science buddies_ how to build and use a subsonic wind tunnel

8/9/2019 Science Buddies_ How to Build and Use a Subsonic Wind Tunnel

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30/1/2015 Science Buddies: How to Build and Use a Subsonic W ind Tunnel

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Figure B.The Wright Flyer. (Wikipedia, 2009.

Today, wind tunnels are used by NASA, Boeing, Northrop Grumman, and every other organization that makes

aircraft and spacecraft. In fact, NASA AMES, in Moffet Field, California, has the most wind tunnels at any one

location in the world, and also has the largest wind tunnel on Earth! Wind tunnels are also used for educational

purposes. For instance, schools use them to demonstrate how planes fly. They are also used in research projects,

as students design aircraft models and test their performance.

Figure C.NASA AMES. The prominent structure in the foreground is the world's largest wind tunnel. (Pincta-cal,

2008.)

Science Buddies is proud to offer this how-to guide, which provides the design for an educational wind tunnel that

you can build yourself. Though it's certainly not big enough or sophisticated enough to study a space shuttle or the

new Boeing 787 Dreamliner, it is excellent for classroom use and for science fair projects. The design has been

rigorously tested, and while you will learn the precise method for building a working wind tunnel, we thought it also

important to share the obstacles we encountered along the way, so you can avoid the same pitfalls. In addition,

once you've completed the wind tunnel, Science Buddies has several aerodynamics-related Project Ideas, which

require the use of a wind tunnel, so be sure to look those up and try them out when you're ready for testing! Here are

a couple of examples: Which Wing Design Creates the Greatest Lift? (http:// www.sciencebuddies.org/science-fair-

projects/project_ideas/Aero_p004.shtml) and Winglets in Wind Tunnels (http:// www.sciencebuddies.org/science-fair-

projects/project_ideas/Aero_p012.shtml)
http://www.sciencebuddies.org/science-fair-projects/project_ideas/Aero_p012.shtmlhttp://www.sciencebuddies.org/science-fair-projects/project_ideas/Aero_p004.shtml


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Figure D.This is an example of an educational wind tunnel used on a college campus, which is over 60 feet long

and 10 feet high. (UC Davis Wind Tunnel Facility, 2009.)

Before you get started, there are a few basics to learn about wind tunnels. First, there are two main types of wind

tunnels: closed-circuit and open-circuit. A closed-circuit wind tunnel is a very large and expensive type of wind

tunnel. It gives engineers and scientists the greatest control over the flow of air, and produces the most efficient

and precise results when models are tested. The wind tunnel you will build is not a closed-circuit wind tunnel

because this would be very expensive and difficult to make without sophisticated equipment and materials. Instead,

you will be making an open-circuit tunnel, which is more like a tube that is open at both ends (see Figure D). While

this design doesn't allow you to have completecontrol of the airflow, it is less expensive to build and run, it can bemade using readily available materials, and it is an effective design for educational purposes.

Second, you need to understand just how an open-circuit wind tunnel works and wht it measures. The wind tunnel in

this project has a fan, which will draw air into the tunnel and pass it over a model, and then release the air out of the

other end. When the air flows around the model, it simulates flight. The model pulls on sensors, which will tell you

how well it pulls up (lift) and how much the air blows it backward (drag). These two measurements are what you are

most interested in as an aeronautical engineeryou want to know how well your plane lifts itself (stays in the air)

and how well it cuts through the air and resists the force of the air pushing back against the plane (how fast it

travels). Lift measurement gives you the force of upwardmotion of the air on the model, and drag gives you the

force of backwardmotion of the air on the model. Basically, a plane is only able to fly because it can control theway that the air pushes on it. This is why a wind tunnel is importantyou want to study how air moves around your

model before you actually build the plane and send it into the sky.

Third, it is important to know that the wind tunnel fan is similar to the engines on a plane. The engines push the

plane forward, which makes wind pass over the wings. The fan draws air backward, so that it passes over the wings

in the same way as if the model were propelling itself forward. The proper way to describe this is that the fan

simulates thrust.

Fourth, wind tunnels are not perfect simulations of environmental conditions. The degree to which they are flawed

representations of actual flight is measured by a calculation called Reynolds Number. The wind tunnel which you

can build using this how-to guide is a student-quality, high school budget tunnel, and so it is important for you to

understand that the data and results it yields are for instructional and educational purposes only. It is possible that

the data will disagree with established data-in order to find out just how inaccurate the tunnel may be, you should

research the concept of Reynolds Number and determine what the RN is for your wind tunnel.

Finally, you should know the five basic parts of the wind tunnel. From front to back, they are: The Settling Chamber,

the Contraction Cone, the Test Section, the Diffuser, and the Drive Section. (see Figure E).


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Figure E.Diagram of an open-circuit wind tunnel. Although the diagram has many different numbered items, you

only need to be familiar with the few mentioned here.

The Settling Chamber(number 12 in Figure E) is at the very front of the wind tunnel, and is made up of screens and

honeycomb-shaped mesh, which straighten out the air and reduce turbulence. The Contraction Cone (numbers 11

and 13 in Figure E) forces a large volume of air through a small opening in order to increase the wind velocity in thetunnel (for more on this, study the continuity equation). The Test Section (number 10) is the place where a model is

mounted on sensors. The Diffuser(number 8 in Figure E) is at the end of the Test Section, and keeps the air

running smoothly as it goes toward the back. It also increases in volume in order to slow the air down as it exits the

tunnel. Finally, the Drive Section (number 7 in Figure E) is at the very back of the wind tunnel, and it is where the

fan is housed. At first, it might seem odd that the fan is at the backof the tunnel, facing outward, instead of at the

front but this is actually the best placement, because it will draw air into the wind tunnel by blowing air out of it.

Drawing air in is better than blowing air in, because it doesn't produce as much turbulence, and it allows for greater

control of the airflow through the tunnel.

