Flow Over a Broad-crested Weir Exp4

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CONTENTS : INTRODUCTION OBJECTIVE THEORY APPARATUS PROCEDURE RESULT CALCULATION DISCUSSION CONCLUSION RECOMMENDATION REFERENCE APPENDIX

PAGES : 2 3 3-4 5 5 6-10 11-13 14 14 15 15 16

Introduction 1

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A weir is usually or commonly used in open channel for controlling upstream water levels and measuring discharge. For both task it acts as an obstruction which promotes a condition of minimum specific energy in sub-critical flow. When used for the latter purpose all weirs must be calibrated because theoretical predictions of discharge are rendered inadequate by the effect of viscosity and the variations of flow geometry with upstream depth. Broad crested weirs are generally constructed from reinforced concrete and are widely used for flow measurement and regulation of water depth in river, canals, and the other natural open channel.

Objective To study the behaviour of non uniform and the use of a broadcrested weir to measure the discharge in a rectangular open channel. Theory A weir in generally can take on many shapes, however broad crested weir operate more effectively than their sharp crested counterparts under higher downstream water levels, and can be used to measure the discharge of river since the parallel flow caused by the weir allows it to be accurately analyzed by the use of energy principles and critical depth relationships. It works on the principle that subcritical flow upstream flow upstream of the weir moves over the obstruction and this height of the weir causes critical flow, accelerating the liquid which then transition into supercritical flow napped after the weir is crossed downstream. This critical depth required to cause critical flow is not easily measured because it is exact location is not easy to determine and may vary with the flow rate. However, the upstream depth can be used to determine the flow rate through mass conservation which is a more reliable measurement. Experimentally, broad crested weirs can be used as a flow rate measuring device and has the advantage that is simple to be constructed and has no edge that can wear and thus after the coefficient. ******************** insert photo***************************** 2

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Apparatus 1. Open channel flume 2. Broad – Crested weir 3. Flow meter for measuring flow rate ( or any other method ) 4. Point gauge for measuring depth

Procedure

1. The dimensions of the channel were recorded. 2. Then, the channel width was adjusted to a mild slope. 3. The Broad-Crested Weir installed and let the water flow over the weir. 4. Ensured the chocking occurs and the water behind the weir increases in depth. 5. After that the flow rate in the channel was measured by using the flow rate meter. 6. Then the height of the broad-crested weir was measured. 7. Measured the depth behind the weir and deduce the value of H. 8. Finally, the discharge is calculated by using the Broad-Crested weir equation. 9. Repeated for the different flow rates.

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Result Dimension: Lw = 0.345 m Pw = 0.1 m Breadh, B = 0.077 m Recorded table : Experim water Critic Veloci ent No : depth al ty, behind Depth V weir, H , Y c (m/s) (m) (m)

Weir Dischar ge Coeffici ent, C wd

Discharge, Q (Taking from flow rate meter) ( x10⁻ᶟ mᶟ/s )

Discharge, Q Experiment * Higher weir in case. ( x10⁻ᶟ mᶟ/s )

0.531

1.250

0.779

0.533

1.150

0.759

0.534

1.100

0.737

0.536

1.050

0.717

0.542

1.000

0.657

0.544

0.950

0.637

0.545

0.900

0.616

0.549

0.850

0.577

broad

1

0.050

2

0.049

3

0.048

4

0.047

5

0.044

6

0.043

7

0.042

8

0.040

0.034 0 0.032 0 0.031 5 0.030 0 0.028 8 0.028 2 0.026 5 0.026

0.577 5 0.560 3 0.555 9 0.542 5 0.531 5 0.526 0 0.509 9 0.509

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0.039

10

0.037

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9 0.497 2 0.485 2

0.551

0.800

0.557

0.555

0.750

0.519

# Sample calculation for reading 1. Parameter Velocity , vc

Formula Vc = √gyc

Calculate = √ 9.81 x 0.034 = 0.5775 m/s

Weir Discharge Coefficient, C wd broad

C wd broad =

Discharge, Q * Higher weir in case.

Q = C wd broad B√g(2/3)ᶟ̷ ² H ᶟ̷ ²

0.65 =

0.65 √(1+0.50/0.1) = 0.531

√(1+H/Pw)

= 0.531 X 0.077 √9.81 X з√ 2/3 X з√ 0.050 = 0.779 X 10 ⁻ᶟ mᶟ/s

Discussion Based on the experiment that was carried out by our group, the magnitude of the flow rate, Q and the effect on the weir discharge coefficient Cwd broad founded that as the magnitude of the flow rate increased, so did the discharge coefficient. Moreover, this may have been due to the the shape of the weir which had a rectangular control section. Since the height of the water increased with the increased flow, so more friction are losses may have occurred. Furthermore, as we can see the relationship between Cwd broad increased with increasing the flow rate, Q. Meanwhile, the relation between the magnitude of the flow rate, Q and the effect on the velocity coefficient Cv was founded that as the flow rate increased, so did was the velocity coefficient. Experiment data also showed that Cv increased with increasing the flow rate.

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Conclusion

Recommendation / Precaution This experiment must have several precautions to avoid error, so much more accuracy can be obtained from this experiment: 1) Error due to the parallax in reading the vernier scale and tank. 2) The flow may not have been fully stabilized when the reading were taken. 3) It was assumed that the density was for pure water however it should be noted the water in the experiment was brown indicating it may have contain other substance and impurities which may have caused erroneous momentum and energy values.

Reference WEBSITE: •

Bernoulli’s Equation.(n.d). Retrieved March,30, 2013,fromhttp://www.physicsforums.com/library.php? do=view_item&itemid=115

•

Aerodynamic Principlesn.d). RetrievedMarch,30,2013,http://library.thinkquest.org/18033/aerodyn.html

•

Bernoulli’s Theorem.(n.d). Retrieved March,23, 2013, from, http://www.nistepkscience.com/Physics/8Bernoullistheorem.pdf 6

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BOOK : • •

Suhaimi Abdul Talib,Hamidon Ahmad,Turahim Abdul Hamid,Junaidah Ariffin. ( 1997). Fluid Mechanics 2nd Edition. Norashikin Ahmad Kamal, Hamidon Ahmad, Wardah Tahir, Irma Noorazurah Mohamad, Fauzi Baharuddin. ( 2011). Fluid Mechanics and Hydraulics Laboratory Manual (OBE).

Appendix

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1. OPEN CHANNEL FLUME

2. WATER JUMP INTO

3. FLOW METER TO MEASURE FLOW RATE

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