Hi, I'm just trying to get a feel for a subject that I plan to take but I'm unsure about the meaning of a certain term which frequently arises. The term 'flow' comes up a lot and I'm wondering what it could mean (or refers to) in the context of a pipe flow problem. I know my question is vague but as an example if I'm given a PDE (for the function u(z,t) say) for a pipe flow problem, what am I actually solving for? Would u(z,t) be the velocity of the fluid? Any help would be good thanks.