Cover Page for Lab Report Group Portion Drag on Spheres Prepared by Professor J. M. Cimbala, Penn State University Latest revision: 29 September 2017 Name 1: Name 2: Name 3: [Name 4: ] Date: Section number: ME 325. Group # Score (For instructor or TA use only): Lab experiment and results, plots, tables, etc. - Procedure portion Discussion Neatness & grammar TOTAL / 50 / 10 / 10 / 70 Comments (For instructor or TA use only):
Procedure and Presentations of Results A. Calibration of the electronic pressure transducer The two most common devices for measuring small differences in pressure are liquid manometers and electronic pressure transducers. When a pitot-static probe is used to measure flow velocity, either of these devices can be used. The water manometer used in this experiment has an inclined portion which permits accurate positioning of the meniscus. When used properly, the inclined micromanometer in the Fluids Lab has an accuracy of ±0.001 inches of water. The water in the manometer is actually a colored mixture which has a specific gravity of 1.000, i.e. its density is 1000. kg/m 3. The electronic pressure transducer is a device that provides a voltage output directly proportional to the pressure difference. For velocity measurements with a pitot-static probe, the electronic transducer is preferred over a liquid manometer for several reasons. First, its response time is much faster. Second, its output can be read directly from a digital voltmeter display. Measurements are much easier because no adjustments are necessary once the unit is calibrated. Finally, since the output is a voltage, it can easily be connected to a computer-controlled data acquisition system. In this lab experiment, an electronic pressure transducer is used for all flow measurements. However, it is first necessary to calibrate the transducer. The Validyne transducer used in this experiment can be adjusted to any desired ratio of voltage output to inches of water. Normally, the unit is adjusted so that 1.0 Volts on the display corresponds to 1.0 inches of water column pressure. However, the pressure difference encountered by the pitot-static probe in our wind tunnel can exceed 5.0 inches of water column; meanwhile, the computerized data acquisition system is set up for inputs in the range of -5.0 to 5.0 Volts. (Voltages above or below these limits are clipped.) Therefore, in order to avoid clipping by the data acquisition system, we will calibrate the transducer such that 1.0 Volts corresponds to a pressure of 4.0 inches of water column. Use the following procedure to calibrate the electronic pressure transducer: 1. First, the manometer must be leveled. Inspect the two level bubbles on the base of the instrument. If the bubbles are not exactly within the hairline marks, use the leveling screws on the base to level the instrument. 2. Next, the manometer itself must be zeroed. With both sides of the manometer open (atmospheric pressure on both sides, meaning zero pressure difference, crank the large knob on the right side of the instrument until the ruler pointer on the manometer is at exactly zero, as illustrated in Figure 2. Do NOT raise the pointer above the zero mark (doing so will bend or break the pointer.) While viewing the Meniscus Zero mark Ruler (inches) Inclined manometer tube round fine-adjustment scale on the bottom of the manometer, rotate the crank slowly to ensure that the pointer is at exactly zero (the fine adjustment scale should also read zero.) Now, using the large wheel-knob on the back side of the manometer, adjust the water level so that it intersects the zero mark as also sketched in Figure 2. Note that there is a very slow time response for the system. Wait a minute or so for everything to stabilize. Ask for assistance from the instructor or TA if necessary. 2. Crank down the water manometer so that the pointer is at exactly 4.00 inches. Again use the round fineadjustment scale near the bottom of the manometer for greatest accuracy. Pointer Figure 2. Manometer at zero inches of head. 3. Connect the hand pump to the manometer and to the pressure transducer as sketched in Figure 3. To avoid leaks, make sure that the black rubber gaskets are in place in the hose connectors. 4. Make sure the wind tunnel is off, and the pressure release on the hand pump is open (pushed in). Under these conditions there is zero pressure difference across the manometer and across the transducer head (P = P atm at both ports). 5. With the Validyne transducer switched to the Lo range, adjust the Zero adjust potentiometer to achieve a zero reading. 6. Close (release) the pressure release valve on the hand pump, and pump slowly until the liquid level is again exactly at the zero mark on the inclined portion of the water manometer. The opposite side of the hand pump contains a knob which can be turned either clockwise or counterclockwise for fine adjustment. Sometimes the small black washer on the blue quick connect falls out and causes a leak. Check for leaks. At this point, there is exactly 4.00 inches of water pressure (head) on the high (+) side of the electronic pressure transducer head, with the low (-) side open to atmospheric pressure. 7. With the Validyne transducer still set on the Lo range, adjust the Span adjust potentiometer to achieve a reading of 1.00 volts. The electronic pressure transducer is now calibrated to read exactly 1.00 volts per 4.00 inches of differential water pressure. 0 1
