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UI Wind Tunnel Facility The purpose of this manual is to provide information about using and maintaining the UI Wind Tunnel Facility, Fig. 1. This manual is divided into two parts, a user’s manual and a technician’s manual. The user’s manual is intended to educate students and those using the user-friendly features of the facility about applications and using the facility. The technician’s manual contains detailed information on the electronic displays, settings, wiring diagrams, calibration procedures, and research applications.

Figure 1: The UI Wind Tunnel Facility. Users Manual I. Introduction to the wind tunnel

Engineers, scientists, and inventors have used wind tunnels to perform aerodynamic tests for over a century. The Wright Brothers used a wind tunnel to their airplanes before flight. A very complete history of wind tunnels is available at the web site www.hq.nasa.gov/office/pao/History/SP-440/contents.htm. Wind tunnels can be used to test all kinds of aerodynamic bodies, from cars and airplanes to golf balls and Olympic skiers. The wind tunnels at Nasa’s Ames Research Center are some of the most famous in the world. Lists of several applications and some pictures from tests in these wind tunnels can be viewed at the web site, http://windtunnels.arc.nasa.gov/WindTunnels/index.html.

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The UI wind tunnel is designed to allow students to test artifacts quickly and easily. Some past applications include testing rockets, parachutes, a model of the FutureTruck, and model gliders. There are several wind tunnel manufacturers. Two manufacturers are Engineering Laboratory Design and Aerolab. It is also possible to build a wind tunnel from scratch. Several references and a plan to build a wind tunnel can be viewed at the web site, www.grc.nasa.gov/WWW/K-12/windtunnel/html. The UI wind tunnel was built by Engineering Laboratory Design in Minnesota. This tunnel is an Eiffel type tunnel, Fig. 2, which means the fan “pulls” air through the test section instead of “pushing” it through as the test section, as some tunnels do. The fan is driven by a 50 Hp AC motor that is controlled by a variable frequency drive. The wind tunnel can produce a test section air velocity of 160 mph.

Fig. 1 is a schematic of the UI wind tunnel. Air enters the entrance because of the lower test section pressure and resulting pressure differential created by the fan exhaust. The air passes through flow straighteners, which consist of a honeycomb and a set of screens. The contraction changes the cross section of the tunnel from the entrance to the test section, and directs the air into the test section. It increases the air velocity and ensures that the flow in the test section is parallel with the centerline of the tunnel. The test section is made of ¾ inch thick Plexiglas, has a removable ceiling and floor, and two access ports in the floor. From the test section, the air flows into the expansion and square to round transformation. The expansion and transformation sections are designed to prevent separation in the flow and lower the air velocity for better fan efficiency. The fan section is attached to the wind tunnel by a flexible coupling to prevent fan vibrations from being transmitted to the tunnel. After the fan, the air passes through an acoustic diffuser, which lowers the tunnel’s operating noise.

Fig 2: Schematic diagram of the UI wind tunnel.

Entrance

Contraction Honeycomb and screens

Test Section Expansion and

Square to Round Transition

Flexible Coupling

Fan and Motor

Acoustic Diffuser

Exit

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II. Quick Start Guide Model Installation and Removal 1. Make sure the wind tunnel test section velocity is zero and the fan is not spinning. 2. Remove the test section ceiling. This operation requires two people. 3. Install the model on the mount attached to the end of the Aerolab electronic

balance. Note: Be careful not to overload the force balance. It is rated for ± 10 pounds force in drag and ± 25 pounds force in lift or side force.

4. Replace the test section ceiling, making sure to align all of the alignment tabs, Fig. 4, on the quick release snaps, and secure all 8 of the quick release snaps.

Wind Tunnel Speed Control Operation 1. Make sure your mentor turns the tunnel power supply on. 2. Press the LOCAL/REMOTE button until the green LED is lit. This indicates the

remote keypad has control of the frequency controller. 3. Enter the desired frequency for the motor, using the up and down arrows. 4. Once the desired frequency is entered, press the READ/WRITE button. 5. Press the RUN button. 6. To change the frequency setting during operation, repeat steps 3 –5. 7. To stop the drive: first reduce the frequency, using the down arrow (10 Hz is a

good rule of thumb), and then press the STOP/CLEAR button.

Force Balance Displays 1. Turn on the appropriate display for the force you would like to measure (Lift,

Drag, or Pitching Moment). You should turn on the force balance displays you wish to use at least 15 minutes in advance so they can warm up to operating temperature.

