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.
Figure 3A. The full Contraction Cone Assembly.
For this assembly, you will need the following parts:
2' x 4' oak boards (all 4)
Rubber gaskets (leftover from the Diffuser assembly)
Wooden corner guard
Screws, washers, lock washers, bolts, etc.
Drawer handles (2)
Part A: Contraction Cone
First, cut 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 for a visual of the way the board should be cut.
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 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.
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.
Once you have connected all four boards, you should let the Contraction Cone dry. It should look like the Contraction Cone in the Figure 3B.
Figure 3B. This is the Contraction Cone Assembly.
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 completely cover 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.
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
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. Below are visuals of a Base design example.
Figure 3E. Side view of the Contraction Cone Base.
Figure 3F. Underside of the Contraction Cone Base.
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 and 2J.
Also, the 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
Finally, you need to build the Settling Chamber. This is actually a simple component—it 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.
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 A 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. The Science 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.
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 for a visual.
You can find this page online at: http://www.sciencebuddies.org/science-fair-projects/wind-tunnel-section3.shtml
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