CHE241 - Lab Report Solteq Flow Over Weirs FM26 (2015)

SOLTEQ® FLOW OVER WIRES (FM 26) Page 1

TABLE OF CONTENT

No Title Page

1 Abstract………………………………. 2

2 Introduction………………………….. 3

3 Objectives…………………………….. 4

4 Theory………………………………... 5 – 6

5 Material And Apparatus……………… 7

6 Methodology…………………………. 8

7 Data and Results……………………… 9 – 11

8 Calculations…………………………... 12 – 15

9 Discussion……………………………. 16 – 18

10 Conclusion……………………………. 19

11 Recommendations……………………. 20

12 Reference……………………………... 21

13 Appendix……………………………... 22


1.0 ABSTRACT

The Flow over Weirs experiment was conducted to investigate the characteristics of flow over a

rectangular notch and triangular notch. The difference in flow rate of water that flows into both

of the channel was observed. Other than that, the experiment was capable to study the discharge

coefficient of fluid flow that determined by calculation of this experiment.

The experiment was started as follows the procedures, with the depth of water with

different height was tested by recording the time taken to collect 3L of water, which later will be

used to calculate the flow rate of the flow. The data obtained were further tabulated by

calculating the discharge coefficient, using the equation provided.

Then, graphs were constructed to analyze the characteristics of the flow. From the

constructed graphs, rectangular notch graph shows the discharge coefficient decrease slowly

before a constant value is reached. Meanwhile, the triangular notch graph shows the discharge

decrease smoothly, but the values are higher than the rectangular notch.

To conclude, triangular notch has a higher discharge coefficient than rectangular notch.

The experiment was successfully accomplished as all the objectives were gained.


2.0 INTRODUCTION

The flow rate in pipes and ducts is controlled by various kinds of valves. Liquid flow in open

channels, however, is not confined, and thus the flow rate is controlled by partially blocking the

channel. This is done by either allowing the liquid to flow over the obstruction or under it. An

obstruction that allows the liquid to flow over it is called a weir, and an obstruction with an

adjustable opening at the bottom that allows the liquid to flow underneath it is called an

underflow gate. Such devices can be used to control the flow rate through the channel as well as

to measure it. A weir is a flow control device in which the water flows over the obstruction.

In this experiment, the rectangular weirs and triangular weirs are been used. Rectangular

weirs and triangular or v-notch weirs are often used in water supply, wastewater and sewage

systems. They consist of a sharp edged plate with a rectangular, triangular or v-notch 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 with broad-

crested weirs will meet accuracy requirements only if they are calibrated.


3.0 OBJECTIVE

The main objectives of this experiment is to observe the flow characteristics over a

rectangular notch and a V-notch. Besides, the other objectives is to determine the discharge

coefficients of the fluid flow. Then, to teach student how to plot a graph of

against H, Log Q

against Log H and against H on rectangular notch. While on V-notch, learn how to plot graph

of

against H. Next, to estimate an average value of for the range of the test. Last but not

least, to compare values between rectangular notch and V-notch.


4.0 THEORY

Flow over Weirs

Weirs are hydraulic structures consisting of an obstruction placed across a water channel with a

specially shaped opening or notch. The weir structure will increase the water level, which will be

measured. Water level-discharge relationships are available for standard-shaped openings or

notches.

Two types of weirs are widely used:

1. Rectangular shape opening

2. V-notch.

Stilling baffle is used to ensure minimum turbulence. The stilling baffle will act as a reservoir to

collect water volume and slowly disperse the water from the opening at the bottom of the stilling

baffle.

Types of Weirs

Rectangular Weir

The rectangular weir is able to measure higher flows than the v-notch weir and over a wider

operating range.

Q eq. (1)

Where Cd = Coefficient of discharge

B = Width of notch

H = Head above bottom of notch

Q =Flow rate

= 2 3

2 3

2 H g B C d

B, 3 3 mm

89mm


V-Notch

The V-notch weir is a notch with a V shape opening. V-notch weir is typically used to

measure low flows within a narrow operating range. Typical Cd values for V-notch are in the

range of 0.58 to 0.62.

√

Where,

Cd = Coefficient of discharge

= Half the enclosed angle of the vee

H = Head above bottom notch

Q = Flow Rate

90 °

50 mm


5.0 MATERIAL AND APPARATUS

MATERIAL

1) Water

APPARATUS

1) F1-13 Stilling baffle

2) Rectangular notch

3) V- notch

4) Stopwatch

5) Spirit level

6) F1-10 Hydraulics Bench

Figure 1: F1-10 Hydraulics Bench

1. Stilling Baffle 5. Hydraulic Bench (FM 110)

2. Vernier 6. Flow Control Valve

3. Hook 7. Water Channel

4. Weir Plate (V or rectangular) 8. Sump Tank

1

8

7

6

5

4

3

2


6.0 METHODOLOGY

Experimental Procedures

1) The hydraulic bench pump were confirmed is securely connected.

2) The collection tank drain valve is ensured left OPEN to allowed flow discharged back

into sump tank.

3) The flow over weirs apparatus were set up on the hydraulic bench.

