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EXPERIMENT (6)

FLOW OVER WEIRS

By:Eng. Motasem M. Abushaban.

Eng. Fedaa M. Fayyad.

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INTRODUCTION

Weir is defined as a barrier over which the

water flows in an open channel

A weir is a notch on a larger scale – usually

found in rivers.

It is used as both a flow measuring device

and a device to raise water levels.

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INTRODUCTION

Crest

Is the edge or surface over which the water flows.

Nappe

Is the overflowing sheet of

water.

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INTRODUCTION

levelDownstrea m

Upstream levelH

PWeircrestWeir

NappeV = 2gh

L 4H

dhh

levelDownstream

Upstream level

H

P

W eircrest

Weir

Nappe

V = 2gh

L 4H

dhh

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INTRODUCTION

Usually named for the shape of the overflow

opening Rectangular Triangular Trapezoidal

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RECTANGULAR WEIR

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TRIANGULAR WEIRS

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PURPOSE

To observe characteristics of flow over a weir.

To determine the head-discharge relationship

of two different shapes of weirs, and to

compare the experimental results with their

corresponding theoretical expressions.

Calculating the coefficient of discharge (Cd).

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APPARATUS

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To determine an expression for the theoretical flow through a notch we will consider a horizontal strip of width b and depth h below the free surface, as shown in the figure

Velocity through the strip

Discharge through the strip,

Integrating from the free surface, h = 0, to the weir crest, h = H gives the expression for the total theoretical discharge,

gh2 V

ghhbAVQ 2

dhbhgH

0

ltheoretica2

12 Q

THEORY

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Rectangular WeirFor a rectangular weir the width does not change with depth so there is no relationship between b and depth h.We have the equation, b = constant = B.Substituting this with the general weir equation gives:

dhhgBQH

Oltheoretica 2

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232

32 HgB

To calculate the actual discharge we introduce a coefficient of discharge, Cd, which accounts for losses at the edges of the weir and contractions in the area of flow, giving :

timevolumeHgBCQ dactual 2

3232

THEORY

THEORY In practice the flow through the notch will not

be normal to the plane of the weir. The viscosity and surface tension will have an effect. There will be a considerable change in the shape of the nappe as it passes through the notch with curvature of the stream lines in both vertical and horizontal planes

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THEORY The discharge from a rectangular notch will

be considerably less.

)H 2gB 32ln(Cd=)ln(Q 3/2

act

)ln(H)2gB 32ln(Cd=)ln(Q 3/2

act

)ln(H23)2g

32ln(Cd=)ln(Qact

axisyintercept=y axis-

)232ln(int gBCercept d

gB

eCercept

d

232

int

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EQUATIONS For rectangular weir :

For Triangular weir :

gB

eCercept

d

232

int

)2

tan(2158

int

g

eCercept

d

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PROCEDURE

Place the flow stilling basket of glass spheres into the left end of the weir channel and attach the hose from the bench regulating valve to the inlet connection into the stilling basket.

Place the specific weir plate which is to be tested first and hold it using the five thumb nuts.

Ensure that the square edge of the weir faces upstream.

Start the pump and slowly open the bench regulating valve until the water level reaches the crest of the weir and measure the water level to determine the datum level Hzero.

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Adjust the bench regulating valve to give the first required head level of approximately 10mm. Measure the flow rate using the volumetric tank or the rotameter. Observe the shape of the nappe.

Increase the flow by opening the bench regulating valve to set up heads above the datum level in steps of approximately 10mm until the regulating valve is fully open. At each condition measure the flow rate and observe the shape of the nappe.

Close the regulating valve, stop the pump and then replace the weir with the next weir to be tested. Repeat the test procedure

PROCEDURE

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RESULT AND CALCULATION

Record the results on a copy of the results sheet.

Plot a graph of loge (Q) against loge (H) for each weir.

Measure the slopes and the intercepts.

From the intercept calculate the coefficients of discharge and from the slopes of the graphs confirm that the index is approximately 1.5 for the rectangular weir and 2.5 for the triangular weirs.

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• Data & Results:

  1 2 3

H (mm)

V (L)

T (sec)

Qact (m3/s)

ln (Qact)

ln(H) (m)

Qth (m3/s)

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QUESTIONS