CIVE1400: Fluid Mechanics Impact of Jets Lab
1st Year Fluids - Impact of a Jet
Objective
To demonstrate the applicability of Newton’s law of motion to a fluid. To investigative the effects of a jet flow on differently
shaped targets, comparing theoretical predictions with actual measurements.
Theory
An example of the application of the momentum equation arises with the impact of jets and their subsequent deflection on
targets of various shapes. In the experiments in the laboratory a vertical water jet is aimed at a target. The vertical force
exerted on the target by the water is measured by placing weights on a weight pan until the force of the jet is matched.
By Newton's second law, momentum, force = rate of change of momentum:
(
)
2121
vvQmvmvMg
−
=
−
=
ρ
(where M is the mass on the weight pan, Q is the flow, g is acceleration due to gravity,
ρ
the density of the fluid and
v
1
and v
2
are the initial and final velocities respectively).
M
g
v
2
v
1
Horizontal, 90
°
Deflector: After hitting the target the jet is deflected at 90
0
and
no longer has any momentum in the y direction, hence the component of v
2
= 0.
Resolving in the y-direction:
()
121
90cos QvvvQMg
ρ
ρ
=−=
as Q = vA where A is the area of the jet,
Mg Q
A
=
ρ
2
M
g
v
2
v
1
120
°
Deflector: Resolving in the y-direction:
A
Q
A
Q
A
Q
QvvQMg
2
3
2
)120cos(
2
21
ρ
ρρ
=
−−=−=
180
0
Deflector: The water returns in the same direction it has come, so:
M
g
v
2
A
Q
A
Q
A
Q
QvvQMg
2
21
2
)180cos(
ρ
ρρ
=
−−=−=
v
1
Plotting graphs of M against Q
2
for these three equations will give straight lines with the following gradients:
Horizontal, 90
°
, target gradient =
ρ
gA
,
120
°
target gradient = 3
2
ρ
gA
Hemispherical, 180
°
target gradient = 2
ρ
gA
1
M
Q2
ρ
gA
1
M
Q2
3
ρ
2g
A
1
M
Q2
2
ρ
gA
Horizontal, 90
°
, Target 120
°
Target Hemispherical Target
1
