, you can have a velocity inlet (with negative or whatever velocity) and a pressure inlet. The inlet/outlet are purely names. You don't need to use a pressure outlet and then deal with the mental gymnastics of reversed flow. If you know the flow is going inward at that boundary, just make it a pressure inlet even if you have no "outlet" BCs.

In general you should be using the mass flow outlet on the left side because a velocity outlet is a little silly because how would you ever know the velocity at an outlet? Your only decision here is whether you want to use a velocity BC on the left or a mass flow BC. But you're going to be using pressure inlets for the right.

Thanks for the clear answer, very helpful.