lectures on wind tunnel

7/28/2019 Lectures on Wind Tunnel

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The E80 Wind Tunnel Experiment

byProfessor Duron

Spring 2006

the experience will blow you away


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The Experiment

Objectives To familiarize the student with the basic operation and

instrumentation of the HMC wind tunnel

To examine the lift and drag forces on airfoil shapes in a flowfield, and obtain a qualitative relationship between lift anddrag coefficients

To understand the significance of the Reynolds Number, anduse Reynolds Number matching to utilize the wind tunnel for

modeling of a submersible design problem.


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How do planes fly?

Airfoils provide lift

But how?


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Bernoullis Equation


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The Bernoulli Effect

The air across the top of an airfoil experiences constricted flowlines and increased air speed relative to the wing.

According to Bernoulli, this results in a decrease in pressure alongthe top of the airfoil and provides a lift force.

So, lift is produced by a pressure difference across thewing.


6/27

Newtons 3 rd Law

Lift results from the angle of attack.

The boundary layer of air on the airfoil and resulting downwash of air behind the airfoil gives the air a downward force.

According to Newtons 3 rd Law, the airfoil experiences a force inthe opposite direction lifting the airfoil.

So, lift is produced by a reaction force on a body caused bydeflecting a flow of gas.


7/27

Bernoulli or Newton for Lift?

Many mis-apply Bernoulli and Newton's equations and over-simplify the description of the problem of aerodynamic lift.

The most popular incorrect theory of lift is known as the "equaltransit time" or "longer path" theory.

Wings are designed with the upper surface longer than the lowersurface, to generate higher velocities on the upper surfacebecause the molecules of gas on the upper surface have to reachthe trailing edge at the same time as the molecules on the lowersurface.

The error in this theory involves the specification of the velocity

on the upper surface. In reality, the velocity on the upper surface of a lifting wing ismuch higher than the velocity which produces an equal transittime.
http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html


8/27

Actual Lift is Complicated

For a gas, we have to simultaneously conserve the mass ,momentum , and energy in the flow.

Newton's laws of motion are statements concerning theconservation of momentum.

Bernoulli's equation is derived by considering conservation of energy.

The simultaneous conservation of mass, momentum, and energyof a fluid (while neglecting the effects of air viscosity ) aredescribed by Euler Equations .

If we include the effects of viscosity, we have the Navier-Stokes

Equations .
http://www.grc.nasa.gov/WWW/K-12/airplane/mass.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/conmo.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/thermo1f.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/airsim.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/eulereqs.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/nseqs.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/nseqs.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/nseqs.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/nseqs.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/nseqs.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/nseqs.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/eulereqs.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/airsim.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/thermo1f.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/conmo.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/mass.html


9/27

Lift and Drag

Drag resists the planes motion. A plane with lower drag can fly atfaster speeds than a higher drag plane with the same power.

Drag is influenced by the fluids viscosity or resistance to flow and bypressure differences within the flow field.

Lift and Drag are represented by nondimensional coefficients defined bythe ratio of lift or drag force and fluid kinetic energy.

where is the lift or drag force, measured in pounds or Newtons; is the fluid density (in slugs/ft 3 or kg/m 3 ), V is the average flow speed(in ft/s or m/s), and A is the area (ft 2 or m 2 ) normal to the force: eitherthe projected frontal area in the case of lift or the wing area for drag.

AV

F C D L D L 2

21

,,

D L F ,


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Downwash


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Experimental Fluid Dynamics

Osborne Reynolds (University of Manchester, 1883) discovered that, if the same atmospheric pressure was used for experiments with wind

tunnel models as a full-size airplane would encounter under actualconditions, the results would be invalid.

For the results to be valid,

the air density inside the wind tunnel must be increased by the sameproportion as the model is smaller than the actual airplane. Practically, if a model is 1/10th the size of a full size plane, the air density

(number of atmospheres) inside the wind tunnel or the flow velocity mustbe increased by a factor of 10 to get wind tunnel results that are valid inregular atmospheric conditions with a full size plane.

Two similarly shaped but different sized objectswould have the same aerodynamics as long as theReynolds Numbers for the two objects matched.


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What is a boundary layer?

Aerodynamic forces depend on the viscosity of the air. As anobject moves through the air, air molecules stick to the objectssurface.