This introduction provides only a glimpse into the field of aerodynamic design, so before you begin building the wind

tunnel, you may want to do additional research about basic aerodynamic principles. At the end of this guide is a

bibliography of the sources consulted when this design was made, so make sure you check out at least a few of

them before you get started. We highly recommend taking a look at The Illustrated Guide to Aerodynamics, 2nd

Editionby H.C. "Skip" Smith NASA's Beginner's Guide to Wind Tunnels website (http://www.grc.nasa.gov/WWW/K-

12/windtunnel.html) and the Wind Tunnel Design website (http://www-htgl.stanford.edu/bradshaw/tunnel/index.html). If you can find a

copy, also check out Low Speed Wind Tunnel Testing, 3rd Editionby Barlow, Pope, and Rae. This book might be

difficult to find unless you use a local university library. Since you are building an open-circuit wind tunnel, you don't

need to study anything that these resources say about closed-circuit wind tunnels.

Important Notes Before You BeginThis design is for an open-circuit, closed-test-section wind tunnel that is about 8 ft. long, and can be taken

apart for storage and transportation, such as for display in a science fair.

If you run into any problems or need any extra help beyond the instructions of this guide, visit our SciBud

Wind Tunnel Questions (http://www.stage.sciencebuddies.org/science-fair-projects/phpBB3/viewtopic.php?f=2&t=5002), set up just for

this Wind Tunnel How-to Guide.

This wind tunnel has three basic parts:

Test Section (#testsection)

Diffuser(#diffuser)

Cone (#contractionconeContraction)

Key terms that you should research before you begin:

Angle of attack

Assemblies (engineering context)

Closed-circuit wind tunnel
http://www-htgl.stanford.edu/bradshaw/tunnel/index.htmlhttp://www.stage.sciencebuddies.org/science-fair-projects/phpBB3/viewtopic.php?f=2&t=5002http://www.grc.nasa.gov/WWW/K-12/windtunnel.html


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Contraction cone

Diffuser

Drag

Drive section

Lift

Open-circuit wind tunnel

Parts (engineering context)

Test SectionThrust

Settling chamber

Variable speed drive section

Voltmeter

The materials you will need to gather to build your wind tunnel are discussed in the next tab (#materials).

Cost

A key element t o consider when building this wind tunnel is the cost. The Science Buddies wind tunnel design cost

about $275. This price estimate is the result of shopping around for the least-expensive products available thatwould still work properly in the wind tunnel. Another cost-cutting idea is using scrap wood, so make sure you collect

all of the unused wood you can before you start buying parts for the wind tunnel. The two most-expensive

components are the Drive Section fan and the Plexiglas sheet(s) used to build the Test Section.

Materials and Equipment

Material cost is around $250$300, but you could ask your school to help pay for it.

If available, you can use force sensors and equipment found in your physics classroom.

The next thing to think about is the list of materials. Nearly everything you'll need to buy will come from a local

hardware store. The materials for the Science Buddies wind tunnel (see Figure A) were almost exclusively bought

from Lowe's Home Improvement, The Home Depot, and a locally based hardware store, but they are readily

available and can be found at a variety of locations.

First, review the following materials list and see if you have any of the listed parts already (such as scrap wood,

screws, etc.). Make sure that you use whatever good materials you and the school already have so that you don't

have to spend more money than you need to spend. For instance, you will need to use two force sensors, which

your school's physics department most likely already has. Most high school force sensors cost about $100 each, so

it is best to use whatever your school already has. If your school doesn't have any sensors, you will have to buy

them.

Here is a basic list of the materials needed, with cost approximations:

Primary Materials

Material Quantity

Estimated

Cost of

Total

Quantity

Notes

Plexiglass sheet,

2' x 4' 1

Low ($20-

$50)

Plywood board, 4' Low ($20- "


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x 8' $50) , .

Plywood board,

17" x 17" 1

Low ($20-

$50)

This will most likely have to be cut from a larger board

although unusual, the dimensions are based on

calculations and must be used for best results,

thickness should be as great as possible.

Fan with blade

diameter of 14 in.

and entire-fan

diameter no

greater than 17 in.

1 Average

($50-$100)

For best results, CFM rating should be between 1600

and 2000, and fan should be designed for gable

mounting.

Oak boards, 2' x

4' 4

Low ($20-

$50) For best results, thickness should be greater than 1/4".

Egg crate mesh

board 2

Low ($20-

$50) Basically an array of deep cells (see Figure A).

Wooden lattice,

1.5" x 1/4" x 8' 1

Very low

(under $20)

Wooden corner

guard, 8' 1

Very low

(under $20)

Roll of aluminum

screen 1

Very low

(under $20) About 36" x 48"

Small handheld

personal cooling

fan

1 Very low

(under $20)

Cut of wire sheet,

enough to cover a17" x 17" area

1 Very low

(under $20)

This will block the exit of the fan so that children do not

put their hands into the fan. Make sure that the wire cellsare small enough to prevent fingers from getting through.

Adhesive rubber

gasket stripping,

at least 8 ft.

1

Very low

(under $20)

to low ($20-

$50)

This will be used at the openings of the three assemblies

to ensure that air does not escape. It should be an

adhesive gasket stripping.

Springs 8 Low ($20-

$50)

Medium-duty coil springs, 3 ft. or so, with closed loops

at each end (you will be latching each end onto hooks).

With

permission,


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materials

school or

someone

you know

Electrical wiring

materials

With

permission,

borrow fromschool or

someone

you know

Small L-brackets

with two screw

holes

8 Very low

(under $20)

Small drawerhandles

4 Low (under$20)

Scrap wood

With

permission,

obtain from

school,

home, or

someone

you know.

This will depend on what you have, or what your school

or someone you know has available. If you can't obtain

any scrap wood, you can always buy extra pieces of

wood. You will use the scrap wood to make stands and

bases for your wind tunnel.

Secondary Materials

Material Quantity Estimated Cost Notes

Saws

With permission,

borrow from school,

home, or someoneyou know.

The types of saws you use is up

to the carpenter/woodshop

teacher who will be helping you.

Drill 1

With permission,

borrow from school,

home, or someone

you know.

Wood glue 1 bottle Very low (under $20)

Screws, wood screws, nuts,

bolts, washers, lock washers,

screwdrivers, related materials

With permission,

obtain from school,

home, or someone

Again, the number and ty pe of

these materials you need will be

up to your carpenter/woodshop


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. .

