Advanced Physics Laboratory — PHYS 301W/302S

Syllabus

Wind Tunnel

In 2007-08 three physics and physics/math student constructed an open-circuit low-speed wind tunnel. They experimented to verify the work of Bernoulli, Navier-Stokes, Reynolds, and others. An simplified drawing of the tunnel is below with the main components labeled.

The Contraction Cone

The contraction cone is designed to increase the velocity of incoming air. The incoming cross section is 2X2 ft2, and the entrance to the test section is 1X1 ft2, giving a contraction ratio of four. We are hoping for a fan that can produce 5-50 mph flows so that the test section will be 20-200 mph.

Many methods can be used to build the contraction cone, including contracting it to a local sheet metal shop or cutting each side from cardboard and gluing it all together. We wanted to keep the project entirely in our own hands, and we wanted the final product to be solid. The contraction cone was sculped from high density styrofoam (home insulation material). First the desired curvature was projected on the wall for the desired 4-to-1 contraction. We used the equation

where H(x), Hin, and Hout are the heights from the centerline for the position along the contraction, the inlet, and the outlet, respectively. The variable α See references 2 and 3 for a discussion of optimizing the contraction cone shape. This curve was projected to the appropriate dimensions and transferred to paper and then to 2 X 2 X 0.167 ft3 sheets of styrofoam. The smaller contraction edge was cut as a square in the styrofoam sheets. The sheets were then glued together with P300 adhesive. The continuous curvature of the cone was achieved by sanding the adjoined pieces of styrofoam to the desired curvature. A layer of BondoTM was applied to smooth the surface of the sanded styrofoam. This was sanded, and a final layer of wall plaster was applied because the BondoTM had bubbled as it cured. The plaster was sanded to produce a final smooth surface in the contraction cone. Finally, a frame was made to hold two pieces of fluorescent light diffuser with 1/2 inch squares. This acted as a honeycomb for producing laminar air flow. We found that two pieces greatly improved our flow quality, especially at high speeds.

The Test Section

The test section is where all the physical measurements take place. The test section was built from plexiglass to be 1'X1'X2'. Originally we attempted to hold it together with homemade brackets, but we found this to be structurally weak. We added L-shaped wood trim glued with 5 minute epoxy to frame the entire test section as well as 1"X2" wood framing on the ends of the test section. The test section was connected to the contraction cone by the 1"X2" wood frame around the test section and glued to the contraction cone. Any gaps were filled with plaster and sanded smooth.

The Diffuser Section

The diffuser section was constructed from plywood. It is approximately 6" long. The length was determined by using a piece of string and identifying the minimum distance from the fan (at high speed) that kept the string from spiraling in the air flow. We assume this was enough distance to remove turbulent effects from the fan. The diffuser also contained honeycomb to further reduce turbulence. The diffuser, test section, and fan were not permanently connected to one another to provide access to the test section. During experiments these components were pressure fitted together and sealed by weather stripping.

The Fan

We were fortunate to get a donated 3-speed air conditioning fan. This fan provided test section wind speeds of up to ~30 mph (~14 m/s). It was electrically connected to three switches. The entire tunnel was built on a wooden platform which we had previously wired with all of the electrical connections and switches for the fan speed.

Air/Fluid Measurements

We constructed a manometer and pitot tubes, but we found that the poor sensitivity made it difficult to make good pressure measurements. In the end we used a Skymate anemometer for wind speed measurements and a Vernier pressure transducer connected to LoggerPro to measure pressure inside the test section. Bernoulli's Principle was applied to verify that there was not significant air compression due to the contraction cone. Smoke tracing was used to qualitatively evaluate the laminar flow of the system.

Airfoil Force Measurements

A Vernier Student Force Sensor connected to Logger Pro. Measurements were made for attack angle of the air foil into the air stream with the sensor set up for drag and lift. A sample set of drag data is shown in the slideshow images.

Budget

The total budget for this project was about $250-300. A good fan will increase the cost since we obtained ours for free. Please contact tmessina<at>centenary<dot>edu if you are interested in project details or knowing some of the pitfalls of building a low-speed tunnel.

References

  1. William H. Rae, Jr. and Alan Pope, "Low-Speed Wind Tunnel Testing," John Wiley and Sons, New York, NY 1984.

  2. Daniel Brassard, "Transformation of a Polynomial for a Contraction Wall Profile," Undergraduate Student Project, Lakehead University, Ontario, Canada, Feb 24, 2003.

  3. R. D. Mehta and P. Bradshaw, “Design Rules for Small Low-Speed Wind Tunnels," Aero. J. (Royal Aeronautical Society) 73, 443 (1979).

  4. John H. Huckans , Nathan A. Kurz, Dean C. Walker, Carla Zembal-Saul, Milton W. Cole, Kimber H. Mitchell, and Diane S. Reed, "A Wind Tunnel in Your Classroom: The Design and Implementation of a Portable Wind Tunnel for Use in the Science Classroom," Science for Children, (2002).

  5. John Downie and Francis Barnes, "A small wind tunnel made of polystyrene," Phys. Educ. 14, 112 (1979). (Not sure about this source. We found it online, and the source journal was difficult to read.

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