A layer of air is created near the surface that is referred to as aboundary layer. This boundary layer, in effect, changes the shape

of the object since the flow reacts to the edge of the boundarylayer as if it was the physical surface of the object.

It is also possible for the boundary layer to lift off or evenseparate from the body creating an effective shape much differentthan the objects physical shape.

Boundary layers are very important in determining the lift anddrag of an object.

To determine and predict these conditions,aerodynamicists rely on wind tunnel testing and verysophisticated computer analyses.


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Reynolds Number

The Reynolds Number quantifies the relative significance of inertia (fluidacceleration) and viscous effects (e.g. drag force, or boundary layerthickness around an object).

If a model flow has the same Reynolds Number as the flow it is meant torepresent, the flow patterns and quantitative pressures and forces will beequivalent.

Reynolds Number is defined

where are defined as the fluid density, viscosity (a

measure of the fluids resistance to motion), the average fluid velocity,and some characteristic object length.

Vd Re

d and V ,,,


16/27

A sample calculation Boeing 747-400

Assume the following Maximum speed = 910km/h Wing Span = 67 m (typ. Area=4600sqft,chord length=21ft) Air temperature = -40C

Need to determine properties of air at specified temperature

http://users.wpi.edu/~ierardi/FireTools/air_prop.html http://www.onlineconversion.com http://www.engineeringtoolbox.com/24_162.html

(water properties will be needed for your design)

Reynolds Number associated with one wing

974.1

67*253*Re E

mskg

m sm

Vd

0.0000148

mkg1.520 3
http://users.wpi.edu/~ierardi/FireTools/air_prop.htmlhttp://www.onlineconversion.com/http://www.engineeringtoolbox.com/24_162.htmlhttp://www.engineeringtoolbox.com/24_162.htmlhttp://www.onlineconversion.com/http://users.wpi.edu/~ierardi/FireTools/air_prop.html


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18/27

Mach Number

Aerodynamic forces also depend on the compressibility of the air orfluid.

At low speeds (typically below 200 mph), the density of the fluidremains fairly constant. At high speeds, some of the objects energycompresses the fluid and changes its density and alters the resultingforce on the object.

Near and beyond the speed of sound (approximately 700 mph), shockwaves are produced affecting both lift and drag on the object.

The important similarity parameter for compressibility is the MachNumber defined as the ratio of the objects velocity to the speed of sound

cV

M


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Wind Tunnels (subsonic less than 250 mph)

Open Circuit Tunnels the entry and/or exit is open to the lab atmosphere simple to construct, but use of flow tracers (smoke) is limited difficult to maintain uniform velocity flow in the test section


20/27

Wind Tunnel (cont.)

Close Circuit Tunnels more uniform flow properties more challenging to construct and maintain


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22/27

Wind Tunnel Instrumentation

A standard pitot probe measures pressure difference.

A stagnation point exists at the nose of the probe where the flow velocity is zeroand a small pressure tap measures the pressure at this point.

Flow generally moves around the probe with some average speed andpressure .

A differential manometer measures the difference between the two pressureswhich varies according to Bernoullis equation

o p

V p

2

21 V p po


23/27

Dynamometer

A two component force balance used to measure lift and dragforces.

Forces generated by the model under test cause the deflection of two restrained cantilever beams (along the lift axis and the dragaxis). Measurements of the resulting deflections can be used to

estimate the applied forces. Linear Voltage Displacement Transducers (LVDTs) are used to

measure the beam displacements.


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Experiment - Tasks

Pitot probe and velocity calculations verify fan speed air velocity relationship

Measure lift and drag forces on an airfoil determine optimal lift/drag ratio determine stall angle confirm experimental results using FoilSim

Determine the airfoil area required to maintain depth for asubmersible during operation.

use Reynolds Number to match operating conditions conduct wind tunnel testing on an airfoil to compute required

lift coefficient


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Safety

Follow the Dress Code for E80 Lab Never turn the FAN on without

Making sure the article under test (AUT) is securely fastenedinside the test chamber

Checking to see that no loose objects are in the test chamber Securing the test chamber cover plate Making sure all test personnel are at a safe distance from the

wind tunnel itself (at least 24 in any direction) Making sure the vent is clear

Do not run the FAN at speeds higher than the posted limit. Use common sense when working in the wind tunnel facility.


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Acknowledgements

Dr. Jennifer Rossmann Jake Pinheiro NASA web sites

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