Nail/Staple gun 1

With permission,

borrow from school,

home, or someone

you know.

Silicone 1 tube Low (under $20)

Try to find a special type that is

intended for use with clear

plastics.

Construction sealant 1 tube Low (under $20)

Measuring tape 1

With permission,

borrow from school,

home, or someone

you know.

Zip ties At least

20 Low (under $20)

Safety goggles 1 pair

With permission,

borrow from school,

home, or someone

you know.

Work gloves 1 pair

With permission,

borrow from school,

home, or someone

you know.

Ear plugs 1 pair Very low (under $20)

School-Donated Equipment


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Figure 1A. The full Test Section Assembly.

The Test Section Assembly is composed of the Test Section (Plexiglas) and the Test Section Base (wooden stand

and sensor mounts). Make sure you take a look at these two major features in Figure 1A before you move on.

For this assembly, you will need the following parts:

Plexiglas sheet

L-brackets

Screws

Washers/lock washers, bolts

Drill

Silicone

Scrap wood

Before you begin this section, you need to know what type of sensors you will use to collect force data, and you will

need to work with your carpenter/woodshop teacher to design the proper base, which will depend on what kind ofscrap wood you have available, or how much more wood you need to buy. This how-to guide, for the Science

Buddies wind tunnel, uses two Dual-Range Force Sensors, made by Vernier (http://www.vernier.com) (see Figure 1A).

Part A: Plexiglas Test Section

1. The first step of building this assembly is to cut the large Plexiglas board into four smaller boards. The

Plexiglas portion of the Test Section will be 1' x 1' x 2', so you need to cut the 2' x 4' sheet into four 1' x 2'

sheets. This can easily be done at your local hardware store where you purchased the Plexiglasyou might

even want to have it cut the same day that you buy it, to save time. In the Plexiglas products section of the

store, there should be a cutting station, which usually cuts Plexiglas for free. Bring your board to this station

and make sure that the cutter understands that you need four boards, each with dimensions 1' x 2'.

2. Next, you will connect the four boards together using silicone to make a rectangular prism (the Test Section

part of the Test Section Assembly). You should use a special type of silicone that is best for clear plastics,

available at your local hardware store. This step can be very tricky, so make sure that you work with a
http://www.vernier.com/


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time for the boards to stick together. With your helper, make the Test Section, and leave it standing

on one of the two ends in a safe place for 1224 hours (depending on how long the silicone needs to

fully dry check the label on its container). Once it has fully dried, you can set it on its long sides, as

in Figure 1C. The silicone holds the boards together, but is very pliable and is not very strong, so do

not flex the boards or pick at the silicone. See Figure 1C for a picture of what your Plexiglas Test

Section should look like.

Figure 1C.The Plexiglas Test Section, held together only by silicone.

8. b. Since the silicone is so weak, it is necessary to reinforce the Test Section. To do this, you will screw

corner brackets into the Test Section, at its eight end corners, in order to keep it together and to give

it greater structural integrity. Note that you will need a drill, eight corner brackets, screws, nuts, and

small lock washers. Make sure you let the silicone dry before you reinforce with brackets, as

described in step 8.

9. With your mechanically inclined helper's assist ance, mark the points on each board of the Test Sectionwhere holes will be drilled. The holes should be rather close to the edges of the Test Section, should all have

the same measured distances from the edges, and should all be in the proper places so that corner brackets

can be screwed onto the Test Section. There will be 16 holes in allfour on each of the boards. Pilot (test)

holes should be drilled first, for accuracy, followed by holes large enough that screws can fit in.

10. Once all 16 holes have been marked and drilled, it is t ime to attach the corner brackets. This part could be

tricky, because you half of the screws' rounded heads will be inside the Test Section, while the other half will

be outside of the Test Section. Specifically, the screws heads on the left and right sides of the Test Section

(when it lies on its 2-ft. dimension) will be outside of the Test Section, while the screw heads on the top and

bottom will be inside the Test Section. Although this may be confusing, it is actually very simple once you

define which boards of the Test Section are the top, the bottom, and the two sides.

a. If you are wondering why the screws need to be put into the Test Section two different ways, it has to

do with the way that the three assemblies will connect together in the end to make the full wind tunnel

assembly. The screws with their heads on the outside of the Test Section need to be on the sides of


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Figure 1E.Test Section fully reinforced by corner brackets. Edge-on view of an attached corner bracket.

Once you have completely connected the four Plexiglas boards together, you have finished the Test Section part of

the Test Section Assembly. Now you must make the Base of the Test Section Assembly.

Part B: Base of the Plexiglas Test Section

1. This part of the wind tunnel is dependent upon how much scrap wood you have available to you, and what

kind of force sensors you will use. Talk to your mechanically inclined helper about what design you should go

with, and figure out the best way to build it. What you absolutely need is a Base that will strongly support the

Test Section, and that will be large enough that it cannot easily tip over. Because of this, you will most likely

want a board for the Base with dimensions of about 1.5' x 2' so that it is just as long as the Test Section, but

slightly wider. You also want to have two legs that connect the Test Section to its Base, and they should be

near the front and back ends of the Test Section. Refer back to Figure 1A.

2. Before you attach the Test Section to its Base, you need to make a decision. You will be running a rod, or

some sort of mounting object, through the bottom board of the Test Section, in order to mount a model. This

object will attach to the force sensors underneath it. Decide on what kind of hole you need in the bottom ofthe Test Section, through which your object will be inserted, and decide if it will be easier to make the hole

before or after the Test Section is attached to the Base. For the Science Buddies wind tunnel, we used a

threaded rod with a small diameter, so a small circular hole was drilled in the bottom of the Test Section.

Whatever you use, make sure that the hole is large enough so that the rod does not touch the sides of the

hole, even when the fan is turned on. This will disturb the sensor readings and produce inaccurate results.

