Weirs
UNIVERSITY OF MALAYA
MECHANICAL ENGINEERING DEPARTMENT
FLUID MECHANICS LAB
Title: Weirs
Objective
Characteristics of flow over a rectangular notch
Equipment Set-up
Hydraulics Bench F1-10, Hook and Point Gauge, Stop watch (not supplied), Weir F1-13
(rectangular notch).
SCREW C
FINE ADJUSTMENT NUT
SCALE
VERNIER
INSTRUMENT
CARRIER
STILLING BAFFLE
WEIR PLATE
(V or U)
DELIVERY NOZZLE
SCREW B
THUMB NUTS
SLIDING MAST
WEIR CARRIER
SCREW C
POINT
Summary of Theory
Q = Cd
2
B 2 g H 3/ 2
3
Where C d
= Coefficient of discharge
B
= Width of notch
H
= Head above bottom of notch
1
Weirs
Procedure
Ensure that the hydraulics bench is located on a level floor as the accuracy of the results
will be affected if the bench top is not leveled. Set up the equipment as shown in the
diagram. Set Vernier Height Gauge to a datum reading, by placing the point on the crest
of the weir. Take extreme care not to damage the weir plate with the point gauge.
Position the gauge about half way between the notch plate and stilling baffle. Admit
water to the channel, adjust flow control valve to obtain heads, H, increasing in steps of
about 1cm. For each flow rate, stabilized conditions, measure and record H. take readings
of volume and time using the volumetric tank to determine the flow rate.
Results and Calculations
Tabulate volumes, time and heads.
Compute and tabulate:
QH 3 / 2 C d =
Plot
3Q
2B 2 g H
3/ 2
Q 2 / 3 , log Q, log H
Q2/3 against H
log Q against log H
Cd against H
Conclusions
1. Is C d constant for this notch?
2. Estimate an average value of C d for the range of the test.
3. Can the Q-H relationship be described by and empirical formula Q = kH n ?
If so, find values of k and n.
4. If Cd varies, suggest a functional relationship between C d and
H
.
B
2