Establishment of uniform flow


Today we are going to demonstrate the establishment of uniform flow in an open channel, under two Froude numbers:
1) supercritical, and 2) critical.

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We use the friction equation

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So = f F2

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in which So is the bottom slope, lower case f is the friction factor, and upper case F is the Froude number.

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The friction factor is equal to 1/8 of the Darcy-Weisbach friction factor. It is equal to:

f = g n2 / (k2 R1/3)

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In SI units, g is equal to 9.81 m/s2 and k = 1.

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The wall of the flume is made out of lucite, i.e. acrylic glass. We assume a Manning n equal to 0.008.

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We estimate a hydraulics radius equal to 3 centimeters, that is, 0.03 m.

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Therefore: f = 0.002

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We turn on the pump in the demonstration flume.

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We set the slope at 0.01, a steep slope, and check the level.

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The calculated Froude number is:

F = (So / f )1/2 = (0.01 / 0.002)1/2 = 2.24

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This Froude number corresponds to supercritical flow.

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Now we set the slope at 0.002, the critical slope, and check the level.

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The calculated Froude number is:

F = (So / f )1/2 = (0.002 / 0.002)1/2 = 1.

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We have shown the flow in this channel operating under supercritical and critical conditions.