Chapter 6 Notches and Weirs

7/27/2019 Chapter 6 Notches and Weirs

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NOTCHES AND WEIRS



Learning Outcomes:

1) Describe the notches and weirs

2) Derive an expression for the discharge

through notches and weirs

3) Apply the deduced expression and calculatethe fluid discharge through notches and

weirs



Notches & Weirs

Notch

An opening in the side of a measuring tank or reservoir extending above free surface

A large orifice which has no upper edge, the upper part

of the flow is free surface

Used to measure flow leaving a tank through anopening on the side of the tank.



Notches & WeirsWeirs

• A notch on a large scale

• Use to measure flows in channels and rivers• May be sharp-edged or broad crested

• A broad crested weir is a rectangular block of height Pw and lengthLw that has a horizontal crest over which critical flow occurs.

• Sharp crested weir is a vertical plate placed in a channel that forcesthe liquid to flow through an opening to measure the flow rate.

• Types of weirs: – Rectangular weir – V-notch weir – Triangular weir – Trapezoidal weir – Parabolic weir

• The edge or surface over which the water flows is called the crest of theweir

• The overflowing sheet of water is termed the nappe

• A broad-crested weir may be defined as one having a horizontal or nearly horizontal crest sufficiently long in the direction of flow so that the

nappe will be supported



Examples



Example 1: (Rectangular Notch)

The discharge over a rectangular notch is0.15m3/s when the water level is 25 cm above the

sill. Taking Cd=0.6, calculate the width of the

notch.



Example 2: (Triangular Notch)

A 90o triangular notch is used to measure flowrate in the laboratory flume. Calculate the flow

rate of the flume if the water level above the

notch is 150mm. Assume Cd=0.63



Exercise

Water flows from a reservoir through arectangular opening 2m high and 1.2m wide in

the vertical face of a dam. Calculate the

discharge in m3/s when the free surface in the

reservoir is 0.5m above the top of the openingassuming a coefficient of discharge is 0.64.



Exercise

In an experiment on a 90o V-notch the flow iscollected in a 0.9 m diameter vertical cylindrical

tank. It is found that the depth of water increases

by 0.685 m in 16.8s when the head over the

notch is 0.2m. Determine the coefficient of discharge of the notch.



Exercise

The flow rate of water flowing in a 3-m widechannel is to be measured with a sharp-crested

triangular weir 0.5 m above the channel bottom

with a notch angle of 60˚. If the flow depth

upstream from the weir is 1.5m, determine theflow rate of water through the channel. Take the

weir discharge coefficient to be 0.60.



Exercise

The flow rate of water in a 6-m wide rectangular channel is to be measured using a 1.1 m high

sharp-crested rectangular weir that spans across

the channel. If the head above the weir crest is

0.60 m upstream from the weir, determine theflow rate of water.

Chapter 6 Notches and Weirs

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Transcript of Chapter 6 Notches and Weirs