# Impact of Jets

Purpose:

* To demonstrate the characteristics of flow over weirs.

* To determine the 'Coefficient of Discharge' for each type of weir.

Introduction:

In open channel hydraulics, weirs are commonly used to either regulate or to measure the volumetric flow rate. They are of particular use in large scale situations such as irrigation schemes, canals and rivers. For small scale applications, weirs are often referred to as notches and invariably are sharp edged and manufactured from thin plate material.

Apparatus:

Hydraulics Bench incorporates a weir channel. The rectangular notch weir or (V) vee notch weir to be tested is clamped to the weir carrier in the channel by thumb nuts. 4

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Figure 1: Flow over Weirs - Figure 2: Flow over Weirs -

vee notch weir rectangular notch weir

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Hydraulics BenchBasket of glass spheres

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Weir channelVolumetric measuring tank

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(V) Vee notch weir7

Rectangular weir

Hook & point gauge Hook Gauge and Scale8

There are different shapes of weirs that can be used to measure the volumetric flow rate. These shapes with their dimension are shown in fig 3 below.

Figure 3: Details of weirs

Theory:

* Rectangular Weir:

A rectangular notch is a thin square edged weir plate installed in a weir channel as shown in figure 4.

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Figure 4: Rectangular Notch

Consider the flow in an element of height δh at a depth h below the surface. Assuming that the flow is everywhere normal to the plane of the weir and that the free surface remains horizontal up to the plane of the weir, then velocity through element 2gh

∴ Theoretical discharge through element dQ=v.dA= 2gh.b.dh Integrating between h = 0 and h = H

Total theoretical discharge Qth=0H2gh.B.dh=B2g0Hh12.dh

So, Qth=23B2gH32

In practice the flow through the notch will not be parallel and therefore will not be normal to the plane of the weir. The free...

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