Budget Friendly Cardboard Wind Tunnel

A Wind tunnel simulates controlled airflow around any object which will be going against the wind. For example, Scientists can test a Car in a wind tunnel to see how the air moves and creates friction. From this, the surface can be modified to reduce the drag. Then the car will be able to achieve higher speeds. We can also test toy planes, sport bikes, bike helmets, wind mills and similar objects if we have a Wind tunnel.

So if you want to test your own model car, aero plane, or model science project in your own wind tunnel, you can refer this instructible. This guide does not only make it very affordable, but also a very fun project as well.

I build this to test and verify airflow simulation for my Wind project. I do not have a Wind lab. So I decided to build it to test for free! With the scientific design guides, I built the wind tunnel using very cheap material like cardboards and paper. It is very important to follow the right guides to get the right results which I will inform. So that you don't commit the mistakes I may have made at first.

One can build this for Aerodynamics testing, to be a home scientist, or even win at school science projects. So lets get the ideas flying!

These are the basic items you will need for a working Wind tunnel.

1. Carton Cardboards.
2. Newspaper.
3. Exhaust Fan.
4. Glue (Fevicol, Dendrite or equivalent).
5. Tape.
6. Scissors.
7. Knife.
8. IR Sensor Module Infrared based for Proximity.
9. Arduino Uno and connector.
10. Laptop to record results.
11. Smoke generator:(Plastic container, Plastic pipe 4mm dia, Steel Plate, Smoking item (egg-containers or carboards or cigarettes etc.)
12. 3D models to test.

There are some complex wind tunnel structures to test a machine's performance in a lab. But I wanted to make an affordable, small, wind tunnel good enough for my purpose to test a project I had to finish.

These are some of the references I looked in the internet:

https://www.instructables.com/DIY-Wind-Tunnel-3/

https://www.hackster.io/news/a-diy-wind-tunnel-for-your-desktop-552ab4b3d5da

https://discover.hubpages.com/education/building-a-wind-tunnel

After careful investigation, I selected the following primary conditions and some pitfalls to avoid:

Primary Conditions:

1. Gradually increasing entry speed.
2. Laminar flow inlet: Any air collected from the environment, will have small vortices, irregularities. To make the flow very regular and parallel to each other, we need to guide the airflow by small divided partitions/ honeycombs.
3. A test section 3-5 times the size of the maximum width of sample.
4. A fan to suck the air instead of pushing it.

Mistakes to avoid:

1. Connecting the fan to the wind tunnel structure: When the fan is vibrating, it is vibrating the tunnel and it will have trouble getting the laminar flow. I saw some obvious whirls in the flow because of this in some projects. In real life tunnels, they use separate foundation or damped structure to avoid this. But in our case, as a solution, we will keep a small gap between the fan body and the tunnel. Also I will place the fan on a different support.
2. Putting the fan in the entrance: Unless you have very big fan with slow rotating perfect blades, the fan blade may cause more turbulence in the flow. So we will keep the fan at the exit so it causes suction and maintains the flow.
3. Laminar flow Speed: In some cases the user is running the fan at very high speed. Which is taking the flow to turbulent zones. We need to keep the Reynold's number below 2300 to keep the flow laminar. Because, Re = ρVD / μ, where ρ = density of the fluid, V = velocity, D = pipe diameter (0.3m) , and μ = fluid viscosity. So for air at room temperature and laminar flow where Re=2000, the limiting velocity is only 0.1m/s or 100mm per second. (Check this online Re calculator : https://www.omnicalculator.com/physics/reynolds-number ) So the flow has to be very slow. Any speed higher than this, the laminar flow will break. It should be a very gentle breeze under complete vibrationless tunnel.

This project is to visualize the laminar airflow around a bird like wing foil and a model car (3D printed). At the end the results came out like this. Also we will test the lift force results and compare with theoretical value for accuracy test.

I noticed that in the Car's example, The air is pushed above the car, so the air puts a downward force on the car as per Newton's law of motion, which helps it to grip the road harder.

I can also see, that for the wing foil, the wing pushes the air downwards. This causes the air to lift the wing. Also the flow separates when air speed is increased. This makes vortices at the end similar to the calculation picture. So the experiment successfully demonstrated the flows expected.

I chose a cardboard box/ carton to make the main shape. Then carefully check all the corners. There is one corner where the two sides are joined by glue. I slid the knife between the layers. this gave me a long uncut piece of cardboard.

I folded the cardboard in the shape of the fan. Also put transparent tape over any irregular section of the wall as in the picture. Then I glued the inside edges with fevicol, folded it and made the channel. On the outside I used tape to hold the shape till the glue dries up.

To make the laminar net, you can 3d print it, order it, make it out of plastic pipes joined by layers etc. But I found the cheapest and easiest way to do this was using some old newspapers lying around. The paper , when folded in corners gain triangular strength and become like a strong yet flexible net.

