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

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

Experiment (3): Impact of jet

Introduction:

Impact of jets apparatus enables experiments to be carried out on the reaction force produced on

vanes when a jet of water impacts on to the vane. The study of these reaction forces is an essential

step in the subject of mechanics of fluids which can be applied to hydraulic machinery such as the

Pelton wheel and the impulse turbine.

Purpose:

To investigate the reaction force produced by the impact of a jet of water on to various target vanes.

Apparatus:

1. Impact of jet apparatus (Figure 1).

2. Hydraulic bench.

Figure 1: Impact of jet apparatus

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

Instructors : Dr. Khalil M. Alastal Eng. Mohammed Y. Mousa 2

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.

Figure 2: Illustrative figure of impact of jet apparatus

Theory:

When a jet of water flowing with a steady velocity strikes a solid surface the water is deflected to

flow along the surface. If friction is neglected by assuming an inviscid fluid and it is also assumed

that there are no losses due to shocks then the magnitude of the water velocity is unchanged. The

pressure exerted by the water on the solid surface will everywhere be at right angles to the surface.

Consider a jet of water which impacts on to a target surface causing the direction of the jet to be

changed through an angle as shown in figure 2 below. In the absence of friction the magnitude of

the velocity across the surface is equal to the incident velocity Vi. The impulse force exerted on the

target will be equal and opposite to the force which acts on the water to impart the change in

direction.

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

Instructors : Dr. Khalil M. Alastal Eng. Mohammed Y. Mousa 3

Applying Newton’s second law in the direction of the incident jet

This is the resultant force acting on the fluid in the direction of motion.

This force is made up of three components:

Force exerted in the given direction on the fluid by any solid body touching the

control volume.

Force exerted in the given direction on the fluid by body force (e.g. gravity).

Force exerted in the given direction on the fluid by fluid pressure outside the control

volume.

By Newton’s third law, the fluid will exert an equal and opposite reaction on its surroundings.

The force exerted by the fluid on the solid body touching the control volume is equal and opposite

to FR . So the reaction force R is given by:

Figure 3: Impact of a jet

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

Instructors : Dr. Khalil M. Alastal Eng. Mohammed Y. Mousa 4

Application to impact of jet apparatus

In each case it is assumed that there is no splashing or rebound of the water from the surface so

that the exist angle is parallel to the exit angle of the target.

The jet velocity can be calculated from the measured flow rate and the nozzle exit area.

However, as the nozzle is below the target the impact velocity will be less than the nozzle velocity

due to interchanges between potential energy and kinetic energy so that :

where is the height of target above the nozzle exit.

1. Impact on normal plane target

For the normal plane target = 90°

Therefore

2. Impact on conical target

The cone semi-angle = 45°

Therefore

3. Impact on semi-spherical target

The target exit angle = 135°

Therefore

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

Instructors : Dr. Khalil M. Alastal Eng. Mohammed Y. Mousa 5

Figure 4: Interchangeable target vanes

Procedures:

1. Position the weight carrier on the weight platform and add weights until the top of the target is

clear of the stop and the weight platform is floating in mid position. Move the pointer so that it

is aligned with the weight platform. Record the value of weights on the weight carrier.

2. Start the pump and establish the water flow by steadily opening the bench regulating valve

until it is fully open.

3. The vane will now be deflected by the impact of the jet. Place additional weights onto the

weight carrier until the weight platform is again floating in mid position. Measure the flow rate

and record the result on the test sheet, together with the corresponding value of weight on the

tray. Observe the form of the deflected jet and note its shape.

4. Reduce the weight on the weight carrier in steps and maintain balance of the weight platform

by regulating the flow rate in about three steps, each time recording the value of the flow rate

and weights on the weight carrier.

5. Close the control valve and switch off the pump. Allow the apparatus to drain.

6. Replace the 5mm nozzle with the 8mm diameter nozzle and repeat the tests.

7. Replace the normal vane with the 45° conical vane and repeat the test with both the 5mm and

8mm nozzles.

8. Replace the 45° conical vane with the hemispherical vane and repeat the tests with both the

5mm and 8mm nozzles.

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

Instructors : Dr. Khalil M. Alastal Eng. Mohammed Y. Mousa 6

Results:

1. Record the results on a copy of the results sheet provided.

2. Calculate for each result the flow rate and the nozzle exit velocity. Correct the nozzle velocity

for the height of the target above the nozzle to obtain the impact velocity.

3. Calculate the impact momentum and plot graphs of impact force against impact momentum

and determine the slope of the graphs for each target. Compare with the theoretical

values of 1, 0.2929 and 1.7071 for the normal plane target, conical target and hemispherical

target respectively.

Target

Vanes

(degrees)

Nozzle

Dia.

(mm)

Height of target

above nozzle

(mm)

Additional Weights

(g)

Volume of water

collected

(Liter)

Time

(sec)

°

5

30

90

30

=

30

Fla

t

8

30

30

30

°

5

25

45

25

=

25

Co

nic

al

8

25

25

25

°

5

30

13

5

30

=

30

Sem

i-

sph

eric

al

8

30

30

30

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

Instructors : Dr. Khalil M. Alastal Eng. Mohammed Y. Mousa 7

Target

Vanes

(degrees)

Nozzle

Dia.

(mm)

(m3/s) (m/s) (m/s) (N) 𝝆 (N) Slope

°

5

1

90

=

Fla

t

8

°

5

0.2929

45

=

Co

nic

al

8

°

5

1.7071

13

5

=

Sem

i-

sph

eric

al

8