Semi Trailer Major Drag Device Testing

Testing:

Pitot Tube Accuracy:

To test the accuracy of the Pitot tube, one can compare the measured wind velocity to a calculated prediction.

The wind speed can be predicted based on the RPM of the blower, its capacity and the size of the tube.

The blower is attached to the motor with a pulley ratio of 1.568. The motor rotates at 1725 RPM, so the blower is moving at 1100 RPM.

1725 RPM / 1.568 = 1100 RPM

Given the capacity of the blower as found by the manufacturer and having measured the back pressure as 1 inch of water (dark blue dotted line), this equates to 950 cubic feet per minute (see optimum performance graph below). This is equal to 15.83 cubic feet per second, assuming the blower is operating as well as possible.

The blower's optimum performance (Click for larger version)

950 cubic feet per minute / 60 seconds per minute = 15.83 cubic feet per second

The inside of the diameter of the wind tunnel is 7.2 inches, giving the tube a cross-sectional area of 0.2826 square feet.

Velocity = air flow / cross sectional area

Velocity = 56.01 feet per second = 17.07 meters per second

To test the accuracy of the Pitot tube:

Materials used: constructed Pitot tube, electric drill, wind tunnel, paper, ruler, pen

Using the electric drill, drill a hole roughly halfway from the entrance of the wind tunnel to the door. The hole should be just large enough for the Pitot tube to be inserted.
Push the Pitot tube through the hole until it is at the center of the tube and turn on the blower.
Turn the Pitot tube so that its nozzle is perpendicular to the flow. Confirm that the water level corresponding to both the nozzle end and open end are identical.
Turn the Pitot tube to face into the flow. Record the difference in water height between tubes.
Using Bernoulli’s formula, calculate the flow velocity based on the measured height.
Compare the measured value to the prediction.

Results:

Measured Value:

Predicted Value: 17.07 m/s

The discrepancy between values is likely due to the blower not performing at its optimum level due to wear.The similarity of predicted and measured values indicates that the Pitot tube is functioning properly.

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Wind Tunnel Turbulent Flow Based on Reynolds Number

After calculating the windspeed based on the Pitot tube measurement, the blower capacity and the diameter of the tube, the Reynolds number of the wind tunnel can be calculated as such:

Reynolds numbers above 40,000 are generally considered to be indicative of turbulent flow. As a result, it can be assumed that the flow conditions in the tunnel are turbulent. In order to further prove this, the velocity profile of the tunnel can be tested.

In turbulent pipe flow, the velocity profile is fairly uniform over the diameter of the tube. The flow velocity at the center of the tube is generally higher, although ideally relatively close, to the velocity at the edges. Laminar flow, however, the flow velocity is much greater at the center of the pipe than at the edges. To test that the velocity profile is indicative of turbulent flow:

Materials used: wind tunnel, pitot tube, ruler, paper, pen

Turn on the blower and insert the pitot tube 1 cm into the tunnel.
Arrange the other end of the pitot tube so that it is exposed only to atmospheric pressure.
Turn the pitot tube so that its nozzle faces directly up, or perpendicular to the flow. Confirm that the height of water from each end of the tube is the same. This proves that the static pressure inside the tunnel is equivalent to atmospheric pressure.
Turn the pitot tube until it is parallel with the flow. Record the difference in the height of the water in each tube.
Repeat steps 2 through 5 with the pitot tube inserted 2, 3, 4, 5, 6, 7, 8 and 9 cm into the tunnel.
Calculate, using Bernoulli’s formula, the flow velocity at each section of the diameter.
Graph and compare the velocities. Any boundary layer of lesser velocity should be relatively small and any differences in velocity should be minimal.

Velocity Profile of Wind Tunnel (from one side). Click for larger version

Based on these results, the flow conditions in the tunnel (whose diameter is 18.3 cm) are turbulent.

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Force Measurer Spring Calibration

The spring used in the force measurer needs to be tested in order to determine the relationship between displacement and force applied. This can be done by hanging weights from the spring, measuring the displacement, converting the weights to forces, graphing the data, and determining the relationship.

Materials used: wire, electronic scale, bolts, spring, measuring calipers, support box, spring handle

Hold the spring handle on the support box and allow the spring to hang. Check that the spring is perpendicular to the ground.
Measure the spring’s length from top coil to bottom coil. Record the measurement
Place bolts on the wire until the combined weight of bolts and wire is roughly 50 g.
Hang the wire on the spring and measure its length from top coil to bottom coil. Record the weight and length measurement.
Repeat step 3 and 4, increasing weight by 50 g until a 300 g test has been done.
Subtract from each length measurement the original, contracted length of the spring to calculate the elongation for each weight.
Multiply the weight for each elongation by 9.81 m/s2 to obtain the force.
Graph the forces and elongations, draw a line through the points and determine the equation of the line.

Data for the Spring Calibration (Click for Larger Version)

The data shows that the spring’s elongation relative to force is not constant with very small forces (the first point, in pink on the graph). Beyond very small forces, however, the data yields a linear equation of

F = 0.084d + 0.7233

Where F is the force in Newtons and d is the elongation of the spring in millimeters.

The above equation has an r-squared value of 0.9999, indicating tremendous accuracy. In the use of the force measurer, the above equation will be accurate considering that no measurements will be taken with the spring subjected to a small enough force for it to deviate from the equation.

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Click here to move on to the Experiment Overview page.


