1. Wind Tunnel AerodynamicsBy
Dr. Charles Trefny
20 Sept 2002

2. Aerodynamics
The field of science and engineering dealing with the effects of air moving around solid objects.

3. Aerodynamicists
Predict what will happen in a given situation using physical laws, math, wind-tunnels and computers

4. Air has mass and obeys Newton’s laws.
100 gallons of air has a mass of about 1 pound.
We must speed-up, slow-down, or change the direction of the air to generate a force.
Look for this in the following examples of aerodynamics…
In the examples, see how the objects are designed to move the air in a certain direction to cause a force in the opposite direction. See how shapes are “streamlimed” for low drag. Note the jet and propellor engines that accelerate the air backward to cause forward thrust. Some objects are stationary with wind blowing over them, and some move through the air.

17. Wind-Tunnels come in all shapes and sizes…
Wind tunnels are used to predict the amount of force generated by solid objects.
This helps aerodynamicists choose the proper size for things such as wings, spoilers, and parachutes.
Information obtained in wind tunnels is used to improve the design of anything affected by wind.
Wind-Tunnels come in all shapes and sizes…
Wind tunnels are either open, or closed-return. They can have open or closed “test-sections” where the models are tested. They can be as large as big buildings or fit on tabletops. Can useful information on large objects be gathered in a small wind-tunnel? They use fans, compressors, or high-pressure tanks to move air, or other fluids through the test section. The test sections are round, square, rectangular, or other shapes like hexagonal.

18. Open-Return Type Wind-Tunnel

20. Closed-Return Type Wind-Tunnel

22. University of Washington “Kirsten” Tunnel

27. Drive fans in the NASA Ames NFAC

28. Von-Karman H3 Tunnel (Belgium)
Blow-Down Type Tunnel

29. ONERA S3Ma Blow-Down Tunnel (France)

30. Wind Tunnel Test Examples
Obviously, wind-tunnels are used most often to test airplanes, but many things are also tested in wind-tunnels. In the following examples, notice that sometimes the actual object is tested, and sometimes a small model of an object is tested. Sometimes only a piece of something is tested.

31. Boeing 757 Model in NASA Langley 14x22-Foot Wind Tunnel

32. Space Shuttle Model in NASA Glenn 10x10-Foot Tunnel

33. Shuttle Model in Texas A&M University 7x10-Foot Tunnel

34. Full-Scale Test of F-18 in NASA Ames 80x120-Foot Tunnel

35. Half-span model of 777 in NASA Ames 11x11-Foot Tunnel

36. Scale Model of an F-18 Fighter

37. Wing in Witchita State University 7x10-Foot Tunnel

38. Landing Gear in Witchita State University 7x10-Foot Tunnel

39. Orbiter drag chute in 80x120

40. Race Car in Texas A&M University 7x10-Foot Tunnel

41. Full-Size Tractor-Trailor Rig in NASA Ames 80x120-Foot Tunnel

42. Tractor-Trailer Model in Texas A&M University 7x10-Foot Tunnel

43. Ford F-150 Pick-Up in Witchita State University 7x10-Foot Tunnel

44. Car in University of Maryland Tunnel

45. “Hog” in Witchita State University 7x10-Foot Tunnel

46. Bicycle Racer in Witchita State University 7x10-Foot Tunnel

47. Skier in the 8x12-Foot Tunnel at The University of Washington

48. Tent in the 8x12-Foot Tunnel at The University of Washington

49. Trash Can in Witchita State University 7x10-Foot Tunnel

50. Live Bird in Univ. of Lund (Sweden) Low Speed Tunnel

51. We’ll use a parachute for example…
Similarity Principle
Allows us to test a small model and get information about the full-size object
We’ll use a parachute for example…
Easier, less expensive, and safer to build and modify models. The parachute won’t fit in our small wind tunnel.

52. Will the egg break?…
The forces on the parachute are in balance, and the chute drops at its “terminal velocity.”
Notice that our wind tunnel is too small to test the parachute, and it probably isn’t at the right speed.
What does the drag force depend on?

53. Similarity Principle:
For similar shapes, this ratio always comes out to the same number.
Weight
As long as the shape is the same, this fraction will always come out to the same number.
The size can be smaller than the real thing, and the weight, density and speed can be different too!
Density x Speed x Speed x Diameter x Diameter
Don’t worry, it’s just a FRACTION!!!

54. = C Weight Density x Speed2 x Diameter2
If we knew what the fraction was supposed to be equal to, we could guess-and-check until we found the right speed! (or use algebra).
How will we find out what the fraction is equal to?
(notice “squared” equals a number times itself)

55. All “known” in the wind tunnel!
Do a wind tunnel test to find out what “C” is for the parachute
Weight
C =
Density x Speed2 x Diameter2
Once we know what the fraction is equal to, we can also design a parachute (pick a diameter) for a different speed.
Did we even have to use air for the test?
All “known” in the wind tunnel!

56. Computers can also be used to solve problems in aerodynamics.
The computer solves complex mathematical equations that are based on Newton’s laws of motion
The wing or “airfoil” is a classic problem…
When would you prefer to use a computer simulation instead of a wind tunnel test?