wind tunnel ex

NASA Glenn Research CenterIcing Branch - Van Zante / Dynacs

Wind Tunnel Experiments for Grades 8 - 12

Wind Tunnel Experimentsfor

Grades 8 - 12

Dr. Judy Foss Van ZanteDynacs Engineering Co., Inc.

Cleveland, OH

6/15/99



Contents

Sample Experiments 3

Governing Equations 15

Flow Visualization Techniques 19

How to Make the Measurements 24

Background - Why Test in Wind Tunnels 27

Selected References 31



Sample Experiments



Ideas for Wind Tunnel ExperimentsModel: Airfoil or Flat Plate

• L vs. 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.



Wind Tunnel Test Section with AirfoilMounting Options

Airfoil on Sting Wall-Mounted

Flow



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 (): angle between flow and chord line.

Chord line:straight line between most forward and most aft points

Lift

Flow



scale

Lift vs. Angle (cont.)

Angle

Lif

tVisual: See airfoil lift

as angle increases

Measure: airfoil lift as

a function of angle



Wind Tunnel Experiment Lift vs. Angle Worksheet



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)



Lift vs. (Velocity)2

Velocity

Lif

tVisual: See airfoil lift

as speed increases

scale

Measure: airfoil lift as

a function of speed

V2

L



Wind Tunnel Experiment Lift vs. Velocity Worksheet



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



Wind Tunnel with Drag ObjectsMounting Options

Bluff Bodies Race Cars

Rotating Sting Pulley



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.



Governing Equations



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

D2

L2

C*S*V2

1D

C*S*V2

1L



Nomenclature

Dynamic Pressure, ½ V2

= density (of air); “rho”

V = velocity (speed)

Surface Area, S

S = chord * spanchord is wing length, span is wing width

Coefficient of Lift CL = function (, model, Ma)

Coefficient of Drag CD = function (, model, Ma)



The Lift and Drag can be changed most easily by

changing the angle of attack () or speed (V). Of

course, the surface area (S) can also be

adjusted. If a water tunnel is also available, the

working fluid (), 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.

Governing EquationNotes



Flow VisualizationTechniques



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)



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. It’s great for delta wings.



Yarn Tufts on surfacexx xx xx xx xx

xx xx xx xx xx

xx xx xx xx xx

xx xx xx xx xx

Tuft Probe

Delta

Win

g

Trailing Edge Cone

Flow Visualization TechniquesIllustrated

Tuft Grid



Flow Visualization TechniquesCautions

• For yarn & string: If the inertia (mass) of the yarn/string is too large, it won’t “follow” the flow.

• For smoke: If the airspeed is too high, the smoke and air will mix and “blur”.



How to Make the Measurements



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.

Wind Tunnel Experiment Details



Wind Tunnel Experiment Details

Measuring Velocity

• Pitot-static tube

P = Ptotal - Pstatic

Bernoulli’s Equation: P = (1/2) V2, 1 kg/m3 (units!)

V = 2* P/

• Three-cup anemometer



BackgroundWhy Test in Wind Tunnels?



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



Two of NASA’s Wind Tunnels

Ames 80’ x 120’

Langley



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 = UL/



Selected ReferencesAerodynamics

1. Abbott, Ira A. & von Doenhoff, Albert E., “Theory of Wing Sections,” Dover Publications, 1959.

2. Anderson, John D., “Fundamentals of Aerodynamics,” McGraw-Hill, Inc., 2nd Ed., 1991.

3. Anderson, John D., “Introduction to Flight,” McGraw-Hill, Inc., 3rd Ed., 1989.

4. 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).

6. White, Frank M., “Fluid Mechanics,” McGraw-Hill Inc., 2nd Ed., 1986.

7. 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.

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