Project Info References Acknowledgments Conclusion Sources of Error Further Research Overview Hypothesis Procedure Results Overview Background Objectives Hypothesis Construction/Details Testing Overview Background Plan	Testing: Pitot Tube Accuracy: To test the accuracy of the Pitot tube, one can compare the measured wind velocity to a calculated prediction. The wind speed can be predicted based on the RPM of the blower, its capacity and the size of the tube. The blower is attached to the motor with a pulley ratio of 1.568. The motor rotates at 1725 RPM, so the blower is moving at 1100 RPM. 1725 RPM / 1.568 = 1100 RPM Given the capacity of the blower as found by the manufacturer and having measured the back pressure as 1 inch of water (dark blue dotted line), this equates to 950 cubic feet per minute (see optimum performance graph below). This is equal to 15.83 cubic feet per second, assuming the blower is operating as well as possible. The blower's optimum performance (Click for larger version) 950 cubic feet per minute / 60 seconds per minute = 15.83 cubic feet per second The inside of the diameter of the wind tunnel is 7.2 inches, giving the tube a cross-sectional area of 0.2826 square feet. Velocity = air flow / cross sectional area Velocity = 56.01 feet per second = 17.07 meters per second To test the accuracy of the Pitot tube: Materials used: constructed Pitot tube, electric drill, wind tunnel, paper, ruler, pen Using the electric drill, drill a hole roughly halfway from the entrance of the wind tunnel to the door. The hole should be just large enough for the Pitot tube to be inserted. Push the Pitot tube through the hole until it is at the center of the tube and turn on the blower. Turn the Pitot tube so that its nozzle is perpendicular to the flow. Confirm that the water level corresponding to both the nozzle end and open end are identical. Turn the Pitot tube to face into the flow. Record the difference in water height between tubes. Using Bernoulli’s formula, calculate the flow velocity based on the measured height. Compare the measured value to the prediction. Results: Measured Value: Predicted Value: 17.07 m/s The discrepancy between values is likely due to the blower not performing at its optimum level due to wear.The similarity of predicted and measured values indicates that the Pitot tube is functioning properly. Return to Top Wind Tunnel Turbulent Flow Based on Reynolds Number After calculating the windspeed based on the Pitot tube measurement, the blower capacity and the diameter of the tube, the Reynolds number of the wind tunnel can be calculated as such: Reynolds numbers above 40,000 are generally considered to be indicative of turbulent flow. As a result, it can be assumed that the flow conditions in the tunnel are turbulent. In order to further prove this, the velocity profile of the tunnel can be tested. In turbulent pipe flow, the velocity profile is fairly uniform over the diameter of the tube. The flow velocity at the center of the tube is generally higher, although ideally relatively close, to the velocity at the edges. Laminar flow, however, the flow velocity is much greater at the center of the pipe than at the edges. To test that the velocity profile is indicative of turbulent flow: Materials used: wind tunnel, pitot tube, ruler, paper, pen Turn on the blower and insert the pitot tube 1 cm into the tunnel. Arrange the other end of the pitot tube so that it is exposed only to atmospheric pressure. Turn the pitot tube so that its nozzle faces directly up, or perpendicular to the flow. Confirm that the height of water from each end of the tube is the same. This proves that the static pressure inside the tunnel is equivalent to atmospheric pressure. Turn the pitot tube until it is parallel with the flow. Record the difference in the height of the water in each tube. Repeat steps 2 through 5 with the pitot tube inserted 2, 3, 4, 5, 6, 7, 8 and 9 cm into the tunnel. Calculate, using Bernoulli’s formula, the flow velocity at each section of the diameter. Graph and compare the velocities. Any boundary layer of lesser velocity should be relatively small and any differences in velocity should be minimal. Velocity Profile of Wind Tunnel (from one side). Click for larger version Based on these results, the flow conditions in the tunnel (whose diameter is 18.3 cm) are turbulent. Return to Top Force Measurer Spring Calibration The spring used in the force measurer needs to be tested in order to determine the relationship between displacement and force applied. This can be done by hanging weights from the spring, measuring the displacement, converting the weights to forces, graphing the data, and determining the relationship. Materials used: wire, electronic scale, bolts, spring, measuring calipers, support box, spring handle Hold the spring handle on the support box and allow the spring to hang. Check that the spring is perpendicular to the ground. Measure the spring’s length from top coil to bottom coil. Record the measurement Place bolts on the wire until the combined weight of bolts and wire is roughly 50 g. Hang the wire on the spring and measure its length from top coil to bottom coil. Record the weight and length measurement. Repeat step 3 and 4, increasing weight by 50 g until a 300 g test has been done. Subtract from each length measurement the original, contracted length of the spring to calculate the elongation for each weight. Multiply the weight for each elongation by 9.81 m/s2 to obtain the force. Graph the forces and elongations, draw a line through the points and determine the equation of the line. Data for the Spring Calibration (Click for Larger Version) The data shows that the spring’s elongation relative to force is not constant with very small forces (the first point, in pink on the graph). Beyond very small forces, however, the data yields a linear equation of F = 0.084d + 0.7233 Where F is the force in Newtons and d is the elongation of the spring in millimeters. The above equation has an r-squared value of 0.9999, indicating tremendous accuracy. In the use of the force measurer, the above equation will be accurate considering that no measurements will be taken with the spring subjected to a small enough force for it to deviate from the equation. Return to Top Click here to move on to the Experiment Overview page.