Getting in the Wind Tunnel: Design

Dear Fellow Basement Scientists,

Ever wonder why textbooks are so long? Well, I figured it out! Its because there is a lot of information out there to learn and it takes quite a few pieces of paper to fit it all. Well, this blog post is a little longer, though not quite textbook length, because there is a lot of information required in applying that textbook knowledge to building our wind tunnel.

Its time for another day in the lab (basement). Lets review our situation: We built the tunnel, legs for the tunnel, and finished the bell mouth including the honeycomb. Before we go on to the fan and diffuser, we can’t forget the minor detail of accessing the inside of the wind tunnel once it’s built. We must have an opening so we can put our specimens into the wind tunnel. Otherwise we would have a very fancy and long fan. I was hoping for more than that.

Let’s think about this opening. What do we want it to accomplish? It needs to allow our test specimens to pass through it when the tunnel is not operating, and allow for the measurement gauge main rod to pass into the tunnel to hold the specimen while the tunnel is operating. However, we want as little air to pass through the hole during opperation. This is because that extra air will mess up all the hard work we put into the bell mouth to create smooth, turbulence free flow! With these things in mind, lets make a door which you open to insert the specimen, and then close for operation. Makes sense to me. Perhaps there is a better or fancier idea out there, but for our needs we don’t need the absolute best, just something that works, and something that we can design and build rather quickly. That’s Basement Science. Okay, now for details.

Our door needs a small hole for the measurement gauge rod to extend through while it holds the specimen. However, we must also think about how it will get there. We can’t just have a hole, for which will come first, inserting the specimen into the tunnel or passing the rod through the hole? To do that it would require a very large door which would take away too much of the structural strength of the tunnel.

This detailed analysis resulted in choosing the door with a slot

What should the dimensions of the door be? Well, we need to think about what size of specimens we will be putting through the door. Given the tunnel dimensions, I would guess the largest thing we could test is about 6″ wide x 1.5″ tall x 6″ long. Interestingly, as the specimen gets wider and taller it takes up more cross-sectional area of the tunnel, reducing the area for the air to flow through. As you reduce this area, the velocity increases. So, honest for goodness, I don’t know what is an ok amount to reduce the area. We must then make some estimates. Here are some calculations to estimate the velocity change:

Sorry for poor photo quality, I had to increase the contrast a lot

With these size there will be a 14% increase in velocity, which isn’t great, but we will tentatively remember that 1.5″ tall and 6″ wide are about our limits of size. So when we make things smaller than this, the effect won’t be as bad. Also, length won’t affect the velocity, but we’ll cap it at 6″ because that’s the width too.

If we stick half of the width in and then pull up the other half, a 6″x6″specimen should be able to make it through a 3″x6″ door. So let make the door 4″ wide x 6″ to allow for a little leeway.

Image not to scale

Great we approximated our biggest specimen at 6″x1.5″x6″ and determined our door size at 4″x6″. The specimen size is by no means a hard and fast rule, after we finish our wind tunnel and test it out, we may find this limit is too big or too small. If the actual limit is a little smaller than this, that’s ok because we can defiantly make great stuff smaller than those limits. We have given ourselves a little extra space in case we need it. Great. Secondly,  given our 9″ wide boards, a 4″ cut should leave enough stability in the wind tunnel itself.

A few details about the door. We want the door to allow as little air to pass through as possible so we need to be creative with the seal around the door. How are we going to seal the door. We can create some pressure around the door if the edges are cut at an angle. So if we put some rubbery material at the joint, it will be squished when the door is shut and seal it up! Great! Ok, what else do we need to consider? We have the seal, size, and slot. What is left is attaching it. What’s available? I bought some hinges earlier at Home Depot  which will let the door to swing open, but we also need a latch or something to hold it shut. I have a metal strap with a hole which will do. We can screw this down and rotate it to latch the door shut.

Wow. That was a lot of detailed planning, but that is what it takes to run an experiment. We are in the heart of experimental planning. It is exciting and a lot of fun when building things, but not as fun as the experiments themselves. We will get there soon enough. Next we must actually build the door which we have been doing so much planning for. Save that for next time! Until then, keep exploring.

There really is so much exploring to do in this world. Building a wind tunnel is just one way to do that.

Best Regards,
Ben Washington

Disclaimer: I may have said this before, but now is a good time to reiterate it. If this were high quality university research, things would be done differently. For one, the bulk of the material would not be purchased at the 85% off section in home depot, and we would not be asking “what’s available”, but “what do we need”. Therefore, our results will not have the accuracy of university research. HOWEVER, and this is the key part, that is ok. In Basement Science our level of accuracy will be sufficient for our needs. We must consider the sources of error and address them, but I believe we can have less than 10% error using our philosophy of Basement Science. That is not enough to verify the viscosity of air to a few decimal places, but it is enough to get an idea of what is going on in our system and draw conclusions. We are going for the highest accuracy given our time and monetary budgets! Live long and explore fellow Basement Scientists.

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