Fundamentals of Flow Through Open Channel
Introduction
Parameters
of Fluid
Flow
References
Introduction
An open channel is a duct in which the liquid flows with a free surface. The flow in a pipe takes place due to difference of pressure, where as in an open channel it is due to the slope of the channel.
Note: Formulae and equations, which are used in pipe flow, are also used for open channel flow.
Parameters of Fluid Flow:
Flow measurements in open channel: -
A. Measurement of velocity:
Current meter: A current meter consists of a horizontal wheel on which buckets are fixed. When the current meter is held in flowing liquid, the liquid strikes with the bucket and wheel starts rotating. Electric current is made to flow from a battery to wheel. The battery is kept above the free surface and connected to the wheel by electrical wires. A commutator is fixed to the shaft of the wheel. The commutator makes and breaks the circuit once in each revolution. The No. of revolutions per minute can be counted by means of earphone or any other device. The current meter is calibrated before use. The calibration is done in a channel in which the velocity is either known or can be calculated. By varying the velocity, a calibration chart between number of revolutions per minute and velocity is obtained. The velocity V can be expressed as V= C N where C is the constant of the meter obtained from calibration chart and N is speed in revolution per minute. Using calibration chart or equation, the velocity at any point can be obtained from the number of revolutions per minute.
B. Measurement of discharge (irregular channels):
Venturi flume: - It is suitable for channels, which are regular in cross section e.g. canals. A venturi flume is based on the principle of venturi meter used for pipe. It consists of converging approach, a throat and diverging exit. The pressure everywhere is atmospheric.
Q= C [(A1 A2/ÖA12-A22)] (Ö2gh)
In which C= coefficient of venturi flume.
A1=area of c/s at the entrance.
A2=area of c/s at throat.
H=depth of liquid at entrance.
h=depth of liquid at the throat.
C. Measurement of discharge in rivers (irregular channels):
1. Segment method: The width of river is divided into a number of vertical segments. The mean velocity in each segment can be determined by measuring the velocity at 0.6 D from the free surface, where D is the depth of segment. For greater accuracy, the velocity should be measured at several depths in the segment and the mean velocity is taken as the average of these velocities. Knowing the velocity in each segment, the discharge in the segment is obtained as the product of the mean velocity and the area of the segment. Total discharge in the channel is obtained by adding the discharges in the segments.
2. Chemical method: In this method, a salt solution of the known strength and weight is added at a section in the channel. The solution mixes with the water in the channel. The strength of the solution is changed. A sample of water is taken at another section down stream and its strength is measured.
Discharge in the river, Q=q w/W (cumecs).
Where:
q=quantity of salt solution in cumecs.
w=weight (kg) of salt per cubic meter of solution injected.
W=weight (kg) of salt per cubic meter of stream water at lower section.
This method is not reliable if the water does not mix thoroughly with the solution before it reaches the lower section.
Reference:
1. Fluid Mechanics, Hydraulics And Hydraulic Machines –K.R.Arora.