What is the most effective way to make an sub-critical open-channel flow switch to super-critical?
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Put a sluice gate in the channel. Fluid upstream of the gate will pile up, building significant pressure at the base of the gate. When the gate is raised slightly, the water will shoot out in a super-critical state. Downstream of that, a hydraulic jump will occur, and the flow will again return to sub-critical. But for a short distance, you'll have sustained super-critical flow.
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Both previous answers are correct, but you could also steepen the gradient of the channel bed.
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The most effective way is to either narrow the channel or to shallow up the depth.