058:080 Experimental Engineering
Lab 1a
Wind Tunnel Testing Principles & Drag Coefficients of Golf balls
Objectives
- To perform air flow measurement using the wind tunnel.
- To compare measured and theoretical velocities for various area changes along the flow path of a Venturi insert.
- To calculate and compare the drag coefficient of a dimpled and non-dimpled golf ball.
- To perform uncertainty analysis, including bias and precision errors for the pressure measurement and uncertainty of drag coefficients.
Equipment
Name / Model / S/NFlowtek Wind Tunnel
Venturi inserts
Pressure Transducers
Golf balls
NI DAQ Card / PCI-6024E
Computer
The wind tunnel flow measurement workstation consists of 8 pressure tubes connected to pressure transducers to measure pressure relative to atmospheric pressure, Flowtek wind tunnel with control panel to control air flow, and data acquisition system (DAQ NI PCI-6024E and LabView 2010 software). In this experiment, you will perform air flow measurement along path of Venturi inserts, and the dimpled and non-dimpled golf ball.
This lab was designed to be completed in 2 lab sessions. Data processing may take 1 or 2 more sessions. Data processing should be performed as data is taken, to guarantee good results.
Required reading
Read reference [1] for introduction and operation of the wind tunnel. For further reference see [2].
PRE-LAB QUESTIONS (10% of the total grade of the lab, 2% each)
a) For all the values in Table 1, calculate the velocity of air at each pressure tap given that A0 = 2.323x10-2 m2 and V0 = 6 m/s. A*V = A0*V0 = constant.
b) What is drag coefficient? How is it related to the resistance force of an advancing object?
c) What is Reynolds number?
d) How can you tell a sphere ball is experiencing turbulent flow?
e) How does the Flotek wind tunnel work to create air flow? What is the direction of the air flow?
Procedure
1. Write detailed explanation of how the different components of the experiment work. Be sure to include information about:
- How can you control the fan of the tunnel?
- What DAQ channel does each of the pressure taps connect to?
- Describe in detailed how to mount the rake with pressure tube.
2. A LabView program (lab1a.vi) will be used to control the measurement of this experiment using the DAQ. The LabView VI is available on the course website under the Lab 1a section of the Laboratories page. After opening the VI, you should be able to find the device PCI-6024E by clicking Measurement & Automation under tools in LabView and then expanding the Devices and Interfaces list.
Figure 1 in Appendix.A shows the front panel of the program. By pressing “Ctrl + e” you can switch between the front panel view and the block diagram view. Explore the program to understand its functionalities before conducting the experiment.
Note: The data saved in the specified file will be overwritten if the file name is not changed for each run of the program. Also, click the “stop” button on the front panel to stop the program instead of “abort execution” in the menu bar.
In the front panel, 0 to 7 denotes the corresponding channel measurement (i.e. 0 is channel 0 measurement, 1 is channel 1 measurement, etc.). The “Original measurement” (V0 to V7) provide the voltages measured by the transducers, with instant and mean values. “Pressure” (Pressure 0 to Pressure 7) show the pressure in Paschals (Pa) relative to atmospheric pressure. “Velocity” (Velocity 0 to Velocity 7) show the velocity in m/s. The “Motor RPM Controller” is adjustable and controls the percent of full-scale operation (% FSO) for the wind tunnel fan.
i. Copy the downloaded folder containing the LabView VIs a place of your preference such that you can play around with it without modifying the original. Use the copy for the entire experiment.
ii. Test the program. Change the user input options if necessary in the front panel, such as the file name and path where your data is to be saved.
iii. The “Measure” button takes a single point sample of all the pressure and velocity and records the data into the file that you specified in step 2-ii. The “Read_Counter” will tell you how many times the data is recorded. Note that the data will be recorded at the time when you release the mouse, i.e., click and hold the mouse will NOT take any data.
iv. Look over the wind tunnel control panel (of the wind tunnel station). The “Control Power” key turns on the power to the control panel. The “Fan Enable” button starts the tunnel fan, while the E-Stop button stops it. On the back of the control panel box, there is a switch that controls the method in which the motor RPMs are adjusted. If the switch is toggled up, the RPMs are adjusted using the Fan Speed dial on the panel. If the switch is toggled down, the RPMs are adjusted using the “Motor RPM Controller” on the front panel of LabView software. For the whole experiment, the RPMs will be adjusted using the computer.
