Subsonic Wind Tunnel

• Introduction

Wind tunnels can be as simple or as complicated as the builder desires. As such, there are numerous sources out there for both industrial wind tunnel applications and homemade DIY tunnel applications. This guide combines a reasonable mix of the two. While it is certainly not up to any sort of professional standard, it is a relatively simple DIY home project that combines many design cues from more professional-grade sources. Most wind tunnels are semester or year long projects. I created this in about two weeks.

Sections

This tunnel, as with most, has been split up into 3 main sections: The Contraction Cone, Test Section, and Diffuser.

• Contraction ConeThe contraction cone is the entry point of air into the tunnel. It draws air in from a wide area, and contracts it down to the size needed for the inlet of the test section. The contraction cone is at the front, or upstream, section of the wind tunnel.
• Test SectionThe test section is where the airfoil or model is placed. This tunnel uses plexiglass sides in order to see the model in the test section. Generally, the load cell set up is mounted beneath the test section in order to gather data. The test section is in the middle of the wind tunnel.
• DiffuserWith the increased velocity of the air through the test section, a lower pressure is created according to Bernoulli's equation. The diffuser serves to slowly return the pressure to its static pressure outside the tunnel before exiting through the fan. It is important that the fan is placed in the diffuser section to pull the air through the tunnel. This creates far less turbulence through the entire wind tunnel. The diffuser is at the end, or downstream, section of the wind tunnel.

Materials

- 20"x48" plywood board x4 (diffuser walls)

- 20"x20" plywood board (diffuser base/outlet)

- Wood Glue

- Wood Screws

- 2 Speed on/off switch (for the fan)

- Gable Mount Attic Fan (I used a 1945 CFM QuietCool Attic Fan)

Procedure

Step 1

First, grab your 20"x20" plywood board for the diffuser base (also known as the outlet). The size of this board, and the overall size of your diffuser, will depend on the diameter of the fan. The QuietCool I picked up was roughly 17" in diameter. So I went for a 20"x20" diffuser outlet to account for this. If you use a smaller fan, you may use a smaller diffuser size. I would recommend at least a half-inch thick board for this. It will be holding a lot of weight.

Cut a circular hole in this board with a diameter the size of the fan.

Step 2

Now mount your attic fan on the diffuser outlet board. Make sure to line up the fan with the hole, and be sure that the fan blows air out of the outlet hole, rather than pulling it in. Also be sure that none of the mounting arms extend past the outlet board.

Step 3

Next it is time to create the diffuser walls. Take your 20"x48" plywood boards, and cut them into trapezoids. Once again, the size of this trapezoid depends on the size of your diffuser outlet board, and your test section size. If you are following this guide exactly, the test section will have a cross section size of 10 inches, and the diffuser outlet board has a cross section size of 20 inches. This means the trapezoid will go from a base size of 10" to a base size of 20", creating roughly a 5.5 degree diffuser angle, which falls within the recommended angle size, while keeping the length of the diffuser relatively small.

("To prevent reverse flow, they suggested a maximum divergence angle of 7° and a diffuser length at least 5 times greater than the small diameter. A divergence angle within the 5°–15° range was suggested as the optimum geometry for a diffuser to achieve the best pressure recovery.") (source)

Step 4

Attach these diffuser walls to the diffuser outlet board. Be sure not to use screws or nails, as any obstructions into the tunnel may cause disturbance to airflow. For this, I used Liquid Nails, which was extremely effective in attaching the diffuser boards together. Attach them so that they create a trapezoidal prism, with a 10 square inch opening (inlet) and a 20 square inch outlet attached to the diffuser outlet board. The fan should be inside of this diffuser, once again so that it sucks air through the diffuser, and out the outlet.

Step 5

Drill a hole through one of the diffuser boards so you may feed the fan wires through it. Be sure to make the hole as small as practical so that no air may seep through, and seal it with silicone and/or electrical tape when done. Wire this to an on/off switch. Since my fan was 2-speed, I wired it to a 2-speed switch, and used Command Velcro to secure it to the outside of the diffuser.

Step 6

Finally, use silicone sealant to line the corners of the diffuser box with, to seal any possible holes in construction. Then, run electrical tape along the inside of the corners to improve airflow, and get rid of any possible obstructions or leaks.

Materials

- Plexiglass sheets x2

- L-brackets

- Straight brackets

- Wood boards (for frame)

- Wood screws

- Wood Glue

- 10"x24" plywood

Procedure

Step 1

To start, build a frame to house the plexiglass. This can be done in any manner, but keep in mind that there should be no screws intruding into the test section (hence the need for metal brackets), and that the test section cross section should be 10" x 10". This means that you should also save some room in the width of the frame for the installment of plexiglass. Also, keep a large space open in the ceiling of the test section for a lid.

Step 2

Using Liquid Nails, or any other construction glue, attach the plexiglass onto the inside of the frame. Make sure the inside of the test chamber has an area of 10"x10".

Step 3

Create a lid for the test section by cutting out a piece of plywood exactly the size and shape of the space in the frame ceiling. Put electrical tape over the sides of both the lid and the gap in the frame to create a tighter seal. It is important that the lid is both flush and airtight. Then, attach handles to the lid. I did this by screwing handles into a separate piece of wood that had the dimensions of the entire top of the test section, then gluing that flat against the piece I created for the lid.

Step 4

Seal the entire inside of the test section using both silicone sealant, and electrical tape along the inside corners. Cover any intruding screws or imperfections with electrical tape to keep the flow smooth.

