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Whilst working on an experiment with pipe flow from a basin, I noticed that with large enough pipe diameters, the fluid would begin to form open channel flow instead of filling the pipe entirely. Does anyone know the causes of open channel-flow, or could point me towards relevant literature?

Thank you.

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    $\begingroup$ look up "Froude Number" on wikipedia; it's relevant for understanding the transition between pipe flow and open-channel flow. $\endgroup$ Sep 25, 2020 at 18:10

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In the classic case of open-channel flow, the movement of the fluid is driven by gravity (the open channel has a downward slope) and resisted by fluid friction. You cannot pump fluid through an open channel at any rate you desire because for high flow rates the channel simply overflows.

To prevent overflow, the channel is equipped with a top surface and it becomes a pipe instead of an open channel. Because it now can withstand hydrostatic pressure, you can force fluid through it at speed by pressurizing its inlet, and there are no free surfaces remaining inside it: all the flow cross-section is occupied by fluid, and you have pipe flow.

When you release the pressure driving the flow, it slows down and reverts to being driven by downslope gravity and the pipe once again is capable of supporting the establishment of a free surface.

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