chapter 6 notches and weirs
TRANSCRIPT
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NOTCHES AND WEIRS
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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
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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.
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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
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Examples
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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.
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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
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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.
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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.
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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.
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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.