Science Buddies: "Ask an Expert"

I am currently upgrading my old wind tunnel. One of its upgrades is a larger variable speed fan. I need help designing a new contraction zone. The diameter of the fan blades is 24 inches or about 60.96 cm. The contraction is to a 15inch by 15inch square or about 38.1 cm by 38.1cm square. There is no real length restraint with in reason.

The two basic problems are joining a circular fan cross section to a square testing cross secton and if a curved shape should be used rather than a sraight one. For the first problem I was thinking of an octogonal cross section near the fan. The corner faces would taper off into isosceles triangles leaving four rectangular faces to join forming a square cross section. This however does not work so well with a curved contraction zone. If you have any suggestions on how to join the circular cross section and the square one I would greatly appreciate it. *I would like to know what works best a curved shape or a staight one and if it is better by a small or large margine. Also for a curved shape any equations or ratio guidelines are welcome.

As for material I was considering a wooden frame with cloth between to cut down costs. If that is a bad idea I can use wood, plastic, or fiberglass in the construction. Other material suggestions are also welcome.

Thank you for your valued input.

*Note: In the design of the contraction zone sub sonic flows of varying velocity that will run through it must be considered.

Fundamentally, your transistion section problem is similar to forced air furnace plenum design. How many cfm is your fan capable of producing? This one number will provide a lot of insight into how sophisticated your transistion element needs to be in order to minimize the transition turbulence to allow a reduction in the length of the section prior to the test location where you require laminar air flow.

A simple cube pressure chamber with a circular inlet and a square outlet with vertical and horizontal vanes will work well at low air flows; however, it won't maximize the air speed possible with a given fan because of the relatively high back pressure. What longer tapered transition sections provide is less drag (back pressure) so they are more efficient. If I remember correctly, the simple cube pressure chamber transition efficiency goes down with something like the square of the air speed so the higher the air speed desired, the more important it is to have a better transition design.

A 24 inch diameter fan is about 452 square inches and a 15 inch square is 225 square inches so your compression ratio is only 452/225 or 2.

Your idea of an octagon to square transition piece is significantly better than a simple cube and probably good enough as long as the length of the transition is chosen appropriately. You will also want to make sure that the inside of your octagon is outscribed (each side is tangent to the outside of) a 24 inch diameter circle.

The transition of each side is (24 - 15)/2 or 4.5 inches. Common rules of thumb for smoooth transition lengths for furnace plenums are 3 to 4 times the transition half widths so you need to be using 15 to 24 inches of length.

If you use cloth as your transition elements, you will definitely want to apply a coating that will harden. Ripples in the fabric will cause a significant amount of drag and could easily make the transition behave worse than the simple cubic pressure chamber approach.

I am currently working on a Project Guide for the Science Buddies site dedicated to subsonic wind tunnel design. I have a long list of sources which may be useful to you if you need to do some more research. In case it is of use to you, here is my list of resources (I highly recommend the first one, which is an extensive book by Barlow and Pope):

Print:
• Barlow, Jewel B., Alan Pope, and William H. Rae, Jr. Low-Speed Wind Tunnel Testing. 3rd ed. New York: John Wiley & Sons, Inc., 1999.
• Collins, Luke, James Flint, Ben Morgan, Clint Witchalls, and Chris Woodford. Cool Stuff and How It Works. 1st ed. New York: DK Publishing, Inc., 2005.
• Mehta, R. D., P. Bradshaw. "Design Rules for Small Low Speed Wind Tunnels." Aeronautical Journal (1979): 443-449.
• Smith, H. C. "Skip". The Illustrated Guide to Aerodynamics. 2nd. New York: TAB Books, 1992.

