Experiment Name: Flow measurement in open channel by V-Notch & rectangular weir.
V-Notch & rectangular weir Laboratory Setup.
Objective:
(i) To determine the coefficient of discharge of V-notch and rectangular weir for different rates of
flow.
(ii) To compare the results by plotting Qactual vs H and Qₜₕₑₒ vs H in log-log graph for each results.
Apparatus Used:
1. An Open channel attached with V-notch, Rectangular weir.
2. Volumetric tank.
3. Stop watch
Theory:
Weirs are the instruments, which used to measure the discharge in open channel flow. Triangular weir which is often called V-notch is used when the discharge is small. The theoretical discharge formula for the V-notch is,
Cd=coefficient of discharge (Qactual/Qₜₕₑₒ)
θ=Angle of V-notch,
H=Head over the weir.
For a particular weir Cd and θ are constant, the flow equation can be written as,
Q=KH5/2
For rectangular Weir,
Cd=coefficient of discharge (Qactual/Qₜₕₑₒ)
For a constant value of b, the flow equation can be written as,
Q=KH3/2
Experimental procedure:
1. Fit the required notch in the flow channel.
2. Close the gate valve at the channel and fill up the water in the channel up to sill level.
3. Check if water leaks from the notch.
4. Take down the initial reading of crest level (sill level) by the piezometer.
5. Then open the gate valve (By pass valve) slowly so that water starts flowing over the notch.
6. Let the water level become stable and note down the height of water surface at the upstream side by the sliding depth gauge.
7. Close the drain valve of measuring tank, direct the collector into the measuring tank and measure the discharge.
8. Take several readings for different flow rates.
Observations:
Angle of the V-notch (θ)= 45°,
Cross sectional area of the volumetric tank (A)= 0.3× 0.3= 0.09 m²,
Width of the rectangular weir (b)= 10.7cm.
Total six readings for each V-notch and Rectangular weir is shown below,
Required Graph to be plotted:
(i) ‘Q’ vs ‘H’ in log-log graph paper. [Both Qₜₕₑₒ and Qactual] [2× 3 cycle graph]
(ii) ‘Cd’ vs ‘H’ in Normal graph paper.
Figure- Q vs H in log-log graph.
Figure- Cd vs H in Normal graph.
FAQ:
1. Draw the schematic diagram of the setup.
2. Show sample calculation.
3. State assumptions taken during experiment.
a. Flow is steady,
b. Flow is laminar,
c. Velocity of approach is assumed small so that kinetic energy can be neglected.
4. Define velocity of approach. State its effect on the flow measurement.
Velocity of approach can be defined as the velocity with which the water reaches the weir or notch before it flows over it.
The velocity of approach is used to find an additional head (hₐ= V²/2g). Again the Q is calculated to get more accurate value.
Written by- SOUMIK.