You can control the RPM of the fan either by hit/sliding the controller, or increment/decrement or enter the exact percentage of the RPM (0 – 100) in the controller’s window. The controller of the fan is connected to channel ao0 of the DAQ.
v. Look over the pressure transducers in the back of the instrumentation box on the wind tunnel. A total of 8 pressure tubes can be traced to pressure transducers located behind the tunnel. These eight transducers are connected to the computer and correspond to the eight pressure and velocity measurement on the front panel of the program. One of the pressure transducers has a tube labeled “0” which connects at the place where the incoming air flow starts, thus will provide reference for other tap measurement for the experiment.
vi. Important: Make sure there are no loose objects within the tunnel upon start-up.
Turn on the wind tunnel by turning the key from Lock to Run. Do not press “Fan Enable” yet. Run the LabView program. Notice the indicator “Initializing” in the bottom left corner saying “Wait” with the red button. Wait until the button turns green and the indicator says “Ready” before continuing (it takes one minute). During this time, the program is initializing all the pressures to zero. This measurement initialization is very important. Be sure to perform this at the beginning of each run of the program. Once the indicator turns green, then press the “Fan Enable” button on the wind tunnel control panel. Notice the changes that take place on the screen. Slowly increase the RPMs by sliding up the Motor RPM slider in small increments and then releasing the mouse button (or by other ways to change the RPM in step 2-iv). Note: Do not make sudden and large increases in the RPMs.
vii. For each of RPM level, wait for the air flow to reach steady state (monitor it from readings of the front panel, such as the original, pressure, or velocity measurement.) Record several data at the steady state of each RPM (e.g. 10%, 20%, 30% FSO) by hitting and releasing the “Measure” button. Notice the change in the indicator “Read_Counter”.
viii. Set the RPM back to 0 (without sudden changes) and turn off the wind tunnel by pressing the E-Stop button on the wind tunnel control panel. The E-Stop button will always shut off the motor.
ix. Stop the LabView program. Open the saved file and check the pressures and velocities. Since tap 0 is the only tap connected to the wind tunnel, readings from tap 0 should give you the correct measured information.
Now you should be able to perform the air flow measurement.
Calibration of pressure transducers
3. Turn on the wind tunnel and run the LabView program. Perform the measurement initialization as in step 2-vi. Wait until the indicator turns green and says “Ready”.
4. Detach pressure tap 0 from the wind tunnel, connect it to the laboratory standard pressure calibrator. Turn the calibrator on by pressing “kPa”.
5. Press the button on the side of the pump to release any pressurized air.
6. Turn the dial on the pump to create a vacuum within the pump. Adjust the pressure differential so the calibrator indicates some non-zero value (E.g. -0.5 kPa). There is no need to pump the hand pump.
7. Record the value of the corresponding pressure measurement in LabView. Denote this value as ΔPmeasured, and the value on the calibrator as ΔPstandard. They are measurements of the pressure difference between the static pressure and the ambient pressure from LabView program and the lab standard.
8. Calculate the percentage error of the measured pressure and lab standard using equation 1.
%= ΔPmeasured-ΔPstandardΔPstandard×100 / (1)9. Repeat steps 3 through 8 for the other seven transducers.
Air flow measurement along Venturi insert
10. In the wind tunnel, note the small holes on the topside of the Venturi insert. Underneath the bottom insert, attach pressure taps to some of the holes as shown in Figure 2. Note that tap 0 locates at the wind tunnel right before the Venturi inserts and serves as reference tap.
Figure 2 Pressure Tap Locations for Venturi Insert
11. Table 1 shows the wind tunnel cross-sectional area at the location of the pressure taps seen in Figure 2. Also given is the area of the cross section prior to the Venturi insert (Tap 0).