I had a tunnel approximately 300mm wide, So I cut flat newspaper pieces slightly bigger than 300mm wide. Also for the triangular portion I had to cut slightly bigger than 600mm wide newspaper pieces and then folded in pyramid shape.

After this, I used a quick setting glue 'dendrite' on the crests and glued the papers layerwise as shown. I don't know anyone making in this way but it fortunately worked and saved me few bucks and cutting effort.

When it was done, I put it through the entry side of the tunnel. The Net size was larger hence it got compressed from the sides. Since the curvature was parallel to the flow, it will not affect a laminar flow. Rather it will keep the net in place because of the side forces.

I cut two sheets of cardboard same as the length of the diagonals of the tunnel. Then cut halfway at the middle. I joined them and made a 'X' and put it through at the entry point. This will stop the tunnel from getting distorted diagonally. After this, I cut cardboards in the shape shown and made the gradually reducing entry point as shown. These components will make sure that the laminar flow is achieved.

I cut a small portion out from the mid section of the tunnel. used the cut out as a window frame. Then attached a transparent polythene on the inside of the frame. I can easily open this window to adjust the components inside if I want.

In some instructible I saw people using Vape releases from a vaping pen. I did not want to use it. It will also make the inside of the structure oily.

So I wanted to make a smoke maker instead. To make this, i cut out an old container. turned it upside down. After that i cut a hole through the bottom. Then I fixed a plastic straw bent at 90 degree by heat.

Then I burnt an egg-container made out of condensed paper in a steel plate. It burns and produces a lot of smoke. You can use any similar material like old ropes, cardboards, cigarettes etc. I placed the plate below the collector. When air flows inside the tunnel, it sucks the smoke through the pipe.

Also I found that a focused light shows the smoke lines clearly. So I cut a slit on the top of the tunnel similar to what is shown in the picture. I can simply put a flashlight on top of the slit for a lighting zone focused on the smoke line. I can also illuminate the inside by flashing from the entrance opening. But the flash light must be at a distance from the entry. Whatever I did , I had to make sure that the light hits the specimen correctly so that the smoke is visible.

I attached a black sheet inside of the tunnel just at the opposite of the window. This will help to make a dark background when taking he picture so I can inspect the flow clearly.

The .stl files of the car are attached. I 3D-printed the files at 500% scale. The black papers inside the tunnel makes the car clearly distinguishable under proper lighting.

For the wing test, I just used a bent plastic sheet held at place by a vertical cardboard post. The sheet is transparent so that the smoke lines below the sheet can be illuminated by the lights.

You can use your own models of car, planes, and anything you wish.

After setting the things up, I started the fan at a distance from the exit. So that the airflow inside the tunnel is below 100mm/second. Then I could see the laminar airflow shown by the smoke line. The flow is hugging the car from above and showing how the air induces drag in a car. If you have trouble having the low air speed, you can use a smaller 12V PC exhaust fan also at the outlet.

In the next image you can see a laminar flow over the wing at low speed. But when I increase the airflow speed in the tunnel, small vortices are created as expected in the third image.

The fourth image is showing a simulation where these similar results came out. Hence our experiment is showing good results.

I made a device which will rotate when wind falls on its face. Then connected an infrared object detection module as shown. It detects whenever any face comes close to it. With an arduino coding, the frequency and the gap between the face gives the velocity of wind. You may have to turn the potentiometer in the sensor to get the right blipping as shown.

The steps of the attached code are: calculate 'high' sensor datas > Calculate interval between two successive 'highs' > From the time period and the distance between moving faces, calculate speed> Average the data to get rounded value of wind velocity.

I had to position the device in such a way that the air flows on only one side of the baseplate. The opposite part had to be covered on the four sides as shown in the last picture. this will make sure that the device works without leaking any air to the outside of the tunnel.

First, I measured the speed inside the wind tunnel, it was 1.6m/second on average.

Then I removed the speedometer and placed a flat Airfoil/ surface at 10 degree angle in the same location. The surface width was 9.6cm ,length= 15.5cm. When I ran the wind through the tunnel, it measured a -2.1g on the weight machine.

Now let us see what is the theoretical result.

1. Angle of attack=10 degrees.
2. CL=1 (from the graph).
3. V=1.6m/s.
4. Rho (Wind density) =1.225 kg/m3.
5. Area= 96x155/10^6 m3
6. So, force=0.5*1*1.225*(1.6)^2*(96x155/10^6 m3)=0.0233N=.0023kg=2.3gm.

Our results:2.1gm. Error=0.2/2.3=Only 0.087 out of unit value. Which is good for my project.

Thats it for this project. It is now ready to do tests in my project. I can test models and refine them for no extra cost of renting a lab or making a very expensive wind tunnel. Cardboard box saves the day. Hope you like the project and some of my approaches to solve problems. Let me know what you think or any queries you have! TC.