3. Also, y ou need to figure out the best way to attach your force sensors. You will need twoone for lift and

one for drag. You absolutely must have right angles (as perfect as you can make them) between the drag

sensor and your connection rod, so that no component of the lift force affects the drag measurement. See

Figure 1A for the example from the Science Buddies wind tunnel. Notice how one sensor is mounted so that

the rod comes straight up out of it (lift sensor) and the other is mounted so that its rod is perpendicular to theother force rod (drag sensor).

a. As you can see from Figure 1A, the Base was made so that it has two smooth rods onto which the

sensors can easily clamp. In the same way, you and your helper must decide on the best way to

mount the sensors you have, andmake sure that they will be completely perpendicular so that you


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can get the most accurate readings possible. You must also make sure that the long objects you use

to mount the model are very rigid and will not bend or move around in the wind, because this will also

alter your readings.

4. Finally, you need to figure out a way to mount your model so that it can be given an angle of attack in order

to be tested from more than just the fixed mounting position. For the Science Buddies wind tunnel, the rod

that attaches to the model is bendable so that its angle can be measured before it is inserted into the Test

Section and a model is mounted on it.

Building the Diffuser Assembly

This assembly houses the fan and the wind speed sensor. It is perhaps the easiest of the three assemblies to build.

It will look like the following:

Figure 2A. The full Diffuser Assembly.

This is the largest assembly, and it is the only one that is made up of electronic components and wiring, so you will

need the help of your physics teacher or an electrician, as well as a carpenter or woodshop teacher. It is composed

of the Diffuser, the Drive Section (fan), Drive Section Control (fan switch), Wind Speed Sensor, and the DiffuserBase. Make sure that you identify these components in Figure 2A before moving on.


4' x 8' plywood board

17" x 17" plywood board

Gable-mounted fan

Electrical wiring materials (for an on/off switch)

Small handheld cooling fan

Wire sheet (17" x 17")

Rubber gasket, 4'

Wood glue

Wood screws, washers, lock washers, bolts

Drill


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Nail gun

Scrap wood for the Base

Drawer handles (2)

As with the Test Section, you will need to work with your carpenter/woodshop teacher to design the proper base for

this long assembly. Again, it depends on what kind of scrap wood you have available, or how much more wood you

can buy.

Part A: Drive Section

The first part of the Diffuser Assembly's construction is the Drive Section. This is composed of the fan and the 17"

x 17" board. The board is 17" on all four sides, based on an aeronautical ratio given in Low Speed Wind Tunnel

Testing (Barlow, Pope, and Rae, 1999). The area of the Drive Section (the large end of the Diffuser) should be 23

times that of the area of the end of the Test Section (also the area of the small end of the Diffuser), with best

results if the ratio is 2:1. The area of 17" x 17" is almost exactly twice that of 12" x 12" (289 square inches: 144

square inches is almost equal to 2:1), which is why the board should be 17" x 17". The board should also be rather

thick in order to hold the fan tightly and to keep the Diffuser stable.

1. The board will be the back end of the Diffuser Assembly, so the fan needs to be mounted to it. First, you

need to cut a circular hole in the board that has a diameter just about that of the fan blades, so that all the air

blown by the fan goes through the hole. The hole needs to be centered on the board and should be as circular

as possible.

Figure 2B. You can see that the hole is large enough so that all blown air will go through the circular hole.

2. Next, place the fan face down over the hole to make sure it fits properly, and screw it into place (using

washers) so that it is firmly attached to the board. Once you have done this, you have completed the Drive

Section assembly of the wind tunnel. The Drive Section will serve as the foundation for the rest of the

Diffuser. It is easiest to build up from the Drive Section, so leave it on your work table, face down. The fan

will blow outof the wind tunnel, not into it, which is why the Drive Section is at the back end of the wind

tunnel and why you will build up from the face-down Drive Section.

Part B: Diffuser Body

1. Next, have your 4' x 8' board cut into four pieces. The pieces need to be equal, trapezoidal shapes. One base

of the trapezoid should be 17" in length and the other should be 12" in length. Have your carpenter/woodshop

teacher help you. Make sure to check back to Figure 2A (#Figure2A)if you are confused. These boards will form


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Figure 2D. Run the wires from the fan through a hole in one of the Diffuser walls.

You should use silicone to glue it in place and seal any possible openings through which air could escape.

5. Attach the rest of the boards. When you have glued and drilled them all into the Drive Section, you will have

something that looks like the Diffuser of Figure 2A (#Figure2A), but without the four boards attached to each

other.

6. Wait for the glue to dry before you continue. Your next step is to attach the four boards together so that youhave an enclosed and airtight Diffuser body. This will be a little tricky because you will have to bend the

boards a little, and hold them together. You will use wood glue and a nail gun, so make sure that your

carpenter/woodshop teacher is careful when firing nails into the boards. Your goal is to attach them so that

you have a square hole at the top, with the same dimensions as the Test Section's end, 12" x 12".

7. Begin by recalling the way that the Plexiglas boards were connectedyou used silicone to attach the edge of

one to the face of the other. In the same way, you will attach the edge of one board to the face of the other.

You'll also need to make sure that you have one dimension of the 12" x 12" opening slightly longer than the

other, just as with the Test Section, so that they fit properly together.

In order to do this properly, you need to clearly define which boards are the sides, and which are the top and

bottom of the Diffuser. This should be easy to remember because the board with the hole and wire tube will

be a side board, so keep that in mind. Make sure that the square hole will be flush with the Test Section's

hole, so that the air will have a smooth transition as it passes from the Test Section to the Diffuser.

8. Begin closing the Diffuser by running a line of glue along the edge of one of the top or bottom boards. Then,

before all the glue runs down and drips off, hold the two boards together as best as you can so that the

edges are closed and form a good corner. Have your helper fire a series of nails into the corner, through the

face of the side board, into the edge of the other board, so that the length of the nail goes into wood instead

of pointing out the other side of the boards. You won't be able to seal it at the corner of the Drive Section

because the boards will be bent, but this will be fixed later so don't worry.

9. Attach all four of the boards together in this way, keeping in mind that the top/bottom boards need to be

attached by their edges to the inside faces of the side boards. You should now have a pyramid-like object,

which looks very similar to that in Figure 2A (#Figure2A). Leave it standing on the Drive Section and let it dry for

a while.


