Building a Wind Tunnel: It Will Blow Your Mind

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Building a Wind Tunnel It Will Blow Your Mind!

• Tom Carlone Ben Goldberg

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Why Wind Tunnels?
In a society that is growing dependent on
computers and always moving towards new
technologies, the use of wind tunnels to solve
aerodynamic problems may seem obsolete.
When aerodynamic problems can not be solved on a
computer flow visual offers the ability to
observe the air in many different scenarios.
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Why Wind Tunnels for Our SMP?

• Interest in flight
• Hands on experience
• Introduction to aerodynamics
• Engineering skills

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The Beginning of Powered Flight

• Wright Brothers tested airfoils in gasoline
  powered wind tunnel - build Wright Flyer in 1901
• Used the wind tunnel to measure lift and drag of
  over 200 different airfoils
• In fact, the accurate wind tunnel data we
  developed was so important, it is doubtful if
  anyone would have ever developed a flyable wing
  without first developing this data. Sometimes the
  non-glamorous lab work is absolutely crucial to
  the success of a project. www.wrightflyer.org

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How did the Wright Brothers do it without
computers?!
Multi-axis balance used to measure lift, drag,
yaw, and torque individually
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What did the Wright Brothers learn?

• Curved surfaces produce more lift than flat
  surfaces. The greater the curvature the greater
  the lift.
• Curved surfaces also produce more drag than flat
  surface.
• Amongst curved surfaces, parabolic curves (those
  with the highest camber nearer to the leading
  edge) have better performance than circular arcs.
• Long thin wings (high aspect ratio) have better
  performance than short wide wings (low aspect
  ratio).
• For many of the wings tested, the highest lift
  does not occur at the greatest angle of attack
  the lift peaks at a low angle of attack and then
  decreases. (stall)
• Curved wing tips produce lower drag than
  rectangular wing tips.

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Comparing the lift coefficient and stall angles
of wings with and without flaps and slats.
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How did we get here and where are we going?
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Computational Fluid Dynamics (CFD)

• Computers used to perform calculations to
  determine how surfaces will act in a fluid
• Wind tunnels are used in combination with CFD
  simulations

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Design process with computers
Design process before computers
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A Quick Lesson on Aerodynamics

• Key Terms
• Lift
• Drag
• Stall
• Angle of attack
• Leading Edge
• Trailing Edge
• Sweep Angle
• Symmetrical Airfoil
• Semi-symmetrical
• Non-symmetrical
• Camber
• Dihedral

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LiftDrag Ratio

• This ratio is an important value in aerodynamics
  there are many applications of this value
• maximum range of propeller-driven airplanes
• maximum climb ratio for jets
• maximum power off glide ratio

Glide ratio 451
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Wind Tunnel Design
Open Loop Wind Tunnel
5 Main Components Settling Chamber Contraction
Cone Test Section Diffuser Drive Section
Closed Loop Wind Tunnel

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Reynolds Number

• Compares the inertial force of a fluid to the
  viscous force
• Air has mass like an object
• Also has fluid/viscous properties
• In order to have a direct comparison, the
  Reynolds numbers need to be the same

144 scale
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Flow Visualization

• Smoke
• Delivered through a wand to observe the airflow
  around a test object
• Can detect eddies and vortices which disturb the
  airflow
• Can show a laminar/smooth flow around an object
• French Chalk
• Liquid mixture of kerosene, talc and oil which is
  applied to test object
• Leaves a residue showing the surface streamline
  patterns
• Tufts
• Black or white low-density yarn
• Arranged in a grid pattern after to detect
  eddies/vortices
• Arranged in a grid pattern on the surface of the
  test object to see surface flow

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The practical uses

• Design more efficient cars
• Design better planes
• Design structurally sound buildings
• Understand aerodynamic phenomena

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Applications of wind tunnels
Wind patterns Example Wind accelerated over a
river
Cities Cars Bicycles Planes Trains
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Our Wind Tunnel!
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Plan, prototype, design, build, troubleshoot and
test

• Things to keep in mind
• Time 15 weeks
• Size reasonable to transport
• Materials accessibility of parts
• Cost budget lt500
• Functionality able to move air over a test
  object between 0-15mph
• Intent observe basic aerodynamic concepts using
  flow visualization
• Testing - Test various objects to analyze the
  designs and aerodynamic properties

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Prototyping and using CAD

• Paper models
• Foam core model
• CAD models

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Contraction Cone
Take a large volume of low-velocity air and
reduce it to a small volume of high-velocity air
3 Options -Cubic Function -Trapezoidal -Circular
Target contraction ratio 51
Actual contraction ratio 4.81
Efficiency vs. Ease of Construction -designed in
conjunction with honeycomb -boundary separation
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Settling Chamber
Straightens the airflow
LOTS of options -aluminum honeycomb -light
diffuser -packaging honeycomb -straws
MythBusters used straw method
Length of honeycomb should be 6-8 times cell
diameter Over 3,000 straws!!
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Testing the Straws

• Does the honeycomb work?
• Restricts airflow, reducing efficiency
• Draws more current, limits top speed
• Eliminates swirling motion
• Straightens airflow

