Wind Tunnel Experiments for Grades 8 - 12

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Wind Tunnel ExperimentsforGrades 8 - 12
Dr. Judy Foss Van Zante Dynacs Engineering Co.,
Inc. Cleveland, OH
6/15/99
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Contents
Sample Experiments 3 Governing
Equations 15 Flow Visualization
Techniques 19 How to Make the Measurements 24 Ba
ckground - Why Test in Wind Tunnels 27 Selected
References 31
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Sample Experiments
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Ideas for Wind Tunnel ExperimentsModel Airfoil
or Flat Plate

• L vs. a Lift vs. Angle of Attack
• L vs. V Lift vs. Velocity
• CD vs. Re Drag vs. Reynolds Number i.e., vary
  Speed and/or Size
• Investigate the effects of contamination on the
  leading edge (sand paper, paper mache) to mimic
  ice accretion, bug splat, etc... This should
  reduce max lift increase drag.

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Wind Tunnel Test Section with AirfoilMounting
Options
Airfoil on Sting Wall-Mounted
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Lift vs. Angle of Attack
As the angle of attack increases, so should the
lift - until a certain point (the stall angle of
attack). Angle of attack (a) angle between
flow and chord line. Chord line straight line
between most forward and most aft points
Lift
Flow
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Lift vs. Angle (cont.)
Lift
Angle
Visual See airfoil lift as angle increases
Measure airfoil lift as a function of angle
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Wind Tunnel Experiment Lift vs. Angle Worksheet
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Lift vs. Velocity
As the velocity (speed) increases, so should the
lift. Note Keep the angle of attack constant.
The greater the angle (prior to stall) the
greater the change in lift.
Lift
Velocity (Speed)
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Lift vs. (Velocity)2
Visual See airfoil lift as speed increases
Measure airfoil lift as a function of speed
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Wind Tunnel Experiment Lift vs. Velocity
Worksheet
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Ideas for Wind Tunnel ExperimentsModel Drag Body

• Double Elimination Competitions
• Build two objects. In a head-to-head comparison,
  see which one has the least drag.
• Which way will the object with the most drag
  move?
• Race Cars
• Geometric shapes

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Wind Tunnel with Drag ObjectsMounting Options
Bluff Bodies Race Cars Rotating
Sting Pulley
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Ideas for Wind Tunnel ExperimentModel - Drag Body

• Notes
• The frontal area (the side facing the flow) must
  be the same. Drag is directly related to the
  surface area.
• If using the pivot sting, objects must be
  mounted equally far apart from the pivot point.
  It is important that each object has the same
  moment arm.
• If using the pulley system, it might be better to
  have two pulleys.

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Governing Equations
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Governing Equations

• Lift Drag are equal to the
• Dynamic Pressure Surface Area Coefficient
• These Coefficients are a function of
• Angle of Attack, Model Geometry Mach number

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Nomenclature

• Dynamic Pressure, ½ r V2
• r density (of air) rho
• V velocity (speed)
• Surface Area, S
• S chord span
• chord is wing length, span is wing width
• Coefficient of Lift CL function (a, model,
  Ma)
• Coefficient of Drag CD function (a, model,
  Ma)

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Governing EquationNotes

• The Lift and Drag can be changed most easily by
  changing the angle of attack (a) or speed (V). Of
  course, the surface area (S) can also be
  adjusted. If a water tunnel is also available,
  the working fluid (r), e.g. air to water, can
  also be a variable.
• During the course of one experiment, it is
  important to only change one variable at a time.