Materials

- 24"x24" mat board (contraction cone walls)

- 24"x24" honeycomb mesh

- 24"x24" Wire mesh screen

- 24"x24" Door/window mesh

- Hinges x2

- Zip Ties

- Wood boards (inlet frame)

Procedure

Step 1

The first thing to do is cut out the contraction cone walls. To do this, I used a fifth-order polynomial commonly used as recommended by Bell and Mehta. This polynomial can be found here (among other places). Then, I simply printed out a scale copy and cut out the matboard following that design, taping the four pieces together along the edges.

Alternatively, you could use straight walls made of plywood, though this would lead to less uniform and controlled air flow, as air aggressively overshoots the sharp corners.

Step 2

Now it is time to create the settling chamber. This is part of the contraction cone section that straightens out the airflow before it enters the contraction cone.

To do this, first create a 24"x24" frame with the wood beams. I used 6 inch beams to do this.

Step 3

It is then time to install the hex mesh. I sourced my mesh from SaxonPC by requesting a custom order, as they didn't offer them in such a large size on their site. The hex cells are each 3.175 mm in diameter, and run about 0.75 of an inch deep.

Alternatively, you can create this mesh out of large diameter straws. (This is cheaper, but would take a long time, and yield slightly worse results).

I simply used more liquid nails to attach this, being careful when it comes to accidentally getting glue on the cells themselves.

Step 4

Next is to attach the mesh screens. About an inch behind (downstream of) the hex mesh, use zip ties to secure the wire mesh. While the hex mesh does straighten out airflow, it creates some turbulence of its own. The wire mesh is useful in knocking down that turbulence so it may dissipate faster.

While the second screen is not necessary, it will only improve the reduction of turbulence. Make sure it is a more fine mesh, with smaller holes, than the previous one. I used door/window bug screen. I attached this using staples, with electrical tape to smooth out the protruding staples and imperfections.

Step 5

Build a frame for the contraction cone, so it may attach to the settling chamber. This frame will have to be slightly larger than the settling chamber frame, depending on the thickness of the mat board. This will allow the mat board to line up with the width of the settling chamber frame when you attach them. Using glue, attach the frame to the contraction cone.

Step 6

Using two hinges, attach the settling chamber to the contraction cone. Use a piece of matboard to place between the hinge and the settling chamber frame in order to offset it so that the mat board lines up with the frame.

Materials

- Assorted wood planks/beams

Procedure

Create the base however you see fit. I used beams connecting down to a plank that ran across the entire tunnel, being sure to leave room under the test section for the load cell setup, which I knew the size of beforehand.

As you do this, connect each piece of the tunnel together. Make sure that the contraction cone and diffuser line up with the inlet and outlet of the test section. If they don't align perfectly, that is alright. Just smooth it out as best you can with some electrical tape.

Use electrical tape and silicone sealant to seal any connection points and to improve airflow. Remember this is especially important in the test section, where the pressure is lower than the outside air, so air will be sucked in through gaps.

I left approximately 11 inches underneath the test section for the load cell setup.

I would like to preface this by stating that there are many ways to set up your sensors, a couple of which I will mention. I used an Arduino and a swiveling arm that could measure both lift and drag.

Materials

- Arduino (I used the MEGA 2560)

- HX711 Amplifiers

- Arduino breadboard x2

- Servo

- Potentiometer

- 5V AC Adaptor input

- LCD Screen

- Aluminum

(These components, and the wiring, can be seen in the attached images).

Procedure

The Load Cell Arms

This setup uses aluminum arms that are responsible for measuring lift and drag. The lift arm is mounted as a lever, so that as lift occurs and the sting (the arm that holds the airfoil) moves up, it pushes downward on the load cell. The arm immediately above this is responsible for measuring drag. It simply moves back by the sting as it rotates backward. This applies force directly to the load cell.

The servo is mounted on the lower part of the sting arm, and is attached to the top of the sting via a rod so that it may change the angle of attack of the airfoil. By being a part of the sting arm, it gets rid of any potential issue of the servo restricting movement of the system.

Other Options

In the attached images is a picture of the setup used by Science Buddies in their article on how to build a wind tunnel. I would recommend checking out their how-to guide as well, as it was influential in this tunnel design. They simply used some aluminum rods attached to two different load cells at a 90 degree angle. This is a less complex option that could work well in any tunnel design.

Mounting

I drilled a hole, just big enough for the sting arm, in the middle of the test section. I used a couple of wood boards to mount the load cell system below the test section, making sure that the sting arm was able to protrude into the tunnel by roughly 5 inches. I also had to drill another hole for the servo arm, but if you are not using a servo to change the angle of attack, this is not necessary.

Make sure that neither of these holes restricts the movement of the sting as the airfoil experiences drag and lift. Keep in mind that they should be as small as practical so that as little air seeps through as possible. This is a source of error for tests.

Now, you can test some models!

Some things to look out for include:

- gaps/holes, especially in connection points. Be sure to seal all holes (besides the one through which the sting protrudes) that you can find in the test section. Overall, make sure the build is airtight.

- Make sure your airfoils or models don't take up more than 80% of the tunnel width. Otherwise some wall interference might occur.

- Calibrate your load cells and make sure there is no crossover between drag/lift measured.

- You can also measure airspeed with an anemometer!

Thank you so much for taking the time to look over this.

If you are confused, or need help with any step, be sure to leave a comment or otherwise contact me.

This build was for a High School project, so do not expect it to be perfect in every way.

Hope you enjoyed!