I want to model the flow of liquid through a pipe/tube and calculate the pressure drop over the pipe. What sort of boundary conditions should I be using? I tried with a pressure inlet and a pressure outlet, but I'm not sure if that is the right way to go. Pressure at inlet = 1.8bar, pressure at outlet = 1 bar (open to atmosphere). When I check the results, I have a pressure of about 1.4bar in the middle of the pipe length. This seems intuitive as we are going from 1.8bar to 1 bar, but using pressure loss theory, shouldn't the pressure loss be more non linear than that due to friction factors? Like how Darcy-Weisbach equation describes. The pressure of the fluid at the exit won't be exactly atmospheric, because the liquid has a velocity as it exits and therefore a pressure is required to generate that velocity. What am I missing?