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10. Once the glue has dried, you can set the Diffuser on one of its sides. Inspect all of the points of contact of

all the various boards, and make sure that they are airtight. Fire more nails into weak connections to

reinforce them, and use silicone anywhere that does not seem airtight (except for the four large holes at the

corners of the Drive Section). You may want to fire a series of nails into the Drive Section to reinforce the

drilled screws and so that the boards are firmly attached to the Drive Section, which is relatively heavy.

11. In order to seal the large holes in the four corners, get your construction sealant and have your teacher

carefully seal the holes, as shown in Figure 2E.

Figure 2E.Fully sealed Diffuser corner.

12. Your Diffuser should now be fully sealed. Before you move on, there is one final airflow concern to address.

Look down into your Diffuser to see the Drive Section fan. Notice that the fan is circular, but the end is a

square, and that the corners of the board are actually walls that will disturb the airflow. This can cause

turbulence in the Diffuser and affect the airflow in the rest of the wind tunnel. However, your fan will negate

almost all of this turbulence by drawing the corner air to the circle and blowing it out. Look down into the

inside corner of the Diffuser, through the Drive Section's fan hole (as in Figure 2F). If your fan is like the one

used by Science Buddies, then there is plenty of open space through which the fan will draw the disturbed

corner air and blow it out before it affects the air in the rest of the wind tunnel too much. Make sure you

understand this so that you can explain it later if need be, and cite it as a source of error when you compose

your report for any science fair project you do.


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Figure 2F.The fan gives plenty of space for air to pass from the corners to the blades.

Turbulence created at the corners of the Drive Section is sucked into the fan and blown out of the Diffuser.

13. Before you make your control box for the fan, you need to take care of a potential safety iss ue. The fanspins very quickly and is very sharp, which could severely injure fingers and hands. To protect against this,

take the cut of wire sheet and attach it to the Drive Section. Use screws and large washers, and make sure

that the edges of the wire sheet are not sharp or dangerous. Refer to Figure 2A (#Figure2A) to see how the sheet

looks when attached.

Part C: Electrical Control Box

Now that you have made the Drive Section safe, it is time to put your control box together. Basically, you will use a

switch like those you use to turn on lights in your house. Note that wind tunnels, like the UC Davis wind tunnel,

actually have many speeds for their fans, instead of just on and off. This is known as a variable speed drive

section. While it would be ideal to have a variable speed switch for your wind tunnel, it is more practical to have the

on/off switch, because it is difficult to find a variable speed switch for the purposes of this project. A dimmer will not

work because you need to know the exact speed of your fan and be able to reproduce the exact speed with a

quantitative button, instead of with a qualitative dial. Other types of switches are usually too weak to handle the

voltage of an attic fan. Since it is so difficult to find a variable speed switch, it is best to use an on-off switch and

then plug the fan into a variable AC transformer (see if your physics teacher has a piece of equipment like this).

This way, you can quantitatively set the amount of power that your fan will use, and then turn it on, essentially

giving you variable speed control. This will be mentioned again later.

1. Putting together the on/off switch should be fairly simple for your electrically inclined helper, so allow him or

her to determine the best way to make it. All that you need is an on/off switch for the fan, mounted onto the

side board of the diffuser. See Figure 2A (#Figure2A) for reference.

2. Once you have fully assembled the fan control, it is f inally time to test the fan! Bring the diffuser to an area

where there is an outlet, and plug it in and turn it on. To demonstrate that the fan draws air into the wind


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tunnel, securely hold a light object at the small opening of the Diffuser and see what happens when the fan is

turned on. As in Figure 2G, the object should be drawn into the Diffuser with a considerable amount of force.

Note:Make sure that you hold on tight so that the object doesn't fly out of your hands and get chopped up by

the fan!

Figure 2G.The effect of the Drive Section fan is demonstrated here using a glove.

Part D: Base

Next you need to build a Base for the Diffuser. As with the Test Section, check with your carpenter/woodshop

teacher to determine the best and safest way to do this with the wood that you already have. You may need to buy

more pieces of wood, depending on what kind of scrap you have available. An example of a good Diffuser Base

design is given in the following figure:


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Figure 2I.Rubber gasket stripping on the front edges of the Diffuser.

Figure 2J.Example of a well-covered corner.

This is a picture of a corner from the Contraction Cone, which will be built in the next section.

It serves as an example for the gaskets on the Diffuser.

2. You are almost finished with the Diffuser Assembly! Now you need to add four spring-connecter screws to

the Diffuser, which will be used to connect the three wind tunnel components. Recall from building the Test

Section that you the heads of the screws need to be on the outside so that the springs can hook onto them.

You will use screws, washers, nuts, and lock washers, as before, and the threaded ends will protrude into the

Diffuser. Again, this is a slight disturbance to the airflow, but it is necessary in order to get the springs to

hook the Diffuser to the Test Section. Try to make the screws protrude into the Diffuser as little as possible,

as in Figure 2I, in which only the nut protrudes into the airflow. Make sure that the screws are in the same

positions, relative to the 1' x 1' opening on the Diffuser, as they are on the Test Section. You might want toput the two components side-by-side in order to make sure that the screws are aligned properly. You will

attach the two sections together using springs hooked onto these screws, so their alignment is important.

3. The next step is to attach the wind speed sensor, which is made using the small, handheld personal fan. You


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may be wondering how a fan can be a wind speed monitor, since it is used to blow air instead of to measure

wind. Actually, it can be used for either purpose. If used as a fan, voltage is an input (batteries), and if used

as a monitor, voltage is an output! Basically, you will take the batteries out, and run wires from the motor to a

voltage monitor (called a voltmeteryour physics teacher should have one). This way, when the wind from

the Drive Section causes the sensor's blade to spin, the motor will produce electricity, which will be

measured by the voltmeter. First, you need to take the batteries out of the personal fan, and remove all

parts, except for the body, the fan blade, and the motor. Next, you need to solder your wire to the motor in

order to transfer voltage to a voltmeter. Your physics teacher/mechanically inclined helper should know howto solder, and can determine the best way to wire the sensor. Visit the Science Buddies Electronics Primer:

How to Solder Electronic Components (http://www.sciencebuddies.org/science-fair-projects/project_ideas/Elec_primer-solder.shtml)

page for helpful instructions.