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Flow Visualization

• Low-Density string/tufts
• Burning Mineral Oil
• Incense Sticks
• Smoke-in-a-can
• Dry Ice Sublimation

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Test Section
Airfoils, cars and other objects are placed in
test section to test and observe various
aerodynamic aspects
Looks easy! Its just a clear rectangular prism-
it should have been easy to build! Right? Wrong
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Diffuser
Connects the test section to the drive section.
The air slows down due to the shape of the
diffuser.
Specifications -Sheet Metal -13.75 x 13.75
square -gt 25 diam. Circle transition
Target angle of expansion is between 5 and 10
degrees
Actual angle of expansion 6
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Drive Section aka The Fan
Provides the force that causes air to move
through the tunnel.
Specifications -FlowPro 8400 cfm -25 cooling
fan -Completely variable speed using AC motor
dimmer switch
Normal testing velocity 5mph
Max testing velocity 20mph
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Schematic of Fan Control System
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Testing with our wind tunnel
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Bernoulli Effect

• Faster moving fluids create areas of low pressure
• Fluids move from areas of high pressure to areas
  of low pressure

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Curveball Simulation
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Different Types of Balls

• Different surfaces interact differently with air
• Have varying wakes behind the different balls
• Fuzz on tennis ballsmore drag
• Ability to curve the ball

http//www.nasa.gov/centers/ames/news/releases/20
00/00images/tennisball/Tennispage.html
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Smooth Ball vs. Golf Ball

• Flow Separation causes drag
• Dimples cause a turbulent flow
• Turbulent flow reduces separationless drag
• Golf ball travels farther

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Bernoulli Effect on an Airfoil

• Air moves faster over the top of the wing, it
  travels a longer distance in the same amount of
  time
• Creates area of low pressure
• The higher pressure air below the wing tries to
  move towards the low pressure area, pushing the
  wing up
• http//www.youtube.com/watch?v6UlsArvbTeofeature
  related

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Airfoil
Controls -Ailerons -Flaps -Airbrake/elevator -Ang
le of attack
Flaps Down
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Why does an airplane fly?

• Two main theories
• Bernoulli's Principle
• Air above the airfoil travels a longer distance
  in the same amount of time (must be going faster)
• Only valid for non-symmetrical airfoils
• Newtons Third Law
• For every action there is an equal and opposite
  reaction
• For planes flying with a significant angle of
  attack
• Air has mass, wing pushes air down, air pushes
  wing up

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Stalled Wing

• On a normal airfoil, the plane will stall at an
  angle of attack of about 15 degrees
• Lowering flaps will allow the wing to reach a
  higher angle before stalling
• The flow will separate (no longer a laminar flow)
  and will begin to swirl creating a loss of lift
• As the angle of attack becomes large, the flow
  tends to separate from the top surface of the
  airfoil creating a large wake of relatively dead
  air behind the airfoil

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Air Foil Tested at Different Angles of Attack
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Flat airfoils

• A flat airfoil is not the best design for
  efficiency/gliding purposes
• According to Newtons 3rd law, a flat airfoil
  will still create lift
• Needs to fly with an angle of attack
• However flat airfoils stall at a lower angle of
  attack

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A flat airfoil
Vortex is produced at a flat, triangular wingtip
Produces lift but stalls at a lower angle
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Wingtips

• Low pressure on top of wing
• High Pressure on bottom of wing
• Travels from high to low pressure and swirls as
  it is bush backward, creating the vortex

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Micro Flyer
Has a camber to the wing, similar to original
Wright Flyer
Wing tips are curved to reduce vortices, which
reduces drag and improves efficiency
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Whats Drag?

• Pressure drag
• Flow separation causes eddies, creating areas of
  high velocity air, thus a lower pressure
• Air moves from high to low pressure, resisting
  the objects forward movement

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BMW Lemans

• Designed with wind tunnel testing
• Focus on down force
• Streamlined to minimize drag
• Low to ground to reduce turbulence

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McLaren Formula 1

• Down force on front and rear fin
• Increase in disturbed air behind car
• Extra focus on down force on rear fin
• Designed to reduce drag

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Pickup Truck Better gas mileage with the hitch
down or up?

• Air trapped in back, creates smooth flow over top
  of truck

• Without pocket or air, there is more contact and
  turbulence created

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Tractor Trailer
Restores smooth laminar flow saves 3434 gallons
of fuel per year at 65 mph
Air gets pulled down into crevasse between cab
and trailer creates drag
National research council in Canada
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Professional solution
Specially designed airtab
airtab is a part designed by NASA using wind
tunnels that creates two controlled vortexes of
air. By making strong controlled air currents it
can direct air over the gap or past the back of
the truck to reduce air resistance.
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A Brief Overview of what weve Talked About

• http//youtube.com/watch?vz2LsalyVq68

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Wind Tunnels Still Serve an Important Role

• Today wind tunnels are often combined with
  computers to form a strong combination that is
  excellent at solving aerodynamic problems.
• Engineers use wind tunnels to design, create and
  test new shapes for planes, trains and
  automobiles to improve safety, efficiency and
  productivity.