4) Thumb nuts were confirmed securing the rectangular notch weir plate is fully secured.

General Shut Down Procedures

1) The hydraulic bench flow control valve were shut and water supply is closed.

2) The residue water from channel and sump tank was emptied by ensuring the drain tank

valve is left open.

Experiment 1: Flow Characteristics over Weirs

1) The weir apparatus on the hydraulic bench were levelled and the rectangular notch weir

is installed.

2) The hydraulic bench flow control valve were slowly opened to admit water to the channel

until the water discharged over the weir plate. The water level is ensured that it is even

with the crest of the weir.

3) The flow control valve is closed and allowed water level to stabilized.

4) The Vernier Gauge was set to gives a datum reading using the top of the hook.

5) The gauge is installed about half way between the notch plate and stilling baffle.

6) Water is admitted to the channel. The water flow were adjusted by using the hydraulic

bench flow control valve to obtain heads (H).

7) After water flow condition is stabilized, heads readings is recorded in every increasing of

about 1 cm.

8) Step 4 and 5 were repeated for different flow rate.

9) The readings of volume and time were taking using the volumetric tank to determined

flow rate.

10) The rectangular notch were replaced by the notch with v-notch.

11) The result was recorded in the tables.


7.0 DATA AND RESULTS

Rectangular Notch

Rectangular

Notch

Volume

(L)

Height

(m)

Time

(s)

Flow Rate,

Q

(m3/s)

Log H

T1 T2 T3 Average

3 0.0105 50 21 42 37.6667 7.9646 x 10-5

-1.9788

3 0.0197 32 10 11 17.6667 1.6981 x 10-4

-1.7055

3 0.0325 24 7 6 12.3333 2.4321 x 10-4

-1.4881

3 0.0434 16 5 7 6.0000 5.0000 x 10-4

-1.3625

Rectangular

Notch

Log Q Q2/3

( )

Cd

-4.0988 1.8512 x 10-3

0.7600 0.3182

-3.7700 3.0665 x 10-3

0.6305 0.5970

-3.6140 3.8963 x 10-3

0.4262 0.9848

-3.3010 6.2996 x 10-3

0.5678 1.3152

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0 0.01 0.02 0.03 0.04 0.05

Q2

/3 (

m2/s

2/3

)

Head Above Bottom of Notch (m)

Graph of Q2/3 Against Head Above Bottom of Notch, H


-4.5

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

-2.5 -2 -1.5 -1 -0.5 0

Log Q

Log H

Graph of Log Q Against Log H

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 0.01 0.02 0.03 0.04 0.05

Cd


Graph of Coefficient of Discharge, Cd Against Head Above

Bottom of Notch, H


V-Notch

V-Notch

Volume

(L)

Height

(m)

Time

(s)

Flow Rate,

Q

(m3/s) T1 T2 T3 Average

3 0.0158 79 27 31 45.6667 6.5693 x 10-5

3 0.0244 35 11 10 18.6667 1.6071 x 10-4

3 0.0297 28 6 8 14.0000 2.1429 x 10-4

3 0.0413 17 4 4 8.3333 3.6000 x 10-4

V-Notch

Q2/5

(

)

Cd

2.1233 x 10-2

0.8866

3.0368 x 10-2

0.7319

3.4072 x 10-2

0.5970

4.1930 x 10-2

0.4398

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045

Q2

/5




8.0 CALCULATIONS

Sample Calculation of Flow Rates , Q

Rectangular Notch V-Notch

Q1

Q1

Q2

Q2

Q3

Q3

Q4

Q4

Sample Calculation of Log Q

Rectangular Notch

Log Q1

Log Q2

Log Q3

Log Q4


Sample Calculation of Log H

Rectangular Notch

Log H1

Log H2

Log H3

Log H4

Sample Calculation of

Rectangular Notch

(

)

(

)

(

)

(

)

(

)

(

)

(

)

(

)


Sample Calculation of Coefficient of Discharge, Cd

Rectangular Notch

√

(

√

)

Where,


B = Width of notch


Q = Flow Rate

Cd1

(

√

)

Cd2

(

√

)

Cd3

(

√

)

Cd4

(

√

)


V-Notch

√

(

√ ( )

)

Where,


= Half the enclosed angle of the vee


Q = Flow Rate

Cd1

(

√ ( )

)

Cd2

(

√ ( )

)

Cd3

(

√ ( )

)

Cd4

(

√ ( )

)


9.0 DISCUSSION

The Cd value is not constant for the rectangular notch. This is because the value for the flow

rates, Q and the head above bottom of notch, H obtained for both rectangular notch and V-notch

are different in each experiment. However, the Cd values of rectangular notch is smaller

compared to the Cd values of V-notch.

Calculation below shows the average value of Cd for rectangular notch for the range of the test:-

Based on the calculations for Cd values of V-notch, the Cd values gain decreases as the

flow rate decreases. Besides, as the head above bottom notch increases , the Cd values gain also

decreases. This shows that, the Cd values is dependent on the value of flow rate, Q and the value

of the head above the notch, H.

