1. Government Engineering College,Bhavnagar
B.E Civil Engineering (Sem-3) Subject: Fluid Mechanics
Topic: Flow Measuring Devices: Orifice
Name Enrollment No.
Chaudhari Tejash D
Bhanshi Manilal S
Gayakwad Pravin S
Kurkutiya Naresh N
Raut Nitin R

2. Contents Introduction Classification of orifice
Flow Through an orifice
Hydraulic coefficient
Experimental determination of hydraulic coefficient
Determination of coefficient of velocity
Determination of coefficient of contraction

3. Introduction
An orifice is a geometric opening having a closed permiter, made in the walls or the bottom of a vessel containing fluid through which the fluid may be discharged.
The opening is regarded as an orifice only if the top edge of the opening lies below the liquid surface in the tank.

4. Classification of orifice
Geometry of orifices
Bell mouthed orifice

5. Orifice can be classified base upon:
(a) Shape – circular, rectangular, square and triangular.
(b) Size and head of liquid from centre - Small if h > d
- Big if h ≤ d
(c) Shape of upstream edge - Sharp edged orifice
- Bell mouth orifice
(d) Discharge conditions - Free discharging orifice
- Drowned or submerged orifice

6. Flow Through an orifice
Consider a tank with a sharp-edged circular orifice which discharges liguid from the tank directly in to the tank directly in to the atmosphere as shown in fig.
Tank with a orifice

7. This section of minimum cross- section area is called venacontracta.
Applying Bernoulli’s equation at point 1 and 2 and neglecting losses we get,
Now, (atmospheric pressure)

8. is very small in comparison to as the area of tank is very large compare to the area of the jet of liquid . So can be assumed as 0. This is theoretical velocity . Actual velocity will be less than these value (due to losses). Equation (1) for theoretical velocity is known as Torricellis equation ( = area of orifice ) (Theoretical discharge)

9. Hydraulic coefficient
1. Coefficient of velocity ( ): 2. Coefficient of contraction ( ):

10. 3. Coefficient of discharge ( ):

11. Discharge through orifice is, Q = Area of jet Velocity of jet Q =

12. Experimental determination of hydraulic coefficient
Determination of coefficient of discharge( )
The water is allowed to flow through an orifice fitted to a tank under a constant head H as shown in fig.
The water is collected in a measuring tank for a known time (t).
The height of water in the measuring tank is noted down.

14. Determination of coefficient of velocity ( )
First method:
Consider a tank provided with a small orifice on one end of it's side as shown in figure. Let,
x = horizontal distance travelled by the particle in time ꞌtꞌ .
y = vertical distance between P and C-C.
V = Actual velocity of jet at vena-contracta.

15. Then horizontal distance
x = V × t …(1)
and vertical distance …(2)
from equation
Substituting the value of t in equation 2 we get

16. Second method (momentum method) :
This method there is a tank with a triangular beam provided with an orifice.
The tank is supported so that the triangular beam rests on knife edge supports.
A horizontal lever is fixed to the tank wall opposite to the wall containing the orifice.
When tank contains water and the orifice is closed, the tank is levelled by placing or removing weights on the lover.
When the orifice is open and the water is discharged in a jet, the liquid will exert a horizontal force P on the wall of tank. By placing additional weight W on the lever the tank is balanced.

17. From momentum equation
Where, Q = discharge through the orifice
V = velocity of the jet at vena contracta
γ = specific weight of water
Let , H = head of water over the orifice
y = Vertical height of the knife edge support
l = horizontal distance between the additional load W and the knife edge support.

18. For condition of equilibrium.

19. Determination of coefficient of contraction :
First method :
Coefficient of discharge and coefficient of the orifice are first determined. Now the coefficient of contraction can be found out by the equation.
Second method :
In this method the area of the jet at vena contracta is measured by contraction gauge.
This instrument consists of a ring provided with four radial screw gauges, equally spaced.
The ring is held at the vena contracta section so that the jet can pass through its centre.

20. The screws of the screw gauges are now adjusted so that their sharp points just touch the surface of the jet.
Now the instrument is removed and the spacing between the opposite screw point are measured accurately.
In actual practice this method is not found to be satisfactory, due to following reason.
The section of the jet is not absolutely circular.
It is practically impossible to adjust all the four screw point in content with the jet simultaneously.

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