Calibration of Rectangular Notch(Fluid Mechanics)

 

Experiment 

Calibration of Rectangular Notch

Aim:To calibrate the Rectangular notch and find out the coefficient of discharge.

1. Theory: The notch is hydraulically defined as an opening provided in the side of a tank in such a way so that the top level of the liquid is below the top edge of the opening. Notches are generally used for measuring the flow of liquid in the channels.

In the unit, a centrifugal pump sucks the water from the sump tank and discharges it into a small flow channel. The notch is fitted at one end of the channel. The water flowing over the notch falls into the collector. The water from the collector is then goes to the measuring tank for the measurement of actual flow.

For a Rectangular Notch, The theoretical discharge is given by

Q_th= (2/3) x √2g xB x H^3/2

Where, H= Head over the notch=(Water level – Sill level) in meter.

      B= Crest width of the notch.

For the given notch, B= 0.20 m

Also, it is given that

Length of the measuring tank (L)= 0.80 m

Width of the measuring tank (B)= 0.60 m

 

3. Apparatus: Rectangular Notch, adjustable channel, sliding depth gauge, measuring tank, stop watch .
4. Procedure:

 1. Fit the required notch in the flow channel

 2. Fill up the water in the sump tank.

 3. Close the water supply gate valve to the channel and fill up the water up to sill              level in the channel.

 4. Check that the water does not leak from the sides of the notch.

 5. Take down the initial reading of the crest level (sill level) by the sliding depth gauge.

6. Now start the valve and open the gate valve slowly so that water starts flowing over the notch.

7. Let the water level become stable and then note down the height of water surface at the upstream side by sliding depth gauge.

8. Close the drain valve of measuring tank, direct the collector into the measuring tank and measure the discharge by measuring the rise of water in the indicator scale attached to the tank in some time ‘t’ measured with a stop watch.

9. Repeat the process and find out various discharges.

 

5. Observation Table:

SR No	Sill Level Height (m)	Water height on upstream side (m)	Height of water above sill (H) in m	Rise in water level in the measuring tank(h) in m	Time taken for rise ‘h’ in the tank(t)in seconds

 

 

 

6. Calculation: 1. , H= Head over the notch=(Water level – Sill level) in meter.

         2. Actual discharge Q_a=0.6x 0.8x h/t m³/sec

         3.Theoretical discharge Q_th= (2/3) x √2g xB x H^3/2, here B= 0.20 m.

         4. Co-efficient of discharge C_d= Q_a/ Q_th

         5. Calculate the average value of C_d for variable flows.

 

 

7.Precaution: