CIVE1400: Fluid Mechanics Rectangular Weir Lab
First Year Fluids - Rectangular Weir
Objective
To investigate the characteristics of flow over a rectangular notch-weir, comparing theoretical predictions with actual
measurements.
Theory
This type of weir is used in practice to measure small free-surface flows. From application of the Bernoulli equation we get the
equation below relating the height of fluid above the weir crest and the weir width to the flow over the weir.
QC B gH
d
=2
32
32/
where C
d
= coefficient of discharge, B = width of notch-weir and H = height of water (head) above the weir crest.
As
CBg
d
2
32
is constant, a graph of Q against H
3/2
would be a straight line intercepting the Q axis at 0 with gradient
CBg
2
32
d
.
To examine the accuracy of the experiment the gradient of the graph can be compared with the theoretical value. See the figure
below:
Q
CBg
d
2
32
1
H
3/2
We can also look at the experimental value of the exponent. Taking logs of equation gives
log log logQCBg
d
=
+
2
323
2H
Thus a plot of log Q against log H gives a straight line of gradient 3/2 which intercepts the log Q axis at
log CBg
d
2
32
.
See the figure below:
3
2
log CBg
d
2
32
log H
log Q
1