8. Record the potentiometer readings on the Validyne display unit: Zero: Span: Tubing High P atm Low Tee Meriam inclined manometer Pressure transducer P atm Low - + High Quick connector Cable 0 0 0 Hi Zero Lo Span Pressure transducer display Hand pump Figure 3. Schematic diagram of how to calibrate the pressure transducer using the inclined manometer. 9. Replace the tee connection on the high (+) side of the transducer head with the total pressure line from the pitotstatic tube. Also, make the connection between the low (-) side of the transducer and the static pressure line from the pitot tube. Measuring the difference between wind tunnel total and static pressures will enable you to monitor the wind tunnel speed. 10. Slowly crank the manometer back up to the zero level so that it is ready for the next group. The water manometer is no longer needed. Carefully return the hand pump to its resting place so that it does not get knocked off. B. Calibration of the drag balance The drag strain gage is not functioning. As such, the strain gage mount has been rotated 90 degrees such that the lift strain gage is oriented in the direction of flow such that it can measure the drag force. Use the lift stain gage setup and calibration created in the Lift on a Wing lab throughout this lab. C. Operation of the Wind Tunnel and the Pitot-Static Probe In this section, you will familiarize yourself with the operation of the wind tunnel, and will practice calculating the wind tunnel velocity from the differential pressure reading of the Pitot-static probe. The following procedure should be performed without a model mounted to the lift balance. Remove any spheres or other models that may be in the wind tunnel. Also be sure that no loose tools or other objects are in the test section. Close and secure the wind tunnel test section door. Before proceeding, make sure the pressure transducer has been calibrated to 1.0 volt per 4.0 inches of water, as described in the previous section. Adjust the zero potentiometer on the Validyne unit, if necessary, since the zero sometimes drifts with temperature.
1. Record the ambient (barometric) pressure from the gauge which is mounted on the wall of the wind tunnel. It is often necessary to lightly tap the casing of the barometer to get a good reading. Be sure to use the proper scale, which is cm of mercury. (Typically in State College, the barometric pressure is around 72 cm of mercury.) P atm = cm of mercury. 2. Record ambient temperature from the glass thermometer located under the barometer. Note: Temperature is also measured by a thermocouple, and is recorded by the digital data acquisition system see Channel 2 of the Techkor unit. To read the temperature, divide the number on the display by 10 this is T in o C (1 volt = 10 o C). Write down both temperatures below they should be close. T ambient, glass thermometer = C T ambient, thermocouple = C. (2) 3. Using the ideal gas law, and the thermocouple temperature, calculate the air density. Show your calculations below. Note: One centimeter of mercury is equivalent to 1333.23 N/m 2 of pressure. The gas constant, R, for air is 287. m 2 /(s 2 K). = kg/m 3. (2) 4. Calculate or look up the coefficient of viscosity,, of the air at the temperature indicated by the thermocouple. = N s/m 2. 5. The speed of the wind tunnel is controlled by a Toshiba Tosvert 130H1 Transistor Inverter. First make sure the remote control unit (mounted to the side of the wind tunnel) is turned to Stop. Turn on the main circuit for the 480 Volt power supply (located on the wall just to the left of the sink). Push in the Start button on the small electric box just below the Toshiba wall-mounted control box. The display should read OFF. From here on, you will use the dial on the remote control to control the wind tunnel speed. (The remote control is located on the wind tunnel below the test section) (5) 6. Flip the bottom switch of the remote control to Manual, set the dial setting to a reading of around 200, and turn on the wind tunnel by rotating the upper switch to Start. Slowly increase the wind tunnel speed, using the dial. As the fan frequency increases, so should the Pitot-static pressure. Make five manual calculations of wind tunnel speed, using Equations (4) and (5), and show your calculations below. The last case should be at maximum wind tunnel speed. Be careful to use the proper SI units, and remember that the Validyne display represents 4.00 inches of water column for every 1.00 volt. Motor frequency (Hz) Validyne display reading (Volts) Pitot-static pressure difference (inches of water column) Velocity (m/s) 7. The maximum wind tunnel speed is m/s. Note: The maximum wind tunnel speed should be in the range between 40 and 60 m/s. If it is not, check your calculations and electronic and pressure line connections. (Often the problem is a leak in one of the blue quick connects or in the pitot-static probe connectors) If the problem is not found, consult your instructor or TA for assistance. 8. Turn the remote control switch to Stop to turn off the wind tunnel. Leave the wall-mounted Toshiba inverter unit on, however, until you are finished with the lab.
