The flow of water in a conduit may be either open channel flow or pipe flow. The two kinds of flow are similar in many ways but differ in one important respect.  Open channel flow ,iust have a free surface whereas pipe flow has none. A free surface is subject to atmopspheric pressure. In Pipe flow there exists no direct atmospheric flow but hydraulic pressure only.

Course notes: CIVE2400 Fluid Mechanics Section 2. Prepared by University of Leeds.

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