4. Now that you have attached wires to the motor, put the personal fan back together and attach it to the inside

of the front of the Diffuser, as shown in Figure 2K. The motor will cause some turbulent disturbance, but this

is allowablenearly every single educational open-circuit tunnel uses a wind speed monitor that blocks

airflow, so the effect on this wind tunnel's accuracy is easily dismissed.

a. When viewing Figure 2K, note that the fan is off to the side, but about halfway from the top and

bottom. Make sure that your wind speed monitor is in the same position, and that there is space all

around it for air to flowdo not mount it to the inside of the Diffuser, because air will not properly flowaround it and its readings will be inaccurate.

Figure 2K.The wind speed monitor is mounted on the inside of the Diffuser at the front, slightly to the side.

Note: There is a handle on the Diffuser, which will be attached later on.

5. b. Make sure that the fan is firmly secured so that it does not tiltit must point directly into the airflow.

c. You also need to drill a very small hole in the Diffuser near the sensor, through which the wires will be

run. As with the hole drilled for the control tube of the fan, this hole must also be small enough to not

let much air through.
http://www.sciencebuddies.org/science-fair-projects/project_ideas/Elec_primer-solder.shtml


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6. Finally, it is t ime to attach the handles. Given the fact that the Diffuser Assembly is rather large and is heavy

only on one end, it is difficult to move, so you need some handles for personal safety. Two people should

always move the Diffuser Assembly together, instead of trying to do it by yourself, because it could hurt your

back and it takes less time to move with teamwork. To attach the handles, simply screw two of the drawer

handles into the Diffuser at either end, and make sure that their screws do not protrude into the Diffuser. See

the following figure for a visual of the handle at the Drive Section end of the Diffuser. Also, refer back to

Figure 2K to note the handle at the Test Section end.

Figure 2L.Drive Section handle

Building the Contraction Cone Assembly

This assembly will be at the forward end of the tunnel, into which the air will flow as it is drawn in by the fan at the

back. This assembly consists of the Contraction Cone and the Settling Chamber.


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Figure 3A.The full Contraction Cone Assembly.


2' x 4' oak boards (all 4)

Honeycomb meshAluminum screening

Rubber gaskets (leftover from the Diffuser assembly)

Wooden lattice

Wooden corner guard

Zip ties

Staple gun

Wood glue

Screws, washers, lock washers, bolts, etc.

Drawer handles (2)

Scrap wood

Part A: Contraction Cone

1. First, c ut your four boards into equal trapezoidal shapes. Like the Diffuser Assembly, the Contraction Cone

Assembly has a large end and a small end. Unlike the Diffuser Assembly, however, the Contraction Cone

Assembly takes a large volume of air and turns it into a small volume. The area ratio of the front to the back

is based on a ratio just like with the Diffuser, except this ratio is 7:1 rather than 2:1. This means that the

edges on the large end of the Contraction Cone Assembly need to be 32" long, because 32" x 32":12" x 12"

is just about 7:1. You now have the base dimensions for your trapezoidal cuts: 32 in. and 12 in. The height

should be 2 ft., so your carpenter should just measure out each base on both of the 4-ft. edges of the boards,

and make sure that they are centered with each other. See Figure 3A (#Figure3A) for a visual of the way the

board should be cut.

2. Next, connect the four boards in the same way that you did with the Diffuser. This will be trickier, though,

because you don't have anything like the Drive Section to connect the boards to. Instead, you will just have


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to connect them to each other, one by one, using wood glue and a nail gun. Remember, as with both of the

other components, you need to make sure that they fit together with minimal airflow interruption as it passes

from the Contraction Cone to the Test Section.

a. The use of the nail gun is why the boards need to be relatively thick , because if you have thin boards,

the nails are likely to not go straight into the wood and will instead poke out into the Contraction Cone,

disrupting airflow.

3. Once you have connected all four boards, you should let the Contraction Cone dry. I t should look like the

Contraction Cone in the Figure 3B.

Figure 3B.This is the Contraction Cone Assembly.

4. Next, you need to reinforce the Contraction Cone. You have probably noticed that it is wobbly and unstable,just like the Test Section was before you reinforced it with brackets. For this, you will need the wooden

corner guard. Cut it into four equal pieces and attach each one to the corners of the Contraction Cone, using

wood glue and a staple gun. You will probably not have enough corner wood to completelycover the corners

of the Contraction Cone, but if you cut it into four equal lengths and connect them to the corners, you will

greatly reinforce the Contraction Cone's structural integrity. See the following figures for visuals.

Figure 3C. Reinforced Contraction Cone.


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Figure 3D. Attach the wooden corner guard using wood glue, as well as staples from the nail gun.

Part B: Base, Spring Hooks, and Gaskets

1. Now that the Contraction Cone is sturdy, you will need to work with your carpenter/woodshop teacher to

make a Base for it, just as with the other parts. Again, this will depend on what scrap wood you have

available, and what other wood you can buy. Make sure that your carpenter keeps track of angles, just as

with the Diffuser Base, in order to allow the Contraction Cone to stand level. Here are visuals of a Base

design example.

Figure 3E.Side view of the Contraction Cone Base.


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Figure 3F. Underside of the Contraction Cone Base.

2. As with the Diffuser, the Contraction Cone also needs to have rubber gaskets attached to its front end so

that it can form an airtight seal with the Test Section. Refer back to Figures 2I (http:// www.sciencebuddies.org/science-

fair-projects/wind-tunnel-toc.s html#diffuser)and 2J (http:// www.sciencebuddies.org/science-fair-projects/wind-tunnel-toc.shtml#diffuser).

3. Also, t he Contraction Cone needs to have connection screws just like on the Test Section and the Diffuser.

They need to be aligned with those of the Test Section, and should be made from the same parts.

