Wind Tunnel Testing and Full Size Similarities?

Question

What needs to be taken into account when testing aerodynamics in awind tunnel, fore example Geometric and Dynamic Similarities, can anyone give a fairly detailed answer or a good webiste to reseach from. Thanks

Answer 1

You run into issues with "similitude".

When you scale a flying model geometrically, the wingspan for example shrinks proportional to scale. But the wing area shrinks to the SQUARE of scale. Volume shrinks to the cube of scale, etc.

So if you are dealing with phenomena that vary with the squares and cubes of geometric scale, you have to be aware of it and prepare to adjust your model accordingly. Thus, your "scale model" for the wind tunnel might be such that the "wing area" is halved (in order to check dynamic loading, for example) regardless of what this does to you geometric scaling.

Same thing is true with scaling buoyancy models.

Another thing that scales poorly is certain properties, like viscosity. A scale model in a wind tunnel will make the air act more viscous than it really is. Everyone who has watched an old Japanese sci-fi movie (think 1960's Godzilla movies) has seen this effect with water; when you see scale models thrashing around in water, filmed in slow motion, it looks very cheesy. Your picks up on the fact that the water is too "viscous" - it doesn't behave correctly.

Similitude is the reason why.

If you work around wind tunnels and such, you will become well versed in this aspect of simulation and modeling very quickly.

Answer 2

It's hard to get everything perfect, so you generally have to pick what's important and fudge a little bit on the rest. Two important aerodynamic factors are Reynolds number (Re) and Mach number. It can be very expensive to make these match your flight conditions (if it's even possible). Using Re as an example, if the Re is too low, the flow will remain laminar for too long. By putting a rough spot in the right place you can cause the flow to trip to turbulent flow as it would in the real airplane (or whatever you're testing). But there really is no perfect wind tunnel test, which is why flight testing is still needed. And in this day and age we still find surprises in flight testing. Check out http://www.aa.washington.edu/uwal/uwalinfo/techguide.htm for more information.

Answer 3

A wind tunnel is a research tool developed to assist with studying the effects of air moving over or around solid objects.

Ways that windspeed and flow are measured in wind tunnels:

Threads can be attached to the surface of study objects to detect flow direction and relative speed of air flow.
Dye or smoke can be injected upstream into the airstream and the streamlines that dye particles follow photographed as the experiment proceeds.
Probes consisting of a Pitot tube can be inserted at specific points in the air flow to measure static and dynamic air pressure.
http://en.wikipedia.org/wiki/Wind_tunnel

http://www.advancedtechnologiesinc.com/wind_tunnel_models.asp
http://www.structuremag.org/archives/2003/november/windtunneltesting.pdf
http://www.arniebakercycling.com/handouts/hev_wind_tunnel_testing.htm
http://www.nasaexplores.com/show_912_teacher_st.php?id=030110101756
http://www.vigyan.com/tunnel.shtml
http://en.wikipedia.org/wiki/Vertical_wind_tunnel
http://www.britannica.com/eb/article-9077168/wind-tunnel
http://www.britannica.com/eb/article-9077168/wind-tunnel

Answer 4

Look for dimensional analysis conversions for aerodynamic testing. These will explain how to convert/super impose test results from a smaller test sample to a life sized aircraft.