Table 1 Wind Tunnel Cross-Sectional Areas for Various Pressure Taps along Venturi Insert
Pressure Tap / Area [m2]0 / 2.323E-02
1 / 1.548 E-02
2 / 1.327 E-02
3 / 1.256 E-02
4 / 1.344 E-02
5 / 1.593 E-02
6 / 1.832 E-02
7 / 2.062 E-02
12. Start LabView program and perform measurement initialization.
13. Turn on the wind tunnel by pressing “Fan Enable” once the LabView says “Ready”. Slowly adjust the RPMs such that the velocity at Tap 0 reads Velocity 0 ~ 6 m/s. Note and record the motor speed in % FSO from the front panel of LabView program and in RPMs from the control panel, as well as Tap 0 velocity reading in your logbook.
14. Make 20 measurements of the velocities by clicking “Measure” 20 times.
15. Repeat steps 13 and 14 for Velocity 0 ~ 12 and 18 m/s. Remember to change your file name to be saved to avoid overwriting the data.
16. Set the Motor RPMs to zero (Do not make sudden change), stop the program, turn off the fan, and turn the key to the “Lock” position. This might be a good stopping point for day 1. However, make sure you perform your data processing of the Venturi insert part and obtain good results (See Analysis, step 27 to 30). Do not wait until the end of the experiment to do data analysis.
Drag coefficient of non-dimpled and dimpled golf ball
17. Measure the diameters of the non-dimpled and dimpled golf ball. Take 10 measurements for each ball so that the precision error can be found.
18. Measure the location of the pressure taps relative to the central tap. The pressure taps will be attached to the rake to measure pressure right after the golf ball which will then be used for deriving the drag coefficient. You may have to detach the rake with the screwdriver to measure the distances with the calipers. These values are radii, ri (i = 2~7), with the central tap (Tap1) being r1 = 0 mm. Note that Tap 0 is for measuring the incoming air flow and is not used for measurement with rake. When attaching the pressure taps, be aware about their locations. See Figure 3 below for further explanation.
Figure 3 Circular cross-section of the measure region (Each dot represents pressure tap location)
19. Reattach the rake if it was taken out.
20. Insert the non-dimpled golf ball, lining it up with the rake.
21. After making sure that everything is secure within the tunnel, close the viewing window. Start LabView program, perform measurement initialization and wait until the program says “Ready”.
22. Increase the Motor RPM such that Velocity 0 = 6 m/s. In your log book, record Velocity 0, the motor RPM with %, and the actual motor RPMs (the red numbers on the tunnel control panel).
23. Record data for 30 times.
24. Repeat steps 22 and 23 for Velocity 0 = 12 m/s and Velocity 0 = 18 m/s.
25. Reduce the Motor RPM to zero, stop LabView program and turn off the fan.
26. Repeat steps 20 through 25 for the dimpled golf ball.
Analysis
For air flow along Venturi inserts:
27. Calculate the average and the precision errors for all the pressure and velocity values. Make comment on accuracy of the pressure transducers.
28. Knowing that A*Vel = A0*Vel0 = constant, calculate the theoretical velocities at each pressure tap and each Velocity 0. Here, for each Velocity 0 case, use Vel0 to be the average velocity at Tap 0 and A0 to be 2.323x10-2 m2.
29. Plot pressure tap vs. velocity for all three Velocity 0. Discuss and compare your theoretical and measured velocities.
30. Discuss any findings of your Venturi inserts measurement, such as the distribution of velocities and pressure along the Venturi inserts.
For golf balls measurement:
31. Calculate the Reynolds number for each Velocity 0 and each ball:
Re=V0×dυ / (2)Where d is the ball diameter and n is the kinematic viscosity of air (1.557×10-5 m2/s at 25 °C).
32. Calculate the drag force, Fdrag, using equation 3:
Fdrag=(Pi-P1)×Ai / (3)Where Pi is the pressure reading of Tap i (i = 2 ~ 7), P1 is the pressure reading at Tap 1, Ai is area of annulus for the Tap i. See Figure 3 for reference. For instance, with the radii of ri (i = 1 ~ 7), A2=π(r22-r12); A3=π(r32-r22), etc. That is, Ai can be calculated with the difference of circular area of tap i and tap (i-1). Make sure you understand why equation 3 can be used to approximate drag force.