2. After placing your model on the mount and turning the tunnel power supply on, but before starting the flow, press the Tare button to zero the display.

3. Set the tunnel to the desired speed and record the force displayed in Newtons.

Air Velocity Display 1. Turn on the Air Velocity Display. 2. After turning the tunnel power supply on, but before starting the airflow, press the

TARE button on the display’s front panel to set the reading to zero. 3. The anemometer is calibrated to display test section air velocity in units of m/sec.

Angle of Attack 1. Turn on the Angle of Attack display. 2. Use the angle of attack adjustment knob on the sting to move the sting until the

scribe mark on the upper horizontal support arm is even with the zero angle of attack indicator, Fig. 7.

3. Press the TARE button on the front panel of the display to zero the display. III. Quick Reference Guide

• Test Section size: 18” x 18” x 36”

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• Maximum Test Section Velocity: 70 m/sec (155 mph) • Maximum Solid Blockage ratio defined as the projected area of the test

object divided by the total cross-sectional area of the test section: 0.10 • Maximum lift force: ± 25 pounds force • Maximum drag force: ± 10 pounds force • Maximum angle of attack: ± 30 degrees • Accuracy of lift measurement: 0.00385N • Accuracy of drag measurement: 0.00454N • Accuracy of velocity measurement: ± 1% of full scale • Accuracy of angle of attack: ± 1 degree

IV. Safety

1. A mentor must be in the room at all times while the power supply to the tunnel is on.

2. Keep all objects, including people, out of all parts of the wind tunnel. 3. Keep trash and debris picked up. If trash begins to move around the room during

a test, stop the test and remove the trash from the room. 4. Make sure the floor is clean before beginning a test. Use the dust mop or wet

mop if necessary. 5. Do not rub the test section or use hard objects that might scratch the Plexiglas. 6. Do not pull or move any of the wires, power supplies or instruments. 7. Keep all loose paper and other lightweight objects covered or tied down during

testing. 8. Do not open any port or cover on the test section while the wind tunnel is

operating. 9. Do not walk in front of the wind tunnel entrance or exit while the tunnel is

operating. Make sure the yellow safety ropes are in place before beginning a test to prevent people from walking in front of the intake or exit.

10. Wear eye protection while in the wind tunnel laboratory. V. Testing Guidelines

A. Physical Restrictions of the Tunnel 1. Test section is 18” x 18” and 36” long. 2. Test section velocity range is 0.5 – 70 m/sec (1 – 155 mph).

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B. Solid Blockage Ratios Solid blockage is the ratio of the frontal area of the test object to the cross sectional area of the test section. When the test object is operated in the atmosphere, the test section walls do not affect the flow around the object and this ratio is essentially infinity. When the test object is tested in the wind tunnel, the presence of the test section walls affect the flow pattern around the model and alter the test results from actual operating conditions. Barlow, Rae, and Pope suggest that blockage ratios should be below 0.10 in order to avoid large differences between test data and operational data. A blockage ratio of 0.05 is typical. C. Wall Correction Factors Wall correction factors are one method, probably the most popular, of dealing with the presence of the test section walls and their affects on the experimental data obtained with the test object. The walls of the test section alter the flow pattern around the test object from the flow pattern that would occur in the atmosphere. The most common flow alteration as a result of the walls is that the streamlines around the test object are much closer together than they would be in the atmosphere. If the streamlines are close together, it means that the velocity is higher. Since the velocity in the test section is usually measured in some global position, localized velocity increases near the test object will create inaccurate test results. Wall correction factors are experimentally determined methods for correcting the velocity near the test object. Low-Speed Wind Tunnel Testing by Barlow, Rae, and Pope covers many general wall correction factors, and the paper “Wall-Interference Corrections for Parachutes in a Closed Wind Tunnel,” by J. Michael Macha and Robert J. Buffington provides some correction factors for parachutes and bluff bodies.

VI. Testing Preparation

A. Force Balance The force balance is the device that you will usually use to mount your model into the wind tunnel test section. It was built by Aerolab, Inc. and is an internal, electronic force balance. This means that the force balance uses small strain gages to detect the force applied to the sting. The sting, Fig 3, is the long, metal, cylinder at the top of the force balance, to which models and fixtures are mounted. For model mounting information and more information on the force balance, see the original owner’s manual UI# 13209A, which is available in the stock room. The strain gages are electronically attached to the digital displays. The digital displays supply power to the force balance and measure the voltage signal from the force balance. As the voltage signal changes, the digital displays are calibrated to display the corresponding force applied to the sting. The force balance has a force range of ±10 lbf on the drag channel and ±25lbf on the lift channel. The error (random uncertainty) on the lift force reading is 0.00385N and the error on the drag force is 0.00454N.