Part C: The Settling Chamber

1. Finally, you need to build the Settling Chamber. This is actually a simple componentit is nothing more than

the honeycomb mesh and one or two screens in the large end of the Contraction Cone, which cut turbulence

to a minimum and improve airflow quality. One screen is acceptable, but it is best to have two. The

honeycomb mesh should go at the mouth of the large opening, and the screen(s) should be inside the cone,

about an inch behind the mesh. If you use two screens, there should be 1 in. between them, so that there is

about 1 in. or so between all three parts of the Settling Chamber. The Science Buddies wind tunnel only has

one screen, but if you can use two, that gives you even more control over airflow.

2. Work with your carpenter/woodshop teacher to figure out the best way to attach the honeycomb mesh and

screen(s) to the front of the Contraction Cone. You don't want them to go too deep into the Contraction Cone.

Refer back to Figure 3A (#Figure3A)and take a close look at the Settling Chamber in the Contraction Cone in the

foreground, and note the honeycomb mesh and the screen behind it by looking closely at the figure. TheScience Buddies wind tunnel uses zip ties to attach the honeycomb mesh to a series of staples on the

outside rim of the Contraction Cone, so the inclusion of zip ties in the materials list is a suggestion. When

putting these two components in the Contraction Cone, make sure that you keep airflow in mind, and avoid

using any jutting objects or rough surfaces.

3. Finally, the Contraction Cone needs handles, just like the Diffuser. Attach the handles at the top of the

Contraction Cone for easy transport. Refer back to Figure 3A (#Figure3A) for a visual.

Putting It All Together

1. Now that you have built the three components of the wind tunnel, it's time to put it together and test it! First,

you need to get eight springs, as shown in the following figure:


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place where you will keep and use the wind tunnel, and reattach them so that they form the full wind tunnel.

4. Recall that in Section 1, Part B, you and your carpenter/physics teacher designed a Base that would hold the

force sensors perpendicular so that they would connect to a single rod coming down from inside the Test

Section. Now that the wind tunnel has been built, attach those sensors. The following figure is an example of

sensors mounted to the base of the Test Section Assembly. Note the rods and the airfoil mounted inside the

Test Section, ready to be tested.

Figure 4C.Two force sensors mounted to the wind tunnel.

One is for lift, one is for drag, and they are perpendicular.

An airfoil is mounted to the sensor rods.

Again, the figure presents only an example. While it is suggested that threaded rods be used to connect the airfoil tothe sensors, it is up to you and your physics teacher to decide the best way to connect the particular sensors you

have to any models you may mount. You should also take into account the best way to mount models onto your

rods the Science Buddies wind tunnel uses models carved out of balsa wood, into which threaded casings may be

drilled so that they can be easily screwed onto the rods. Finally, don't forget that you can also test models with an

angle of attack. For the Science Buddies wind tunnel, the threaded rod can be bent at a predetermined angle and

then inserted into the Test Section. Depending on your ideas for testing angle of attack, it may be best to have

multiple rods so that they can be replaced when bent, instead of trying to bend them back into their original perfectly

straight shape.

5. Now it's time to attach monitoring equipment to the wind speed sensor (recall that this is the small handheldfan that is mounted in the Diffuser). Obtain a voltmeter, and attach the sensor's fans to the voltmeter so that

when the sensor's fan spins, it will produce a voltage in the wires that will register on the voltmeter. See

Figure 4D for an example of a voltmeter.


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Figure 4D.A voltmeter used to determine the wind speed in the tunnel.

Note that the two wires from the wind speed sensor have been attached to the sensor.

The sensor measures voltage, which is indirectly a measure of wind speed. It does not give a velocity reading, but

the voltage reading for each speed in your wind tunnel will be constant, so it is reliable. For instance, if you turn the

fan on full speed and get a particular voltage reading, such as 2.8 V, and then turn the fan to that same speed on

another day, the voltmeter will read just about 2.8 V again. Due to this consistency, your wind speed determinations

will be based on relative voltages for different settings on your wind tunnel fan. You may also be able to determine

the relationship of velocity to voltage, and develop a conversion chart so that you can look up the voltage you read

and determine the wind speed inside the wind tunnel. Talk to your physics teacher about this. It is not necessary to

develop such a chart, but figuring out how to do it would certainly add to your understanding of aerodynamics!

6. The last piece of equipment to attach is the variable AC adapter, which allows you to control the amount of

energy the fan receives and thus have a variable wind speed tunnel. Obtain an adapter, plug it into a power

outlet, and then plug the wind tunnel fan into the adapter. Figure 4E shows an example of an adapter:

Figure 4E.This device allows you to control how much electricity the fan gets, control the airflow.

7. You have now attached all of the necessary equipment to the wind tunnel. Figure 4F shows the sensors of

the wind tunnel: lift force, drag force, and wind speed.


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Figure 5A.A known weight is placed on the model.

This is so that the known-weight value will allow the user to verify the sensor readings.

This is known as calibration.

2. c. Calibrating the force sensor is done in the same way, although you will need to take the sensor off of

its mount on the Test Section Base. Set it upright, so that it is like the lift sensor, and calibrate the

sensor by telling it what 0 force is (when nothing is pushing or pulling on it), and then use a known

weight to finish the calibration. Unlike the lift sensor, however, this is a drag sensor, which means that

the force of drag will be pushing into the sensor, so any weight on it should be defined as positive, notnegative. Once you have calibrated this sensor, remount it on the Test Section Base. Having properly

calibrated your sensors, you are now ready to perform your test and collect data from the model.

3. To test the wind tunnel and start collecting data, make sure that:

a. The springs are holding the wind tunnel together.

b. There is no place for air to escape.

c. The equipment is all plugged in.

d. The force sensors are calibrated.

e. The model is properly mounted inside the Test Section.

Treat these conditions as something of a checklist before you test a model every time. See Figure 5B for thefull setup.


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Figure 5B.The work station in this figure is fully prepared to test.

The sensors are calibrated and ready to collect data.

The model is mounted in the Test Section.

3. Now, before you turn on the fan, adjust the variable AC adapter to the desired level (for a test, you probably

just want to use the highest setting, so that you are essentially just turning the fan on all the way), and thenflip the Drive Section control switch to the "on" position. Let the fan get to its rotation speed, and then begin

collecting data. You will collect two series of small data points, taken at regular intervals of timeone series

for each force sensor. You want to collect data only for a short time, such as 10 sec. or so, and then stop

collecting. Produce graphs of all of the data points so that you can note trends, and use a computer to

perform a statistical analysis of all of your data points, such as average force, standard deviation, etc. Figure

5C is an example graph of data points collected from the Science Buddies wind tunnel, and the caption

contains all of the pertinent information that you will need to record from your own tests.


