Flow over Weir

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  • Topic: Volumetric flow rate, Fluid dynamics, Discharge
  • Pages : 6 (1252 words )
  • Download(s) : 162
  • Published : March 24, 2013
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Introduction

A weir is an opening in the sidewall of a tank at top. The stream of liquid coming out the weir is known as a nappe, sheet or vein. There is no difference between a notch and weir except that the former is a small structure and has sharp edges. A weir generally an overflow structure, with a broad crest, build across an open channel. The terms air and weirs are used synonymously in general. The top of weir wall over which the liquid flows is known as the sill or crest. The head under which the weir is discharging is measure from the crest to the free surface. A weir or notch is generally used for measuring the flow of liquids.

In this experiment, we ar using the rectangular weir and triangle weir. Rectangular weir and triangular weir (v-notch) ore often used in water supply, wastewater and sewage system. They consists of a sharp edge plane with rectangular and triangular profile for the water flow. Broad-crested weirs can be observed in dam spillways where the broad edge is beneath the water surface across the entire stream. Flow measurement installations ith broad-crested weirs will meet accuracy requirement only if they are calibrated.

objective

* To determine the characteristics of open channel flow over; * A rectangular notch
* A triangular (v-notch)
* To determine values of the discharge coefficient for both notches

Method

General features of the flow can be determined by direct observation. Discharge coefficient values can be determined from measurements of the height of the free surface of water above the notch base and corresponding volume flow rate

Equipment

In order to complete the experiment, we need a number of pieces of equipment * The F1-10 haudralics bench which allows us to measure flow by timed volume collection * The F1-13 stilling baffle

* The F1-13 rectangular and Vee notch
* Vernier Height Gauge (supplied with F1-13)
* Stopwatch
* Spiril level

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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
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Rectangular weir
Hook & point gauge Hook Gauge and Scale8
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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

Technical Data

The following dimension from the equipment are used in the appropriate calculation. If required these values may be checked as part of the experimental procedure and replace with your own measurements.

* Width of rectangular notch, b= 0.030
* Angle of vee notch, ø=90 degree

Theory

Because the depth of flow above the base of notch is related to the volume flow rate through it, the notch forms a useful flow measurement device. The classical result for flow over notch are obtained by an application of the Bernoulli equation, from a point well up stream to a point just above the notch. This approach requires a number of very substantial assumptions and it yields the following results:

For Rectangular Notch, the formula is given by;

Qt=Cd23b(2g)H32

For Vee-notch, the formula is given by;

Qt=Cd815tan(θ2)(2g)H52

Where;

Qt= Volume flow rate
H = height above notch base
B = width of rectangular notch
θ = angle of the vee in triangular notch;
Cd= the discharge coefficient, which has to be determined by experiment

These can be rearrange to give;

Cd=3Qt2b2gH32

For rectangular notch

Cd=15Qt8tan∅22gH52

Foe vee notch.
Discussion.

From the experiment, we are about to determine the discharge coefficient of the both rectangular and the triangular notches. The discharge coefficient for the rectangular notch are influence by its...
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