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B. Access to the Test Section Access to the test section is achieved by removing the ceiling, floor, or the ports in the floor. Usually models are installed and removed by removing the test section ceiling. The ports in the test section floor allow the installation of model attachment structures, and instrumentation. The Aerolab force balance is an example of a combined model holding structure and instrumentation package. The sting on the balance supports models and contains the strain gages that measure forces on the model. A special port was machined to install the balance in the test section.

There are several other ports that can be installed in the test section floor. Some of these ports allow the installation of pitot tubes and other velocity measuring devices, in order to calibrate the device against a pitot tube. If there is not a port that meets the needs of model installation or instrumentation, it is possible to make a new port. The contour of the port is designed to fit into the precut holes in the test section floor, and the rest of the port is available for any geometry that is necessary to install a device in the test section. It is also possible to design and build different mounting attachments for the end of the balance. There are a couple of existing model attachments. They are located in the third drawer down, in the green and tan cabinets in the wind tunnel lab. There is an attachment for A-C size rocket motor mounts, and a parachute attachment. Ask your mentor or the instructor to help you determine if there is an existing attachment that will meet your needs or if you must make a new attachment. If it is necessary to build a new attachment, they will help you design and build the proper mounting attachment.

Figure 2: The test section showing two quick release snaps open.

Handles

Quick Release Snaps

Smart Air Velocity Transmitter

Force Balance

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C. Installation of the Aerolab Force Balance

1. Locate the port with the slot in it for the balance arms to slide through, and the piece of foam rubber to seal the port after installation.

2. Place the piece of foam rubber over the balance arms so the flange is towards the base of the sting. Slide the port over the balance and let is rest on the support arms with the flat surface facing down.

3. Remove the port you wish to install the balance in – upstream port for parachutes and other trailing objects and downstream port for rockets and other leading objects. To remove the port, remove the two thumbscrews and the port will fall out of the test section floor so be careful to catch it.

4. Slide the balance into the test section through the open porthole and place the balance support table under the base of the balance. It is best to have two people do this – one to lift the balance, and one to position the support table under the balance.

5. Roughly position the balance in the desired location, then level the base of the balance using the adjustable feet on the support table.

6. Position the balance in the desired position – usually parallel with the flow and as close to the center of the test section as possible. The sting should be parallel with the flow so the forces acting on the sting are only in the axial (drag) and normal (lift) directions. The sting should in the center of the tunnel to minimize the effects the walls have on flow around the test object. Use the level on the sting to make sure it is horizontal, or use the angle of attack indicator, with the angle of attack set to zero as described in section VII-D. Use a square to make sure the sting is parallel with the flow in the horizontal plane, but be careful not to scratch the test section. In many cases, aligning the sting by sight will get it close enough to parallel with the flow.

7. Slide the port up into position in the test section floor and install the two thumbscrews.

8. Slide the foam rubber into the hole in the port around the balance support arms to seal the test section so air does not enter the test section through the port during testing.

D. Model Installation and Removal

5. Make sure the wind tunnel test section velocity is zero and the fan is not

spinning. 6. Remove the test section ceiling. This operation requires two people.

a. Use the stepladders to reach the top of the test section. b. Unlatch the 8 quick release snaps, Fig. 2 that hold the ceiling to the

test section. c. Use the handles at the upstream and downstream end of the test section

ceiling to remove it, fig 2. Be careful not to bump the Smart Air Velocity Transmitter, display enclosure, or the transducer input cables when you remove the test section ceiling.

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7. Install the model on the mount attached to the end of the Aerolab electronic balance.

Note: Be careful not to overload the force balance. It is rated for ± 10 pounds force in drag and ± 25 pounds force in lift or side force.

8. Replace the test section ceiling, making sure to align all of the alignment tabs, fig 4, on the quick release snaps, and secure all 8 of the quick release snaps.

9. Removal of the model is the same as the steps above, except remove the

model in step 3.

Figure 3: The force balance mount for rockets.

Force Balance

Mount

Alignment tabs

Sting

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Figure 4: The quick release alignment tab.