D. Drag Measurement Spheres With the aid of the computerized data acquisition system, you will now measure the drag on various spheres, with and without surface roughness. 1. If not already in place, install the sting mount such that it protrudes horizontally from the top of the drag strut, as sketched in Figure 4. Consult your instructor or TA if necessary. 2. Install the smooth golf ball onto the sting mount. With the wind tunnel off, re-zero both the lift balance and the pressure transducer if necessary. 3. Start program ME325-Lift Measure from the Windows desktop. This program acquires the thermocouple temperature, the voltage from the lift balance, and the voltage from the pressure transducer, from which it calculates wind tunnel velocity, Reynolds number, drag force, and drag coefficient. Click on Enter Parameters, and enter the barometric pressure, along with a length scale for calculation of Reynolds number, and an area for calculation of drag coefficient. For the length scale, use the diameter of the sphere, which is given in the introductory portion of this lab (and also posted on the wind tunnel wall for your convenience). For the area, use the projected frontal area in square inches (also posted on the wind tunnel wall). Finally, enter the pressure transducer calibration constant (1.00 volt = 4.00 inches of water column). 4. Under Make File Selections, click on Calibration File and select the file generated previously. The program will perform a least-squares curve fit again. Verify that the intercept and span are the same as before. If they are not, something is wrong with your calibration data file, and you may need to re-do the balance calibration. 5. Create a new text file for the Save Results File by entering a file name of your choice (again it is suggested that you include a member s name in the file for uniqueness, such as Smith_groupC_smooth_golf_ball.txt. 6. Take many data points as you vary the wind tunnel speed from a moderate value up to full speed. After each adjustment of wind tunnel speed, wait until the wind tunnel blower has reached a constant frequency before taking a data point. The program is written such that it is not necessary to take data in ascending order. In other words, you can vary the wind tunnel speed in arbitrary fashion, going back to fill in some data points where there are gaps. Take as many data points as you feel necessary. Note: Due to zero drift of both the pressure transducer and the drag balance, it is advisable to stop the wind tunnel every so often (perhaps after every ten data points) to rezero the instruments. Make sure the wind tunnel fan has come to a complete stop before re-zeroing. (2) 7. When you are satisfied with the amount of data, exit the program, whereupon it will write all the data into the file that you created. Print out your data file when finished. See Table. (5) 8. Has a drag crisis been observed? In the space below, discuss why or why not. (2) 5. Repeat the drag measurements for the other smooth spheres (small brass, medium brass, and large brass). Each time you run the measurement program, enter a new file name; otherwise, your previous data may be lost! For each sphere, print out your data file, and make sure it is clearly labeled and numbered. See Tables through. (2) 6. Repeat the drag measurements for the roughened balls (one of the golf balls, baseball, and softball). See Tables through.
E. Presentation of the Data Now that you have acquired the data for all the spheres, you will generate plots from your data files so that the data can be analyzed. (10) 1. Plot drag force F D versus velocity V for all the smooth spheres on the same plot (use different symbols or colors to distinguish them, and be sure to include a legend). See Figure. (10) 2. Plot drag coefficient C D versus Reynolds number Re d for all the smooth spheres on the same plot (again be sure to distinguish and label each data set clearly). Include published data in your plot. See Figure. (5) 3. Plot drag coefficient C D versus Reynolds number Re d for all the rough spheres on the same plot (again be sure to distinguish and label each data set clearly). See Figure.
Discussion (5) 1. For which spheres (if any) were you able to observe a drag crisis? Discuss why or why not. Briefly discuss what you have learned about drag crisis, the effect of surface roughness, etc. on the flow over a sphere. (5) 2. Do the predictions of dimensional analysis really work? Specifically, do the smooth sphere data all collapse onto one curve when plotted nondimensionally? Why or why not? Do your data agree reasonably well with published data for spheres? If not, suggest some reasons for the discrepancy. How can this experiment be improved?