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NASA Glenn Research Center

Icing Branch - Van Zante / Dynacs

Wind Tunnel Exper iments for Grades 8 - 12

Wind Tunnel Exper iments

for

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 Experimen ts






Ideas fo r Wind Tunnel Exper iments

Model: A irfoi l o r 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 onthe leading edge (sand paper, paper mache) tomimic ice accretion, bug splat, etc... This shouldreduce max lift & increase drag.






Wind Tunnel Test Sect ion w ith A ir fo i lMount ing Opt ions

Airfoil on Sting Wall-Mounted

Flow






Li f t vs. Ang le of A t tack

As the angle of attackincreases, so should the lift- until a certain point (the

stall angle of attack).

Angle of attack (a):

angle between flow andchord line.

Chord line:

straight line between mostforward and most aft points

a

a

Lift

Flow






scale

L if t vs . Ang le (con t.)

Angle

L i f t

Visual: See airfoil lift

as angle increases

Measure: airfoil lift as

a function of angle






Wind Tunnel Exper imentLi f t vs. Ang le Worksheet

angle Lift(deg) (oz)

0 0






L if t vs. Veloc i ty

As the velocity (speed)increases, so should the lift.

Note: Keep the angle ofattack constant.

The greater the angle (priorto stall) the greater thechange in lift.

Lift

Velocity

(Speed)






L if t vs . (Veloc i ty) 2

Velocity

L i f t

Visual: See airfoil lift

as speed increases

scale

Measure: airfoil lift as

a function of speed

V2

L






Wind Tunnel Exper imentLi f t vs. Veloc i ty Wo rksheet

DP Wt Vel V^2 Lift

(oz) (m/s) (m/s)^2 (oz)

0 0 0 0






Ideas fo r Wind Tunnel Exper iments

Model : Drag Body

Double Elimination Competitions

Build two objects. In a head-to-head comparison,see which one has the least drag.

Which w ay w i l l the object w i th the most d rag

move?

– Race Cars

– Geometric shapes






Wind Tunnel w ith Drag ObjectsMount ing Opt ions

Bluff Bodies Race Cars

Rotating Sting Pulley






Ideas fo r Wind Tunnel Exper iment

Model - Drag Body

Notes:

– The frontal area (the side facing the flow) must be thesame. 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 twopulleys.






Governing Equat ions






Governing Equat ion s

Lift & Drag are equal to the

Dynamic Pressure * Surface Area * Coefficient

These Coefficients are a function ofAngle of Attack, Model Geometry & Mach number

D

2

L

2

C *S *V 2

1 D

C *S *V

2

1 L






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 (a, model, Ma)

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






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 (), e.g . air to water, can also be a

variable.During the course of one exper iment, i t is

important to only change one var iable at a t ime.

Governing Equat ion

Notes






Flow Visualizat ion

Techniques






Flow Visual izat ion Techniques

Flow Visualization illustrates the flow on

or near the object. On the surface, regionsof reverse flow become visible.

• Yarn Tufts, Tuft Probe, Tuft Grid

• Smoke Wand, Smoke Wire• Trailing Edge Cone (String & paper cone)







Yarn

• Yarn Tufts - tape ~1” segments of yarn directly to the

surface.

• Tuft Probe - tape ~3” light-weight (and visible) string to endof 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 surface

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

Tuft Probe

Trailing Edge Cone

Flow Visual izat ion Techn iques

Il lustrated

Tuft Grid







Caut ions

• 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 Exper iment Detai ls






Wind Tunnel Exper iment Detai ls

Measuring Velocity

• Pitot-static tube

DP = Ptotal - Pstatic

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

V = 2* DP/

• Three-cup anemometer






Background

Why Test in Wind Tunnels?






Why Test in Wind Tunnels?

The Ultimate Goal : to Understand the FluidMechanics 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 est imates )

• 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/m





Selected References Aerodynamics

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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