V-Notch Lab Report
V-Notch Lab Report
V-Notch Lab Report
1. Abstract
1
2. Introduction
6-9
4. Conclusion and
Recommendation
10
5. References
11
Appendices:
Appendix 1: Sample Calculations
Appendix 2: Full Observation Table 12 - 13
TABLE OF CONTENT
1.0 ABSTRACT
1
2.0 INTRODUCTION
The weir crest is the top of the weir. For a v notch weir it is the point of the
notch, which is the lowest point of the weir opening. The term nappe is used
for the sheet of water flowing over the weir. The equations to meter flow in this
article require free flow, which takes place when there is air under the nappe.
2
The drawdown is the decrease in water level going over the weir due to the
acceleration of the water. The head over the weir is shown as H in the
diagram; the height of the weir crest is shown as P; and the open channel flow
rate or discharge is shown as Q (Bengtson, Harlan H, 2001).
For calculations according to experiment,
Theoretical discharge, Qt =
(2.1.1)
(2.1.2)
(2.1.3)
3
Triangular V-Notch has many applications in industry wise. The soil
instruments V-notch weir uses the principle of gravitational discharge of water
over a triangular notched weir plate. Discharge is a function of the head of
water at the notch, for a given profile size and shape. Experimentally
determined coefficients relate the head of water to the rate of discharge.
Simple in principle the v notch weir is a low cost and robust instrument idealy
suited to the long dams. Water flow monitoring in open channels is widely
employed in environmental and geotechnical field. V-Notch helps in
measuring the leakage measurement which is one of the most important
indicator of the overall performance of earth or rock-fill and concrete dam. The
leakage rate is a function of the water level in the reservoir and depends
either of the construction and of the behaviour of the dam. Consequently,
leakage monitoring provides data for the evaluation of the long term stability
of the dam constructions. Leakage water is usually impounded downstream of
the dam and diverted to a basin in a weir-station. It is also used in drainage
systems in tunnels and excavations (E.Bryan, 2016).
4
The main advantage of using v-notch is it is The expression for discharge
for a right angle V-notch or weir is very simple. Ventilation of triangular notch
is not necessary. Triangular notch will provide more precise results during
measuring low discharge as compare to result obtained from rectangular
notch. Only one reading of H will be required for determination of discharge in
case of triangular notch. V- notches are easily constructed and well-defined
lab calibration. Moreover, high head-drop required. It is more accuracy at low
flows than rectangular or trapezoidal (Sivaranjith.K. 2018).
The disadvantage of v-notch is it vertex of the notch is not clearly cut and
consequently the zero level is not defined well. Moreover, the crest is not
sharp and the weir is skewed. The central angle is not precise and the edge of
crest is not flat at all. Waves, bubble intrusion and not uniform velocity field in
the measurement profile of V-notch. The crest is placed lower than 5 cm
above the maximum downstream water level (Sivaranjith.K. 2018).
5
Sl Actual Discharge Theoretical Discharge Cd
No
Average Cd = 0.60
Table 3.1: Data Sheet of Triangular V-Notch
The table 3.1 shows the result of triangular V-Notch experiment. The flow
rate of the water is set by assuming the flow of water using a stilling baffle
from high velocity, medium velocity and slow velocity was the manipulated
variable. The volume of water collected has been set to constant at 0.012m3.
The actual discharge(Qa), theoretical discharge,(Qt) and coefficient of
discharge,(Cd) are calculated and tabulated in the table 3.1. In the result table,
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 increases. 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. the lower the rate flow of water, the lower the actual and
the theoretical discharge of the triangular v-notch. This is because the
different velocity distributions of the fluid enters the stilling baffle lowers the
pressure upon the triangular V-Notch. When the velocity of fluid is higher, the
pressure becomes lower, then this changes make difference in the head
above bottom notch readings. So that, the theoretical discharge is higher in
the high velocity water flow rate.
6
Figure 3.2 The relationship between actual discharge (Qa) and
theoretical discharge (Qt).
3.3 Discussion
7
Qa is known as the actual discharge which can obtained from the collected
volume (m3) and the time taken, t (sec) to reach 0.012m 3 volume. Actual
discharge is to determine the water that has to be discharged in the tank. Q t is
known as the theoretical discharge which can be obtained by finding the
difference in height of crest using point gauge. The height of water above the
top the notch is usually used to correlate with flow rate. Q t gives the real time
flow of water into tank due to greater flow rate. The greater the flow rate, the
greater will be the increase in depth of flow. This both Q a (actual discharge)
and Qt (theoretical discharge) readings must to be taken to find the coefficient
of discharge at the end of result.
Qt always higher than Qa. From the result table of this experiment, it is
proved that Qt is always higher than Q a. For example, at 0.0491m height of
water reading, Qa obtained is 0.0008086 m3/s and Qt value is 0.001262 m3/s. It
remained like this for all readings. This is because of the values of the
discharge coefficient increased with increases in the values of the upstream
water depth. The second reason would be the delays depends on the time
needed for closer which put inside valve to close the flow through. The delays
also may occured because of the time needed for the pressure inside stilling
baffle to rise above the pressure needed according to velocity for the opening
of discharge valve. Imperfections in sealing, can lead to leakage whenever
the difference between pressures present and water depth. However, the
leakage is negligible. Leakage through the valve due to small orifice can be
considered laminar (Sestan & Virag, 2014, p.4).
8
The discharge coefficient of a V-notch is typically 0.5 to 0.65 according to
the width and height of notch. It cannot exceed more than 1.0. If it exceeds
then it means high non-uniformity of velocity distribution. The higher the non-
uniformity of velocity distribution the greater will be values of coefficients.
(Barger, V. and Olsson, M, 1973).
From the table result it is proved that the value of average coefficient
discharge is 0.60 which is between the range from 0.5 to 0.65. From the slope
of the graph, it is confirmed that range of coefficient discharge is 0.64. The
fourth plot is slightly run out from the line and has not been intersected. This is
maybe because of some errors such as parallax error since this experiment
was carried in a group. The parallax error may be due to the difference in the
height of the readings and time take to collect the water which may cause
difference in the calculated value. Both values of coefficient discharge from
the table and from the slope of graph slightly different but do not exceed the
limit and maintained from 0.5 to 0.65.
9
4.0 CONCLUSION AND RECOMMENDATION
Parallex error can be avoided by checking the height of water level awhich
should be above the water surface. Last but not least, ensure to drain the
water in volumetric tank to avoid overflow of the water and damage the
experimental setup.
10
REFERENCES
1) Zoebelein, M., How V-Notch work, Endeavor Business Media, LLC, United
States, 2013, p1-p20.
3) Bengtson, Harlan H. 2001. Open Channel Flow III – Sharp Crested Weirs,
Brighthubengineering.com.vlog.
Available from World Wide Web:
https://www.brighthubengineering.com/hydraulics-civil-engineering/65701-
open-channel-flow-measurement-4-the-v-notch-weir/.
11
APPENDIX I
SAMPLE CALCULATIONS (READING NO. _3__)
1. Actual discharge, Qa
2. Theoretical discharge , Qt
=
8 90 5
= 2 9.81 tan 0.0386 2
15 2
= 0.000692 m3/s
3. Coefficient discharge, Cd
= actual discharge
theoretical discharge
= 0.0004107 m3/s
0.000692 m3/s
= 0.59
12
13