Web
• Baals, D. (n.d.). Wind Tunnels of NASA. Retrieved August 2008 from http://www.hq.nasa.gov/office/pao/Histo ... ntents.htm
• Benson, T. (July 14 2008). Beginner's Guide to Wind Tunnels. Retrieved August 2008 from http://www.grc.nasa.gov/WWW/K-12/windtunnel.html
• Bradshaw, P and Rabi Mehta. (n.d.). Wind Tunnel Design. Retrieved August 2008 from http://www-htgl.stanford.edu/bradshaw/tunnel/index.html
• Bugg, R., Randy Cyron, Michele Houser, Scott Oberlitner, Joseph Saracen. Miniature-Portable-Subsonic Wind Tunnel for Aerodynamic and Heat Transfer Measurements Designed for Classroom Demonstration Purposes. Retrieved August 2008 from http://www.mne.psu.edu/me415/spring05/ME2/
• Cipolla, J. (2008). Wind Tunnel Consulting. Retrieved August 2008 from http://www.aerorocket.com/offer.html
• Cleghorn, C. (February 2004). NASA's Wind Tunnels Fact Sheet. Retrieved August 2008 from http://oea.larc.nasa.gov/PAIS/WindTunnel.html
• Clifford, R. C. (June 2003). Home-Made Wind Tunnel. Retrieved July 2008 from http://www.theplanpage.com/months/2306/tunnel.htm
• Clifford, R. C. (July 2003). Home-Made Wind Tunnel, Part 2. Retrieved July 2008 from http://www.theplanpage.com/Months/2307/wt_pt2.htm
• Erwin, B. (n.d.). The Baals Wind Tunnel. Retrieved August 2008 from http://web.archive.org/web/200202021135 ... unnel.html
• G.U.N.T. Gerätebau GmbH. (2005). Educational Wind Tunnel. Retrieved August 2008 from http://www.gunt.de/static/s3399_1.php
• Hodanbosi, C. (n.d.). Abstract: The Wandering Wind Tunnel. Retrieved August 2008 from http://www.grc.nasa.gov/WWW/K-12/WindTu ... tunnel.htm
• U.C. Davis College of Engineering. (n.d.). U.C. Davis Aeronautical Wind Tunnel Facility. Retrieved August 2008 from http://windtunnel.engr.ucdavis.edu/
• Wright Brothers Aeroplane Co. (n.d.). 1901 Wind Tunnel. Retrieved August 2008 from http://www.first-to-fly.com/Adventure/W ... tunnel.htm
• The Wright Experience. (2002). In Depth: The Wind Tunnel. Retrieved August 2008 from http://www.centennialofflight.gov/wbh/w ... /index.htm

Thanks for all your help. I was able to complete my project and wind tunnel testing on fanwings last year (school year). I entered in the LA county and California state Science Fairs and did pretty well. Thanks again for your much appreciated help.

Great! Glad to hear about your success, and that you learned a great deal from your project!

Thanks for all your help so far. I am thinking about upgrading my wind tunnel again. This time I am focusing on data collection. I read the science buddies how-to guide on wind tunnel construction and it says that dual-range force sensors are used. It also suggests to get them from Vernier. I checked the Vernier web site and these sensors cost about $100 dollars apiece. http://www.vernier.com/probes/dfs-bta.html . As I am a High Schools student this is kind of expensive for me.
As an alternative I can get some strain gauges for pretty cheap.
1 I was wondering if anyone has or knows of anyone who has used strain gauges in a wind tunnel.
2 Does anyone have any suggestions on how to rig it up?
3 Does anyone have experience working with strain gauges and can give me some advice (Do's and Do not do's)?

Thanks, your help is most appreciated

Many years ago I did lots of work with strain gauge instrumentation but never in a wind tunnel. Small strain gauges are fairly difficult to get accurate measurements from and there is a real art in applying to something so that you are measuring what you really want to measure. If you glue them down to a metal part, the glue will sometimes act like an elastomer and you measure less tension strain than what the part is under and compression strain can be high or low depending on how the strain guage behaves with bending. The glue can crack and the strain guage will bend and you usually get really noisy and inaccurate measurements. Vibration is your enemy and I would expect things in wind tunnels to vibrate in some conditions and render your measurements inaccurate.