Cover Page for Lab Report – Group Portion
Drag on Spheres
Prepared by Professor J. M. Cimbala, Penn State University
Latest revision: 06 April 2012
Name 1:
___________________________________________________
Name 2:
___________________________________________________
Name 3:
___________________________________________________
[Name 4:
___________________________________________________ ]
Date:
_______________________________
Section number:
ME 325._____
Group letter: (A, B, ...) _____
Score (For instructor or TA use only):
Lab experiment and results, plots, tables, etc. - Procedure portion
Discussion
Neatness & grammar
TOTAL
Comments (For instructor or TA use only):
_____ / 50
_____ / 10
_____ / 10
______ / 70
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.
2.
Inclined
manometer
tube
Meniscus
Pointer
First, the manometer must be leveled. Inspect the two level bubbles
0
on the base of the instrument. If the bubbles are not exactly within
Zero
the hairline marks, use the leveling screws on the base to level the
mark
instrument.
Next, the manometer itself must be zeroed. With both sides of the
Ruler
manometer open (atmospheric pressure on both sides, meaning zero
1
(inches)
pressure difference, crank the large knob on the right side of the
instrument until the ruler pointer on the manometer is at exactly zero,
Figure 2. Manometer at zero inches of
as illustrated in Figure 2. Do NOT raise the pointer above the zero
head.
mark (doing so will bend or break the pointer.) While viewing the
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.
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 = Patm 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.
8.
Record the potentiometer readings on the Validyne display unit:
Zero:
Span:
Patm
Tubing
High
Tee
Low
Meriam
inclined
manometer
Pressure
transducer
Patm
Low
+
High
Quick
connector
Cable
Hand pump
000
Hi
Zero
Lo
Span
Pressure transducer display
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 balance installed in this wind tunnel is a cantilever-type balance with strain gages, mounted as shown in
Figure 4. Briefly, the strain gages measure the amount of deflection of the cantilever, which is directly proportional to drag
force. The Measurements Group strain indicator provides the necessary power supply to the strain gage circuitry. The
switches should be set as follows:




The “Full” yellow button should be pushed in
The “Gage Factor” should be set on 1.8-2.4
The “Excitation” should be dialed to 7
The “Range” button should be 100%
There is also a TSI integrating voltmeter connected to the output. This should be set to “DC Volts” and 10 V range. The time
constant should be at 1 second. This voltmeter will be used to monitor both the calibration of the drag balance and for the
drag measurements in the wind tunnel. Its reading will not be the same as that of the strain indicator, but it is linearly
proportional. The advantage is that it integrates over time, making it easier to read when there is an unsteady drag. Use the
following procedure to calibrate the drag balance:
1.
If the computer is off, turn it on. From the Windows desktop or from the Start Menu, start the program FluidsDrag Calibrate. This is a program which will assist you in calibration of the drag balance. As you apply weights to
the drag balance calibration rig, the voltage output from the strain indicator is read by the computer. When
finished with the calibration, the program then performs a least-squares curve fit of the calibration data, which is
then used in the subsequent drag measurements of the various models.
2.
The two strings that come out below the test section are for use in calibration of the drag balance.
The one labeled DRAG should be used for the drag calibration program and the one labeled LIFT should be used
for the lift calibration program, used in a different lab.
Sting mount
Strut
Fairing
1/4-20 threads
Floor of wind tunnel
Strain gages
Drag and lift balance
Figure 4. Schematic diagram of the drag balance with sting mount.
3.
Click on Select File for Saving Data to enter a file name for the output data. By default, all files go to the C:\temp
directory. The file must be a text (*.txt) file. Pick a file name that is unique to your group, e.g. include a member’s
last name, such as “Smith_groupC_drag_calib.txt”. Now you are ready to calibrate.
4.
With no weight on the string, adjust the potentiometer labeled “Balance” on the strain indicator until the voltmeter
reads zero. Enter “0” as the applied mass, and ample the first data point reading of the drag calibration program.
NOTE: The strain indicator is a Wheatstone bridge type circuit, which occasionally drifts with temperature. It may
be necessary to re-adjust the zero balance on the strain indicator occasionally.
5.
Carefully add masses of various values and take more data points. Typically, about ten data points (ranging from
20 grams to 1000 grams) are sufficient for a good calibration, and they do not have to be applied in increasing
order.
Note: The data acquisition board in the computer is set for a range of -5.0 to 5.0 volts. Monitor the voltage
displayed on the TSI integrating voltmeter. Do not exceed 5.0 Volts, or your signal will be clipped, and the
reading will be erroneous.
6.
When finished, End Calibration – Get Best-Fit Line. The computer then calculates a linear least-squares curve fit,
and displays the coefficients. Write the calibration coefficients here:
Slope = _______________________ N/V
Intercept = _____________________ N
(5)
7.
Open the data file that you generated. Check the last line in the file, which should look like this:
25 10000.0 10000.0
where “25” is just a place holder here – it should be the last row number, depending on how many points you
took. If that line is not there, you need to add it; it is required to signify the end of the data.
8.
Finally, using the data file you created, generate a plot of the drag balance calibration (drag force as a function of
voltage), using Microsoft Excel. Use symbols for the data points, and use a solid line for the curve fit. Ask your
instructor or TA for help generating the plot if necessary. Show your plot to your TA or instructor for his/her
initials before continuing. Attach the approved plot to your report. Save your data file and your Excel file in case
you need them later.
See Figure _____________ .
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 drag 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.)
Patm = ______________ 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 oC (1 volt = 10oC).
Write down both temperatures below – they should be close.
Tambient, glass thermometer = ______________ C
(2)
3.
Tambient, thermocouple = ______________ C.
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/m2 of pressure. The gas constant, R, for air is
287. m2/(s2K).
 = ______________ kg/m3.
(2)
4.
Calculate or look up the coefficient of viscosity, , of the air at the temperature indicated by the thermocouple.
 = _______________ Ns/m2.
(5)
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)
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)
7.
Validyne display
reading (Volts)
Pitot-static pressure difference
(inches of water column)
The maximum wind tunnel speed is ___________ m/s.
Velocity (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.
D.
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.
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.
(2)
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 drag balance and the
pressure transducer if necessary.
3.
Start program Fluids-Drag Measure from the Windows desktop. This program acquires the thermocouple
temperature, the voltage from the drag 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 Drag 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 drag 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.
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 drag 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 FD 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 CD versus Reynolds number Red 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 CD versus Reynolds number Red 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?