negative input for velocity inlet to make an outlet?
 

Hi all, just a couple of very basic questions. I will appreciate all the help. By the way I just started learning CFD so right now I am only concerned with simple incompressible laminar pipe flows. Thanks in advance.:)

1. so let's say we have a long cylinder. In most of the basic examples we are taught to set up velocity inlet on one end and pressure outlet on the other. So what if I can only measure the pressure at the inlet and the flow at the outlet? Can I use velocity inlet and give it a negative value to make it become an outlet?

2. what if the flow direction in the pipe is constantly changing? Again I know the pressure at one end and I know the flow value inside the pipe at all time points. and now I want to caculate the pressure at the other end. To make this unsteady flow simulation possible can I use UDF to assign the velocity values (which are changing between positive and negative values with time) to the aforementioned velocity inlet to get what I need?

3. According to the Fluent manual for pressure inlets we need to give total pressures, and if backflow occurs (so the inlets become outlets) Fluent will treat the specified value as static pressures. While for the pressure oulets, Fluent asks for static pressures in normal conditions and treat the specified pressure value as total pressure in case backflow occurs. So does that mean both of them are actually the same thing? And what is so special about giving total pressures to the pressure inlets? Why can't we just assign static pressures to all the pressure based boundary conditions?

answer 1.) You can define pressure BC both at inlet and outlet.
And also you can give -ve value of velocity to define velocity outlet.
But in practice we generally define mass flow outlet rather than velocity oulet.