Wright State University
Department of Mechanical and Materials Engineering
Fall 2004
ME 317: FLUID DYNAMICS LABORATORY
Experiment 3: Linear Momentum Principle Demonstrator
Objective: The objective of this experiment is to compare the force measured when a jet
of water strikes a flat plate to the force calculated using the linear momentum equation.
References: Fox, McDonald and Pritchard, Introduction to Fluid Mechanics, 6th edn.,
Section 4-4: Momentum Equation for Inertial Control Volume, and Ex. Prob. 4.4.
Preparation: Read the reference material and these instructions, then perform the
experiment with your assigned group.
Equipment Needed: Flow bench, stopwatch, measuring tank
Experiment Procedure: The centrifugal pump forces water through the nozzle (d =
15/32  1/64 inch) at a flow rate which is controllable by two valves. As the water flows
out the nozzle, it has linear momentum in the y-direction. Therefore, as the water hits the
plate, it creates a force in the y-direction, which can be determined indirectly from the
spring scale. The flow of water can be fed into the measuring tank over a time period, t.
This will give the volumetric flow rate, which can be used to calculate the force acting on
the plate from the water jet using the linear momentum equation.
1. Measurements
a. Close all valves.
b. Turn the pump on.
c. Adjust one of the ball valves until the average spring scale reading is close to
2.0 Newtons. Use the red gate valve for fine adjustment. Estimate the
variation in the spring scale reading on the sheet provided.
d. When the spring scale reading is set correctly, turn the flow diverter valve so
that the water dumps into the measuring tank, and start the timer.
e. Record the time required for the water level in the measuring tank to reach the
black mark (V = 12 liters), and turn the flow diverter valve back to its original
position.
f. Pour the water from the measuring tank back into the main tank.
g. Record four trials at this setting.
h. Adjust the valves such that the spring scale reading is close to the next setting
on the sheet provided, and repeat steps (c) through (g) for each setting.
i. Turn the pump off.
2. Data Reduction
a. The force imposed by the water jet on the pivot arm should be calculated
using the free-body diagram shown in the figure below.
b. Create an Excel spreadsheet for use in calculating the experimental jet force
and the analytical jet force. Also use this spreadsheet to calculate the
experimental uncertainties.
c. Create a graph by plotting both the experimental jet force and the analytical
jet force on the y-axis versus the jet velocity on the x-axis. For the
experimental data, use a scattered plot with no lines, and also use error bars.
Experimental Uncertainty: Use the methods of Appendix F to estimate the
uncertainties in the calculated results for the experimental jet force. Account for the
following uncertainties: Time for the water level to reach the black mark, height of the
water level due to waves on the surface of the water, the nozzle diameter, the variation of
the spring scale reading during the experiment, and the lengths of the pivot arm.
Report: Prepare a brief report in the usual format. The above-mentioned Excel
spreadsheet and the graph are to be included. Answer the following questions:
1. Is the jet force predicted well by the analysis? Why or why not?
2. How repeatable is the data?
3. Briefly discuss the best way to redesign this experiment. What key points would
significantly improve the accuracy of the experimental data?
L1 = 6 ¼ ± 1/16 inch
L2 = 13 13/16 ± 1/16 inch
m = 200 grams
L2
L1
Pivot
Point
mg
Fjet
Spring
Scale
Wright State University
Department of Mechanical and Materials Engineering
Fall 2004
ME 317: FLUID DYNAMICS LABORATORY
Experiment 3: Linear Momentum Demonstrator Data Sheet
Trial Number Fspring scale (N)
1
2.0
2
2.0
3
2.0
4
2.0
1
3.0
2
3.0
3
3.0
4
3.0
1
4.0
2
4.0
3
4.0
4
4.0
1
5.0
2
5.0
3
5.0
4
5.0
Variation (N)
Time (s)