Experiment (5): Flow through small orificessite.iugaza.edu.ps/mymousa/files/Experiment-5.pdf · Experiment (5): Flow through small orifices Introduction: An Orifice is an opening

Hydraulics Lab (ECIV 3122) Islamic University – Gaza (IUG)

Instructors : Dr. Khalil M. Alastal Eng. Mohammed Y. Mousa 1

Experiment (5): Flow through small orifices

Introduction:

An Orifice is an opening in the side or base of tank or reservoir through which fluid is discharge in

the form of a jet. The discharge will depend up on the head of the fluid (H) above the level of the

orifice. The term small orifice means that the diameter of the orifice is small compared with the

head producing flow.

The analysis of the quantity of water which can be discharged through an orifice is arrived at in a

simple, straightforward manner by the application of Bernoulli's equation. However, experimental

tests typically produce a result which is only some 65% of the solution indicated by the simple

analysis. The study of water flow through an orifice is therefore a classic topic to illustrate the need

for a semi-empirical approach which is so often required in Mechanics of Fluids.

Exercise A: Flow through a small orifice

Purpose:

To investigate the discharge characteristics of circular orifices subjected to a constant head.

Apparatus:

1. Constant head inlet tank (Figure 1).

2. Circular orifices with different diameters.

3. Hydraulic bench.



Figure 1: Constant head inlet tank with circular orifice

Equipment set up:

Set up the apparatus on top of the hydraulics bench with the left hand support feet of the impact of

jet apparatus located on the two left hand locating pegs of the hydraulics bench so that the

apparatus straddles the weir channel. Connect the feed tube from the hydraulics bench to the boss

on the rear of the base of the impact of jet apparatus. Fit the 5mm nozzle and the normal flat target.

1. If the hook gauge and scale are to be used to measure the trajectory of horizontal jets then place

the two positioning rails on the worktop of the hydraulics bench engaging them onto the

locating pegs. Ensure that the engraved rail is placed closest to the front of the hydraulics bench

with the engraved side uppermost.

2. Position the constant head inlet tank onto the worktop of the hydraulics bench (over the hook

gauge positioning rails, if fitted) at the left hand side engaging two of the feet of the inlet tank

onto the locating pegs. If the orifice is to be fitted into the side of the inlet tank then it should be

moved to the left so that the right hand support feet engage with the locating pegs.

3. Remove the hexagonal (37mm across flats) bush and adaptor from the side of the inlet tank. Fit

the required orifice into the screwed hole in the side and plug the unusued hole using the

blanking plug provided.



4. Connect the hydraulics bench flexible delivery tube to the connection provided on the rear of

the inlet tank base. Insert the flexible overflow take off pipe, which is connected to the boss on

the front of the inlet tank, into the overflow pipe of the volumetric measuring tank.

5. Remove or refit the overflow extention tube (screwed) in the inlet head tank to obtain a

nominal head of 250mm or 500mm above the side orifice.

A. Setting the overflow: Switch on the pump and control the flow rate by either adjusting the

hydraulics bench delivery valve or by adjusting the pump speed. The flow should be adjusted

carefully to produce a small but constant overflow and then fine adjusted to give 250 or 500mm

head as required.

B. Flow measurement: The discharge from the orifice may be measured using the volumetric

measuring tank and taking the time required to collect a quantity of water. The quantity should

be chosen so that the time to collect the quantity is at least 120 seconds to obtain a sufficiently

accurate result.

C. Measurement of jet trajectory: Use the hook gauge to measure the trajectory of the jet.

D. Measurement of head: The scale attached to the side of the inlet tank has its zero level with the

centre line of the side outlet boss.

Theory:

Consider a small orifice in the side of a vessel with the head of water above the orifice kept

constant.

Figure 2: Discharge through an orifice

Applying Bernoulli's theorem between the surface of the water 1 and the orifice O yields



However = atmospheric pressure

hence substituting these into Bernoulli's equation gives

In other words, the theoretical velocity of the water passing through the orifice is given by

and hence the quantity of water being discharged through the orifice is given by

However in practice the discharge is always less than this theoretical amount due to the viscosity of

the fluid, to surface tension and due to resistance of the air. The disparity between the theoretical

discharge velocity and the actual discharge velocity is allowed for by introducing a factor

known as the coefficien of velocity so that

If the discharge from a sharp edged orifice is examined closely it will be observed that the minimum

diameter of the jet of water discharging from the orifice is smaller than the orifice diameter. The

plane at which this occurs is known as the vena contracta, which is the plane where stream lines

first become parallel. Applying the discharge equation at the vena contracta

which can be written as

Where: .

or more simply as

Where: .

Typical values of Cd range from 0·6 to 0·65, i.e. the actual flow through a sharp edged orifice is

approximately 60% of the theoretical value. The value of the coefficient of discharge may be



determined by measuring the quantity of water discharged over a period of time whilst the head is

maintained at a constant level.

Procedures:

1. Fit the 5mm diameter orifice into the side of the inlet head tank. Remove the overflow

extension pipe. Start the pump and set up an inlet head of 25cm. Measure the flow rate using

the volumetric measuring tank.

2. Replace the overflow extension pipe and set up an inlet head of 50cm. Measure the flow rate.

3. Repeat the procedure using the 8mm orifice.

Results:

1. Record the results on a copy of the result sheet for discharge characteristics.

2. For each result calculate the flowrate

3. Plot a graph of square root of the head against the flow rate for each orifice diameter, the

results should lie on a straight line passing through the origin to confirm that:

Measure the slope of each graph and calculate the coefficient of discharge for each orifice from

D (mm) 5 8

H (cm) 50 25 50 25

(m)

V (L)

T (sec)

(m3/s)



Exercise B: Trajectory of horizontal jet

Purpose:

To investigate the trajectory of a horizontal jet issuing from an orifice and hence determine the

coefficient of velocity for the orifice.

Apparatus:

1. Constant head inlet tank (Figure 1).

2. Circular orifices with different diameters.

3. Hook gauge and scale.

4. Hydraulic bench.

Theory:

Consider the trajectory of a jet formed by the discharge of water through an orifice mounted in the

side of a tank. The jet will be subjected to a downward acceleration of g due to gravity.

Figure 3: Trajectory of horizontal jet

Taking the origin of co-ordinates at the vena-contracta and applying the laws of motion in the

horizontal and vertical planes then ignoring any effect of air resistance on the jet.

In the horizontal direction

In the vertical direction

Solving simultaneously by eliminating t



Procedures:

1. Fit the 5mm diameter orifice into the side of the inlet head tank. Remove the overflow

extension pipe. Start the pump and set up an inlet head of 25cm.

2. Measure the trajectory of the jet using the hook gauge. Record the horizontal and vertical

distances.

3. Replace the overflow extension tube and establish an inlet head of 500mm. Measure the

trajectory of the jet

4. Repeat the experiment using the 8mm diameter orifice.

Results:

1. Draw a graph of y against x to represent the trajectory.

2. Draw a graph of against x and draw the best straight line through the points to represent

the results. Measure the slope of the line and hence calculate the coefficient of velocity from:



D (mm) 5 8

H (cm) 25 50 25 50

x (cm) Vertical distance below orifice center line y (cm)

0

5

10

15

20

25

30

35

40

45

50

Slope of graph

Documents

Experiment (5): Flow through small orificessite.iugaza.edu.ps/mymousa/files/Experiment-5.pdf · Experiment (5): Flow through small orifices Introduction: An Orifice is an opening