How to Build and Use a Subsonic Wind Tunnel: Section 2
Section 2: 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:
|Figure2A. 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 Diffuser Base. Make sure that you identify these components in Figure 2A before moving on.
For this assembly, you will need the following parts:
- 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
- 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 2–3 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.
- 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 air blown by the fan will go through the circular hole.|
- 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 out of 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
- 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 if you are confused. These boards will form the side walls of the Diffuser.
- Once all four have been cut, set aside all of your scrap wood (you may need it later). You will attach all four trapezoidal boards to the Drive Section in order to make the Diffuser. Before you start, make sure you understand what the Diffuser does and what it looks like (see Figure 2A again). The Diffuser takes a small volume of air (passing out of the Test Section through a 1' x 1' area) and expands it into a larger volume of air (this is why the Diffuser has a large end and a small end). The four boards are cut into trapezoids so that they can all be attached on their longest edges to form the pyramid-like shape you see in Figure 2A.
- To get started, make sure you have wood glue, a drill, and a nail gun handy. Hold one of the boards so that the 17" edge is up against the edge of the Drive Section, and so that both are lying down on the table. You will put wood glue on the trapezoidal board and then stand it up so that it becomes attached to the Drive Section. Run a line of wood glue along the face of the trapezoidal board at the 17" end, and then stand it up so that it becomes glued to the edge of the Drive Section. Hold it for a few minutes to allow the glue to dry, and then drill two or three screws with washers into the boards so that they are held together without your help anymore. This will keep the glue in place while it dries, and allow you to attach the other three boards. See Figure 2C for a visual.
|Figure 2C. In this figure, two of the trapezoidal boards have been attached to the Diffuser already.|
Before you attach all four boards, make sure that you cut a hole in one of them
through which you can run a tube, housing the control wires from the fan. See Figure
2D for a visual. Cut the hole to be as small as possible, and so that the tube fits
snugly through it and plugs it to prevent the escape of airflow. If the tube has
a control box on the end, remove it, because you will need to run the tube through
your hole, and you will attach your own control box to the wires later.
|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.|
- 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, but without the four boards attached to each other.
- Wait for the glue to dry before you continue. Your next step is to attach the four boards together so that you have 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".
- Begin by recalling the way that the Plexiglas boards were connected—you 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.
- 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.
- 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. Leave it standing on the Drive Section and let it dry for a while.
- 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.
- 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.|
- 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.
|Figure 2F. The fan gives plenty of space for air to pass from the corners to the blades. Any turbulence created at the corners of the Drive Section is immediately sucked into the fan and blown out of the Diffuser.|
- Before you make your control box for the fan, you need to take care of a potential safety issue. The fan spins 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 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.
- 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 for reference.
- Once you have fully assembled the fan control, it is finally 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 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:
|Figure 2H. An example of a Diffuser Base.|
Note that the Base may be difficult to build, given the shape of the Diffuser. It needs to stand level on the ground, but the bottom side of the Diffuser slants down from front to back. Make sure that your carpenter/woodshop teacher records the angle of slant and that you build the Base so that it attaches at a slant, in order to keep the Diffuser level with the ground and therefore, aligned with the direction of airflow through the Test Section.
Part E: Gaskets, Spring Hooks, Handles, and the Wind Speed Sensor
- Take another look at Figure A in the Objective section of this how-to guide. Recall that the wind tunnel is composed of three main components, which will be held together by springs and can be easily taken apart. This assumes that the components fit with each other at the 1' x 1' openings. Obviously they will not fit perfectly, so to compensate for this, you need to attach rubber gaskets to the edges of the Diffuser and Contraction Cone, which will come into contact with the Test Section. Cut your rubber gasket into four 1' x 1' strips, one for each of the four edges of the Diffuser's front end. Make sure that they are cut to the proper length. Peel off the paper on the back and attach each gasket to its edge, making sure that they do not stick out too much into the 1' x 1' area. If they do, it will disrupt the airflow as it passes from the Test Section to the Diffuser. Also, make sure that the corners where they meet have as little separation as possible. If they are too separated, air could leak out of them and disrupt the airflow. See the following figures for visual aids.
|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).|
- 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 to put 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.
- The next step is to attach the wind speed sensor, which is made using the small, handheld personal fan. You 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 voltmeter—your 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 how to solder, and can determine the best way to wire the sensor. Visit the Science Buddies Electronics Primer: How to Solder Electronic Components page for helpful instructions.
- 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 allowable—nearly
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.
- 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 flow—do not mount it to the inside of the Diffuser, because air will not properly flow around 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 that there is a handle on the Diffuser, which will be attached later on).|
- Make sure that the fan is firmly secured so that it does not tilt—it must point directly into the airflow.
- 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.
- Finally, it is time 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|