# Pressure Drop

by tonyjk
Tags: pressure
 P: 95 Hello... I can't find the difference between the pressure drop in a pipe due to frictionnal loss and the Pressure difference that cause the flow like in Poiseuille Flow.. Thanks
 P: 737 They are completely different ideas...what specifically don't you understand about either? Edit: Oh are you talking about like the losses associated with flows like Poiseuille Flow?
 P: 737 If you are asking how this solution to pressure drop is associated with similar ideas such as the Darcy-Weisbach equation, the answer is that the analytical (experimentally derived) Darcy-Weisbach equation is employable under broader circumstances. The assumptions taken for Poiseuille Flow equations are (from wikipedia) "...that the fluid is viscous and incompressible; the flow is laminar through a pipe of constant circular cross-section that is substantially longer than its diameter; and there is no acceleration of fluid in the pipe". In theory this is a nice equation to look at to understand where the mechanical energy is being lost to, but in practical applications of pressure drop analysis it is rarely, if ever, employed. Basically: Poiseuille equation is theoretical, solutions like Hazen-Williams and Darcy-Weisbach are analytical.
P: 95

## Pressure Drop

we say that gradient pressure cause flow but in pipe flow the pressure different is due to friction loss
 P: 737 To see how people generally use these equations (Poiseuille is mentioned in there), see here A solution to Poiseuille's equation is used to approximate the D-W friction factor for Laminar, fully developed flow in long pipes.
 P: 95 Thank you i understand it