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Flow VisualizationTechniques
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Flow Visualization Techniques

• Flow Visualization illustrates the flow on or
  near the object. On the surface, regions of
  reverse flow become visible.
• Yarn Tufts, Tuft Probe, Tuft Grid
• Smoke Wand, Smoke Wire
• Trailing Edge Cone (String paper cone)

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Flow Visualization TechniquesYarn

• Yarn Tufts - tape 1 segments of yarn directly
  to the surface.
• Tuft Probe - tape 3 light-weight (and visible)
  string to end of rod. Probe the flow.
• Tuft Grid - attach 1 segments of yarn to a
  wire mesh (screen) and place behind object
  (perpendicular orientation to the flow)
• Trailing Edge Cone - tape one end of string to
  paper cone, and the other end to (spanwise) edge
  of model. This illustrates streamwise vorticity,
  if present. Its great for delta wings.

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Flow Visualization TechniquesIllustrated
Yarn Tufts on surface
Tuft Probe
Delta Wing
Trailing Edge Cone
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Flow Visualization TechniquesCautions

• For yarn string If the inertia (mass) of the
  yarn/string is too large, it wont follow the
  flow.
• For smoke If the airspeed is too high, the
  smoke and air will mix and blur.

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How to Make the Measurements
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Wind Tunnel Experiment Details

• Measuring Lift
• For airfoil and sting measured from the scale
  (ounces). Wt0 weight at zero velocity.
• L Wt0 Wt
• Caution try to minimize the friction (binding)
  at the tunnel/sting interface, e.g., with a brass
  bearing.
• For wall mounted measured from a load cell.
• Caution this is a non-trivial pursuit.

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Wind Tunnel Experiment Details

• Measuring Velocity
• Pitot-static tube
• DP Ptotal - Pstatic
• Bernoullis Equation DP (1/2) rV2, r ? 1
  kg/m3 (units!)
• V ? 2 DP/r
• Three-cup anemometer

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BackgroundWhy Test in Wind Tunnels?
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Why Test in Wind Tunnels?

• The Ultimate Goal to Understand the Fluid
  Mechanics or Aerodynamics of an
• Aircraft in Flight
• Submarine in Water
• Automobile on Road
• New Structure (Building, Bridge) in City
• How do you get There from Here?
• Build a model and test it
• In a Wind Tunnel
• On a Computer

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Two of NASAs Wind Tunnels
Langley
Ames 80 x 120
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Types of Wind Tunnels

• Full Scale / Full Geometry (1999 price estimates)
• NASA Glenn 10 x 10 Supersonic 2000/hr
• NASA Ames 80 x 120 1000/hr
• Sub-Scale / Single Component
• NASA Glenn 20 x 30 Low Speed 2/hr
• How does one scale a model?
• Geometric
• Dynamic (e.g. Reynolds Number, Re rUL/m)

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Selected References

• Aerodynamics
• Abbott, Ira A. von Doenhoff, Albert E., Theory
  of Wing Sections, Dover Publications, 1959.
• Anderson, John D., Fundamentals of
  Aerodynamics, McGraw-Hill, Inc., 2nd Ed., 1991.
• Anderson, John D., Introduction to Flight,
  McGraw-Hill, Inc., 3rd Ed., 1989.
• Shevell, Richard S., Fundamentals of Flight,
  Prentice-Hall, Inc., Englewood Cliffs, NJ, 1983.
• Fluid Mechanics
• 5. Potter, Merle C. Foss, John F., Fluid
  Mechanics, The Ronald Press Co., NY, 1975
  (now published by Great Lakes Press).
• White, Frank M., Fluid Mechanics, McGraw-Hill
  Inc., 2nd Ed., 1986.
• Shapiro, Ascher H., Shape and Flow The Fluid
  Dynamics of Drag, Science Study Series, Anchor
  Books, Doubleday Co., Inc.,Garden City, NY,
  1961.
• Flow Visualization
• 8. Van Dyke, Milton, An Album of Fluid Motion,
  Parabolic Press, P.O. Box 3032, Stanford, CA
  94305-0030, 1982.
• 9. Japan Society of Mechanical Engineers,
  Visualized Flow, Pergamon Press, 1988.
• 10. National Committee for Fluid Mechanics Films,
  Illustrated Experiments in Fluid Mechanics, The
  MIT Press, Cambridge, MA and London, England,
  1972.