Impact of water jet
The purpose of this experiment is to demonstrate the application of the momentum equation. The force generated by a jet of water deflected by an impact surface is measured and compared to the momentum change of the jet.
• Direct measurement of impact forces
• Effect of flow rate and velocity
• Effect of various deflection angles
Introduction / Theory
Newton’s Second law states that
The applied forces is equal to the rate of change of momentum
Newton’s Second Law of Motion states that
The Applied Force is equal to the Rate of Change of Momentum.
[pic] if we assume V2 = V1 = Vy1
Where [pic] is the density of water
is the inlet jet velocity in the y direction
is the outlet jet velocity in the y direction
is the volumetric flow rate
is the angle of deflection of jet flow measured from the veritical
The negative sign means the Applied Force is opposite to the direction of the inlet jet stream.
Nozzle impact distance
Bernoulli equation can be used when fluid is considered as an inviscid, incompressible and steady flow.
|[pic] | (1.4) |
With these theory and Bernoulli equation, we can determine not only the force produce by the water jet, but also the velocity of the particle in the flow.
2.1 Momentum Equations
The general momentum equation is
|[pic] | (1.5) |
where [pic] = force, [pic] = velocity, [pic] = unit outward vector on CS, which is the surface of the control volume (CV). For steady flow with a constant density, with discrete inflows and outflows, with essentially constant velocities on each inflow and outflow cross section, and with the velocities normal to the flow areas, the momentum equation can be written as |[pic] | (1.6) |
Where Q = volumetric flow rate and[pic] = cross sectional average velocity vector with magnitude U. 2.2 Deflector Vanes
A jet flowing against a deflector vane is shown schematically in Fig. 5.1. A vertical jet with velocity U1 strikes the vane and leaves all around the vane with velocity U2. If the change in elevation is negligible between cross sections 1 and 2 and if the shear on the vane is negligible (as is the case for this experiment), then Bernoulli's equation can be used to show that U1 = U2. From continuity, Q1 = Q2. For the control volume shown in Fig. 5.1, assuming that the weight of the fluid in the control volume is negligible, the vertical (z) component of the momentum equation (Eq. 5.2) gives
|[pic] | (1.7) | | | |
Fig. 1.2 - Schematic diagram of vertical jet and axisymmetric deflector
Figure 1.3: Diagram of three deflectors with different angles: 60º, 130º and 180º.
Materials and Apparatus
This is the Apparatus used in this experiment to show the application of momentum in water jet experiment.
• Spring supporting lever
• Weighing tank
• Transparent cylinder
• Jockey weight
The top cover of the impact jet assembly was opened.
The flat deflector plate (angle [pic]=90 degrees) was installed.
References: 1. A brief Introduction to FLUID MECHANICS Young | Munson | Okiishi | Huebsch.
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