To prove whether Q and H relationship described by an empirical formula , the

graph of against is plotted, the derivation from the empirical is as follow:-

Where, when the equation is ploted on a graph,

log Q = value on y-axis

log H = value on x-axis

log k = y-intercept

n = the gradient of the graph

Cd

0.7600

0.6305

0.4262

0.5678


Based on the graph,

The gradient (n) of the graph is :-

The value k is :-

Thus, this shows that Q and H does relate and can be describe using the empirical formula

.

-4.5

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

-2.5 -2 -1.5 -1 -0.5 0

Log Q

Log H

Graph of Log Q Against Log H


0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045

Q2

/5



Based on the graph , the values of

increases as the head above the bottom of notch, H

increases. The Cd values can be obtained from the tangent of the graph on each point plotted.

Cd Calculated Using Formula

Cd Gain From Graph

Cd1 = 0.8866

Cd2 = 0.7319

Cd3 = 0.5970

Cd4 = 0.4398

From the data, there is difference between Cd calculated from formula and Cd calculated

using the plotted graph. The reason of this error could be because of the reading from the graph

is not as accurate as the calculation using formula which lead to about 0.2 – 0.4 result error.


10.0 CONCLUSION

The smooth flow to and over the weir is essential to the determination of accurate rates of

flow since the distribution of velocities on the approach flow has a definite influence on the

discharge over the weir. As the flow rate increases, the discharge coefficient becomes more

accurate to the theoretical value. When the flow rate is to low it clings to the notch and flows

down it. This changes the coefficient of discharge because now the water is not only being

affected by gravity it is having to resist viscosity and the friction of the surface of the notch.

The limitations of the theory is it has to be level so the only force on the water is gravity,

there has to be a constant flow, and constant pressure.The theory behind this experiment makes

an assumption that there is a minimum height of water above the notch and any heights below

this start to deviate from theory at an increasing rate.The relationship between the head of the

weir and the discharge of the water over the weir is directly proportional. The lower flow rates

produce lower heights above the notch creating larger changes from the theoretical equations


11.0 RECOMMENDATION

A weir is a barrier across a weirs designed to modify its flow characteristics. Weirs are

commonly used to alter the flow of rivers to prevent flooding, measure discharge, and help

render rivers navigable.

There are several of recommendations to estimate discharge when using a weirs because

it is very important to ensure that all flow enters by travelling over the weir and not around the

weir or under the weir. It must be noted that the weir should be extended into the ground to

minimize groundwater to pass under the weir. To ensure critical flow over the crest of the weir, it

is important to maintain a ‘free outfall’. As long as the flow conditions downstream of the weir

do not affect the flow over the weir, a free outfall is maintained.

First of all, before doing an experiment it is better to learn and understand first on how to

conduct the experiment. Moreover, the result of rectangular-notch and v-notch obtained must be

taken in 4 decimal points to get an accurate values.

Errors can never be ignored when it comes to laboratory work. The aim is to reduce the

error as much as possible to obtain accuracy in work. Ways to reduce the error are by repeating

the experiment for three times or more and then taking the average readings, by being extra

cautious during the experiment, by asking more than one person to record the readings and carry

out the experiment. It is important to keep the voice to a minimum while in a laboratory and

always listen to the instructor. If any guidelines are needed, then refer to the supervisor.


12.0 REFERENCES

Books

1) Kundu, Pijush K.; Cohen, Ira M. (2008), Fluid Mechanics (4th revised ed.), Academic

Press

2) Hamilton Smith, 1886, Hydraulics, the Flow of Water Through Orifices, Over Weirs,

and Through Open Conduits and Pipes

Web

1) http://discoverarmfield.com/en/products/view/f1-13/flow-over-weirs , retrieve in

August 2015.

2) http://www.codecogs.com/library/engineering/fluid_mechanics/weirs/index.php ,

retrieve in August 2015.

3) https://en.wikipedia.org/wiki/Fluid_mechanics , retrieve in August 2015.

4) http://www.engineeringtoolbox.com/weirs-flow-rate-d_592.html , retrieve in August

2015.

5) http://www.lmnoeng.com/Weirs/RectangularWeir.php , retrieve in August 2015.

6) http://www.aquatext.com/calcs/weir%20flow.html , retrieve in August 2015.

7) http://accessengineeringlibrary.com/browse/applied-fluid-mechanics-for-engineers ,

retrieve in August 2015.

8) https://www.scribd.com/doc/34695544/Flow-of-Water-Over-Weirs , retrieve in

August 2015.

http://discoverarmfield.com/en/products/view/f1-13/flow-over-weirs

http://www.codecogs.com/library/engineering/fluid_mechanics/weirs/index.php

https://en.wikipedia.org/wiki/Fluid_mechanics

http://www.engineeringtoolbox.com/weirs-flow-rate-d_592.html

http://www.lmnoeng.com/Weirs/RectangularWeir.php

http://www.aquatext.com/calcs/weir%20flow.html

http://accessengineeringlibrary.com/browse/applied-fluid-mechanics-for-engineers

https://www.scribd.com/doc/34695544/Flow-of-Water-Over-Weirs


13 APPENDIX

Documents

CHE241 - Lab Report Solteq Flow Over Weirs FM26 (2015)