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Here we are, on to more construction. You may say, “What, I thought this was science not wood shop class!” Well, to conduct experimental science you have to build stuff. So, that is what we continue to do, building stuff to conduct our experimental science. Also remember, you have to build stuff right in order to get any good results.
As mentioned last time, it is time to build the legs for the test section. The length of my legs are based on the height difference in the staggered bench top, height of the test section, and diameter of my fan. You who are following along will need to determine the right lengths for you. Here is a sketch with my dimensions.
I bought a 20” diameter fan, which I will discuss more later, and assumed an extra inch for mounting. I also then assumed the middle of the fan will line up with the middle of the test section. By this, the test section must be 7 inches above the upper bench top. The lower bench top is 7.5 inches below this. Add these together and the legs should be about 14.5 inches long. Lets give ourselves a little flexibility incase we need some unexpected extra space and make the legs 16 inches long.
Alright, how are we going to make these legs. I’ve got a 2x4 which seems like a great choice. Okay, we need to determine how we are going to make the joint between the legs and test section. We don’t want to just slap legs to the side of the test section because all the weight would be on the screws, which is never a good idea. We want the test section to actually rest on the legs so here is what I determined:
Above, you can see the legs and the horizontal what I will call tie rods. The material of the tie rods is quarter inch thick strips ripped from a 1x6 I had laying around from an earlier project. Next is a picture from below and you can see the notch cut into the leg. The tie rod then the test section fit into the notch. As you can see in the next picture, the leg is attached to the tie rod and then the tie rod attached to the test section. The reason for the tie rod is because it allows the screws into the test section to be more spread out and for the legs to be connected together. I thought this would make the joints stronger and allow the test section stand with more stability.
The most important distances to get accurate are from the bottom of the notch to bottom of the legs, which is the height of the test section, and depth of the notch, which must contain the tie rod and overlap the test section. Also, make sure the legs are positioned on the tie rod accordingly so that good locations are left for the attachment screws into the test section. I made a little jig on the table saw so that the location of the notch was the same for each leg. I then used a hand saw to make the cut for the notch. Unfortunately, it turns out my bench top is not level so I have to put shims under the legs anyways.
To add extra stability, I connected the bases of opposing legs with an appropriate length of 2x4. For all screws, I pre-drill holes to not split the wood and then use screws with a suitable length.
Good work! Our test section now stands on its own 4 legs. This part was important because if it is unstable or weak we can't attach the bell mouth or ultimately conduct good experiments.
Where we stand now is we have our “tunnel” and our measurement/test stand. We need to complete construction of our tunnel. Let’s now review the few basic remaining parts of a wind tunnel. Please refer to my highly technical drawing below which lays out our plan. The main drawing is in blue, the rest are a few dimensions I was working out.
There are five main parts to this design. For your reference the air moves from the left to right. First, on the far left is the bell mouth, which is like a nozzle. It starts larger and funnels the air into the “tunnel” of our wind tunnel. Second is the “tunnel” which is straight and holds our specimens. From now on, we will be a little more technical and call this the test section. Third, the next part on the right is the diffuser, which provides the transition from the test section to the fan. Fourth, on the far right, but not drawn is the fan which will suck air into our wind tunnel from the left and blow out to the right. The last part is the measurement/test stand, which we just built. It goes beneath the test section, but here is unfortunately drawn backwards…oops. The whole system is on top a staggered bench top, because that is what I had.
Let me explain the purpose of the bell mouth and diffuser. One goal for a wind tunnel design is to have as smooth of air as possible in the test section. This is to have consistent results. If there was no bell mouth, the air flow would come into the test section all whirly twirly, therefore it needs a little help. Whenever we humans need help, we ask for a guide. That is what the air needs too, a guide. The bell mouth starts very large so the air is slow, and guides the air to the shape of the test section as the bell mouth narrows and the air speed increases. Tada, much smoother air flow. In addition to being a guide, it is also a nozzle, which research has found kills turbulence (any extra whirly twirly in the flow). Also, just to make sure, we will add what is called honeycomb at the connection between the bell mouth and test section. This is like what the name suggests, but more information to come during that part of construction.
The reason for the diffuser is then to provide a guide for the air from the test section to the fan. When we get a fan, we want to get one with as much airflow as possible (and we can afford) which usually means as large a diameter as possible. Like before, we would like as gentle of a transition as possible between the test section and fan, so that means a long gradual slope. If there is a dramatic slope and transition, the airflow will separate from the walls and create lots of efficiency killing turbulence, see below. On this end, because we are past the test section, we don’t care about turbulence for consistency of measurements but for efficiency of the system. Greater efficiency means greater air speed because we only have so much power from the fan. There are a few other details in this construction, but more information to come during that part of construction.
Not drawn in the first picture is a few legs we need to add to the test section so its wont fall over. Our next installment of Basement Science will be their design and construction. Until then, live long and explore!
For our force measurement we are going to use springs as discussed earlier. I explained in "Lift and Drag Measurement Gauge" a little about linear springs. But here is a little refresher. Your common daily use springs are pretty closely defined as linear springs. This means that the force exerted by the spring varies linearly with the length stretched by the springs. This idea is captured by the equation F=-k*x. Force equals the negative of the spring coefficient, k, times the length stretched, x. There is a negative because if you pull it to the right, it pushes to the left. Not all springs are linear and even your common springs are not exactly linear, but it is close enough. As you dig into science, you will find that many of the things you learn are not exact but pretty close. Maybe that is more true for engineering than science.
Anyways, the spring coefficient is a constant of each spring. For our measurements we can measure "x", so if we know "k", we can really easily calculate the force "F". This is our goal. But with our new springs the "k" is currently unknown. To find "k", we need a known "x", and "F". This is easy, apply a known force and measure the distance.
Alright, what do we have available. For me, I don't own any weights. I need to make a weight. What I, and hopefully anyone reading this, have readily available is water. The density of water is pretty much a constant at about 998kg/m^3. So we need a known volume to get a known mass. I filled up a 16oz bottle of water and poured the water into a zip lock bag, which we will assume has no mass. Eliciting the help of http://www.onlineconversion.com/volume.htm 16oz is 0.000 473 176 m^3. Great! Therefore, with density times volume we get the mass to be 0.472 kg. The force of gravity is mass times acceleration due to gravity, F=m*a. Therefore, the force on the spring due to our little bag of water is 9.8*0.472=4.62 Newtons, which is the right units to get the spring coefficient.
As a note, I use metric units whenever I can because it is much less confusing. But those of us in America (and perhaps elsewhere, I am not sure) often have to contend with both systems.
To the experimental table! Grab you springs, your bag of water, and your measuring stick. Its easy from here on out. Hold up the measuring stick, hold the top of the spring along side it, record the location of the bottom of the spring, hang the bag from the spring, record the new location of the bottom of the spring. Voilà! With that change in distance you can not easily calculate your spring coefficient.
Easy, fun, and a little bit of theory.
Sorry, no pictures with time, but now we have our springs and completely finished our measurement system for the lift and drag gauges.
On to the next...