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Baals, D. (n.d.). Wind Tunnels of NASA.Retrieved August 2008, from

http://www.hq.nasa.gov/office/pao/History/SP-440/contents.htm (http:// www.hq.nasa.gov/office/pao/History/SP-

440/contents.htm)

Benson, T. (2008, July 14). Beginner's Guide to Wind Tunnels. Retrieved February 26, 20014, from

http://www.grc.nasa.gov/WWW/K-12/airplane/bgt.html (http://www.grc.nasa.gov/WWW/K-12/airplane/bgt.html)

Bradshaw, P and Mehta, R. (n.d.). Wind Tunnel Design.Retrieved August 2008, from http://www-

htgl.stanford.edu/bradshaw/tunnel/index.html (http://www-htgl.stanford.edu/bradshaw/tunnel/index.html)

Bugg, R., Cyron, R., Houser, M., Oberlitner, S., and Saracen, J. (n.d.) Miniature-Portable-Subsonic WindTunnel for Aerodynamic and Heat Transfer Measurements Designed for Classroom Demonstration Purposes.

Retrieved August 2008, from

http://web.archive.org/web/20070404141346/http://www.mne.psu.edu/me415/spring05/ME2/

(http://web.archive.org/web/20070404141346/http://www.mne.psu.edu/me415/spring05/ME2/)

Cleghorn, C. (2004, February). NASA's Wind Tunnels Fact Sheet. Retrieved February 26, 2014, from

http://www.nasa.gov/centers/langley/news/factsheets/WindTunnel.html

(http:// www.nasa.gov/centers/langley/news/factsheets/WindTunnel.html)

Clifford, R. C. (2003, June). Home-Made Wind Tunnel.Retrieved July 2008, from

http://www.theplanpage.com/months/2306/tunnel.htm (http://www.theplanpage.com/months/2306/tunnel.htm)

Clifford, R. C. (2003, July). Home-Made Wind Tunnel, Part 2.Retrieved July 2008, from

http://www.theplanpage.com/Months/2307/wt_pt2.htm (http://www.theplanpage.com/Months/2307/wt_pt2.htm)

Erwin, B. (n.d.). The Baals Wind Tunnel.Retrieved August 2008, from

http://web.archive.org/web/20020202113520/http://ldaps.ivv.nasa.gov/Curriculum/tunnel.html

(http:// web.archive.org/web/20020202113520/http:/ /ldaps.ivv.nasa.gov/Curriculum/tunnel.html)

G.U.N.T. Gertebau GmbH. (2005). Educational Wind Tunnel.Retrieved August 2008, from

http://www.gunt.de/static/s3399_1.php (http:// www.gunt.de/st atic/s3399_1.php)

Hodanbosi, C. (n.d.).Abstract: The Wandering Wind Tunnel.Retrieved August 2008, from

http://www.grc.nasa.gov/WWW/K-12/WindTunnel/wandering_windtunnel.htm (http://www.grc.nasa.gov/WWW/K-

12/WindTunnel/wandering_windtunnel.htm)

U.C. Davis College of Engineering. (n.d.). U.C. Davis Aeronautical Wind Tunnel Facility.Retrieved August

2008, from http://windtunnel.engr.ucdavis.edu/ (http:// windtunnel.engr.ucdavis.edu/)

Wright Brothers Aeroplane Co. (n.d.). 1901 Wind Tunnel.Retrieved August 2008, from http://www.first-to-

fly.com/Adventure/Workshop/1901_wind_tunnel.htm (http://www.first-to-fly.com/Adventure/Workshop/1901_wind_tunnel.htm)

The Wright Experience. (2002). In Depth: The Wind Tunnel. Retrieved February 26, 2014, from

http://www.centennialofflight.net/wbh/wr_experience/tunnel/index.htm

(http://www.centennialofflight.net/wbh/wr_experience/tunnel/index.htm)

Consultants:

Adkins, Jeff. Astronomy and Physics teacher, Director of the ESPACE Academy. Deer Valley High School,

Antioch CA. Design and mechanics consultant, donator of sensory equipment.

Dumin, Ed. Technology teacher. Deer Valley High School, Antioch CA. General consultant.

Soraoka, Daniel. Aeronautical/Mechanical Engineering student at Embry-Riddle University, Prescott AZ.

Design consultant.

Spahn, Jordan. Foreman, Industrial Electrician. Cupertino Electric, San Jose CA. Materials and construction

consultant.

Van Dam, C. P. "Case." Professor of Aerodynamics at U. C. Davis, Director of the U. C. Davis Aeronautical

Wind Tunnel Facility. Davis CA. Design consultant and conceptual editor.

Credits

Justin Spahn, Science Buddies Intern

Rebecca Steelman, Editor
http://www.hq.nasa.gov/office/pao/History/SP-440/contents.htmhttp://www.grc.nasa.gov/WWW/K-12/airplane/bgt.htmlhttp://www.nasa.gov/centers/langley/news/factsheets/WindTunnel.htmlhttp://www.first-to-fly.com/Adventure/Workshop/1901_wind_tunnel.htmhttp://www.gunt.de/static/s3399_1.phphttp://www-htgl.stanford.edu/bradshaw/tunnel/index.htmlhttp://windtunnel.engr.ucdavis.edu/http://web.archive.org/web/20070404141346/http://www.mne.psu.edu/me415/spring05/ME2/http://www.theplanpage.com/months/2306/tunnel.htmhttp://www.theplanpage.com/Months/2307/wt_pt2.htmhttp://www.centennialofflight.net/wbh/wr_experience/tunnel/index.htmhttp://www.grc.nasa.gov/WWW/K-12/WindTunnel/wandering_windtunnel.htmhttp://web.archive.org/web/20020202113520/http://ldaps.ivv.nasa.gov/Curriculum/tunnel.html


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