Notches and Weirs

Notches and weirs

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Students should be able to:

1. Describe the notches and weirs

2. Derive an expression for the discharge through notches and weirs

3. Apply the deduced expression and calculate the fluid discharge through notches and weirs

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A notch is an opening in the side of a measuring tank or reservoir extending above the free surface.

A weir is a notch on a large scale, used, for example, to measure the flow of a river, and may be sharp edged or have a substantial breadth in the direction of flow.

Weirs are typically installed in open channels such as streams to determine discharge (flowrate).

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Free Flow weir plates and weir boxes are designed for high accuracy open channel flow measurement.

Typical applications include : 1. The measurement of irrigation water;

2. Industrial process discharge;

3. Wastewater treatment plant discharge

4. Holding pond overflow.

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Measure low discharges more accurately than horizontal weirs.

is most commonly a 90° opening with the sides of the notch inclined 45° with the vertical

Since the V-notch weir has no crest length, much smaller flows are represented by a given head than for a rectangular weir

V-notch weirs should always be used when frequent low flows are included in the overall range to be measured.

Other angles used for V-notch weirs are 22 1/2°, 30°, 45°, 60° and 120°.

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The rectangular weir is the most commonly used thin plate weir

These weirs have a sharp edge so that the water flows freely over giving accurate measurements beveled at 45°

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The basic principle is that discharge is directly related to the water depth (h). h is known as the "head."

Rectangular weirs can be "suppressed," "partially contracted," or "fully contracted."

Suppressed means there are no contractions

Weir contractions cause the water flow lines to converge through the notch.

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The width does not change with depth so

b = constant = B

Substitute into general equation

To get the actual discharge, coefficient of discharge is introduced, CD in order to account for losses at the edges of the weir and contractions in the area of flow

Water flows from a reservoir through a rectangular opening 2m high and 1.2m wide in the vertical face of a dam. Calculate the discharge in cubic meters

per second when the free surface in the reservoir is 0.5m above the top of the opening assuming a

coefficient of discharge of 0.64.

Answer : Qactual = 8.16 m3/s

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The relationship between width and depth is dependent on the angle of the V

So the discharge is

The actual discharge is obtained by introducing a coefficient of discharge

In an experiment on a 90˚ vee notch, the flow is collected in a 0.9 m diameter vertical cylindrical tank. It is found that the depth of water increases by 0.685 m in 16.8 s when the head over the notch is 0.2 m. Determine the coefficient of discharge of

the notch.

Answer : Cd = 0.613

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1. http://www.cap-az.com/Operations/RechargeProgram/AvraValley.aspx

2. http://www.controlelectronics.com/360PrimDev.html

3. http://www.flowmeterdirectory.com/flowmeter_artc_02021403.html

4. http://www.lmnoeng.com/Weirs/RectangularWeir.htm

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