E. Dwyer Smart Air Velocity Transmitter The anemometer, any device for measuring wind speed, for the wind tunnel is a Dwyer Smart Air Velocity Transmitter. It is a thermal anemometer that uses a heated cylinder to determine the velocity of the air in the test section. The anemometer has a temperature probe and a velocity probe. Electronics inside the meter heat the velocity probe to a set differential temperature above ambient conditions. As the velocity of the air across the sensor increases, more power is required to heat the velocity sensor. Electronics in the Smart Air Velocity Transmitter use a programmed relationship between the power required to heat the sensor and the air velocity to output a voltage signal that is proportional to the air velocity. The sensor’s accuracy is ±1% of the full-scale range and the repeatability is ±0.2% of the full-scale range. The maximum full-scale setting is 18,000 fpm.

VII. Operation

A. Wind Tunnel Speed Control Operation – See Fig 5

8. Make sure your mentor turns the tunnel power supply on. 9. Press the LOCAL/REMOTE button until the green LED is lit. This indicates the

remote keypad has control of the frequency controller. 10. Enter the desired frequency for the motor, using the up and down arrows. Note: The front panel of the speed controller displays the motor drive frequency, not airspeed. Note: It is best to start at a low velocity when testing an object for the first time. Refer to the chart for a graph of drive frequency vs. test section velocity. A good frequency to start with is 10 Hz and work up or down from there. 11. Once the desired frequency is entered, press the READ/WRITE button. 12. Press the RUN button. The display will show the frequency as the frequency

controller adjusts to the desired setting. 13. To change the frequency setting during operation, repeat steps 3 –5. When you

enter a new frequency while the tunnel is in operation, the speed will change as you change the frequency. It is still important that you push the READ/WRITE and RUN button when changing the frequency during operation.

14. To stop the drive: first reduce the frequency, using the down arrow (10 Hz is a good rule of thumb), and then press the STOP/CLEAR button.

Note: If the model or mount develops a problem during a test, immediately press the STOP/CLEAR button to end the test as quickly as possible.

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Figure 5: The remote keypad. B. Wind Tunnel Instrumentation Operating Instructions

1. Force Balance Displays – See Fig 6 1. Turn on the appropriate display for the force you would like to measure (Lift,

Drag, or Pitching Moment). You should turn on the force balance displays you wish to use at least 15 minutes in advance so they can warm up to operating temperature.

4. After placing your model on the mount and turning the tunnel power supply on, but before starting the flow, press the Tare button to zero the display.

5. Set the tunnel to the desired speed and record the force displayed in Newtons. NOTE: Be careful not to overload the force balance when mounting or removing your model. The Drag channel is rated to ± 10 lbfs and the Lift channel is rated to ±25 lbfs.

Green LED (not lit)

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Figure 7: The electronic displays. 2. Air Velocity Display – See Fig 7

1. Turn on the Air Velocity Display. 6. After turning the tunnel power supply on, but before starting the airflow, make

sure that the display reads zero velocity. 7. If the display does not read zero, press the TARE button on the display’s front

panel to set the reading to zero. 8. The anemometer is calibrated to display test section air velocity in units of m/sec.

3. Angle of Attack – See Fig 7-8

4. Turn on the Angle of Attack display. 5. Use the angle of attack adjustment knob on the sting to move the sting until the

scribe mark on the upper horizontal support arm is even with the zero angle of attack indicator, Fig. 7.

6. Press the TARE button on the front panel of the display to zero the display.

NOTE: Make sure the base of the balance is level before attempting to tare the angle of attack display.

On Switch

Tare Button

Voltage Outputs

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7. Voltage Output All of the electronic displays are capable of outputting a 0-10 VDC voltage signal that is proportional the reading displayed on the front panel. The voltage outputs are on the left side of the electronic display enclosure, Fig 6. The voltage signal can be used for data acquisition. Ask your instructor or mentor if you need to use the voltage output signal and they will help you configure the electronic display to output the desired voltage range corresponding to the readings you will encounter.

VIII. Head Loss Plates The tunnel is not capable of attaining and maintaining small test section velocities in normal operating mode. If your test requires a very low velocity – under 10 m/sec – for an extended period of time, you should use the head loss plates to achieve this lower velocity. The head loss plates are pieces of metal with holes punched in them. There are two plates with different size holes. They are designed to be inserted into the tunnel between the expansion and the square to round transformation. Information on the performance of the wind tunnel with the head loss plates installed is available in the wind tunnel owner’s manual from ELD Inc. Ask your mentor or instructor to help you determine if you need the head loss plates and which plate is appropriate for your tests.