Before knowing the difference between open channel flow and pipe flow; we have to know about them.
Open Channel Flow
A conduit for the flow which has a free surface and which is subjected to atmospheric pressure is an open channel and flow in an open channel is known as open channel flow.
The free surface is actually an interface between two fluids of different density and will have constant pressure (atmospheric pressure). In the case of moving fluid, the motion is caused by gravity, and the pressure distribution within the fluid is generally hydrostatic. Open channel flow is mostly turbulent and unaffected by surface tension.
All the natural water bodies that run from one place to another as well as human-made canals come under open channel flow. But the flow of water through the pipe when the level of water in it is below the top of the pipe is also classified as open channel flow.
Examples: flow in streams, rivers, canals, partially filled sewers
A closed conduit which does not have a free surface and which is not subjected to atmospheric pressure but exert hydraulic pressure is known as a pipe. The flow that takes under the pressure force is known as pipe flow.
Pipe flow does not consider all flow within a closed conduit. Energy is expressed as the head which is defined by the Bernoulli equation.
Difference between open channel flow and pipe flow
|Aspects||Open channel flow||Pipe flow|
|Condition||It is usually uncovered, have a free surface at the top and atmospheric pressure at a free surface.||It is covered and has no free surface.|
|Cross-section||It may have any shape. E.g. rectangular, parabolic, triangular, trapezoidal, circular, irregular etc.||Generally, pipe flow has a circular cross-section.|
|Cause of flow||Flow is due to gravity in open channel flow.||Flow is due to pressure in pipe flow.|
|Surface roughness||Its roughness varies between wide limits and also varies from place to place.||Its roughness depends upon the material of the pipe.|
|Velocity distribution||Velocity is maximum at little distance below the water surface in open channel flow. The shape of the velocity profile of the open channel depends on the channel roughness.||Velocity is maximum at the centre of the flow and reduces to zero at the pipe wall. Velocity distribution is symmetrical about the pipe axis in pipe flow.|
|Piezometric head||Z+y where y = depth of flow; HGL coincides with the water surface in open channel flow.||Z+P/y, where p= pressure in the pipe. HGL does not coincide with water surface in pipe flow.|
|Surface head||Negligible||Dominant for small diameter|
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