EXPERIMENT (6)

FLOW OVER WEIRS

By:Eng. Motasem M. Abushaban.

Eng. Fedaa M. Fayyad.

1

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. 2

INTRODUCTION

Crest

Is the edge or surface over which the water flows.

Nappe

Is the overflowing sheet of

water.

3

INTRODUCTION

4

levelDownstream

Upstream levelH

P

Weircrest

Weir

NappeV = 2gh

L 4H

dhh

levelDownstream

Upstream level

H

P

Weircrest

Weir

Nappe

V = 2gh

L 4H

dhh

INTRODUCTION

If the water springs clear of downstream face, acts as

sharp-crested weir

A long, raised channel control crest is a broad-crested weir

5

INTRODUCTION

Usually named for the shape of the overflow opening

Rectangular

Triangular

Trapezoidal

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RECTANGULAR WEIR

7

TRIANGULAR WEIRS

8

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).9

APPARATUS

10

11

To determine an expression for the theoretical flow through a notchwe will consider a horizontal strip of width b and depth h below thefree 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 = Hgives the expression for the total theoretical discharge,

gh2 V =

ghhbAVQ 2δδ ==

dhbhgH

∫=0

ltheoretica2

12 Q

THEORY

12

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

12

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 nappeas 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.

14

)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 +

axisy−+intercept=y axis-

)232ln(int gBCercept d=

gB

eCercept

d

232

int

=

EQUATIONS

For rectangular weir :

For Triangular weir :

15

gB

eCercept

d

232

int

=

)2

tan(2158

int

θg

eCercept

d =

PROCEDURE

Place the flow stilling basket of glass spheres intothe left end of the weir channel and attach thehose from the bench regulating valve to the inletconnection into the stilling basket.

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

Ensure that the square edge of the weir facesupstream.

Start the pump and slowly open the benchregulating valve until the water level reaches thecrest of the weir and measure the water level todetermine the datum level Hzero. 16

Adjust the bench regulating valve to give the firstrequired head level of approximately 10mm.Measure the flow rate using the volumetric tankor the rotameter. Observe the shape of the nappe.

Increase the flow by opening the benchregulating valve to set up heads above the datumlevel in steps of approximately 10mm until theregulating valve is fully open. At each conditionmeasure the flow rate and observe the shape ofthe nappe.

Close the regulating valve, stop the pump andthen replace the weir with the next weir to betested. Repeat the test procedure 17

PROCEDURE

RESULT AND CALCULATION

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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.

• Data & Results:

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1 2 3

H (mm)

V (L)

T (sec)

Qact (m3/s)

ln (Qact)

ln(H) (m)

Qth (m3/s)

20

QUESTIONS