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With the Sincerest Regards,
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...
The last installment brought about the development of the lift and draft measurement mechanism. However, we still needed to build the device hold it.
Before designing our anchor contraption, we must understand all of what it is meant to do. It must let the anchor screw rotate but not move, and anchor the springs while allowing their length to be adjusted. The first is easy, just drill a hole and stick the screw through it. The second is more difficult. My idea was to have instead of a hold, a slot. The spring would then be attached to a long screw which could more back and forth through the slot and tightened in place. Ok, lets build it.
As usual, I have available wood and decided to use it since it is easily shaped. I also decided it would be best to make two identical sides with the levers in the middle, for stability's sake. Again, it is important to know for which dimensions accuracy is important and which are not. Here is what I determined.
-Length between two sides: Doesn't matter, just smaller than the length of my screws
-Position of hole for anchor screw: Not important, just so that there is enough room on the board for the spring screw slots.
-Position and length of spring screw slots: I positioned these off of the hole for the anchor screw. They must be as perpendicular to the corresponding lever as possible and at the position of the spring. Length is not too critical, just long enough for the extent of spring stretch.
-Quality of box construction: This box should be strong and stable for consistent data.
A little preview so you can understand what I am talking about.
Alright, I took a 1x5 and cut two pieces about about 8 inches long and stacked them on top each other to make sure they had equal dimensions. Given the size of the lever mechanism, this one board would not work, but needed another placed lengthwise perpendicular to make room for the other slot for the spring screw. I cut two more similar pieces and placed them in what would be their final position. Holding the lever mechanism up to these pieces, I estimated the locations of the hole for the anchor screw and spring screw slots. Clamping the two sides together, I drilled a hole for the anchor screw. Making the slots now requires a little more precision.
I put the anchor screw through the lever and into the board to simulate its final position. With it in place, I then marked on the board where the spring screw slots must be. This may be breaking some rules, but to make the slots I used the drill press and bit as a router. As basement scientists, you can only use what you have, and for me it is the only thing that would do the trick. To make the slot straight, I clamped a board to the drill press table to act as a guide rail.
Right now, we got all our slots and holes and the four pieces making up the two sides. We need to connect the two sides. I took my 1x5 and ripped a 12in section in two using a table saw. These two pieces will serve to connect the halves, one across the very bottom and another shorter section a little above it. Drill your pilot holes, get your 2" wood screws and we have our measurement contraption!
Congratulations, We have our measurement mechanism. Hooray for basement science! We wont know if it really works until we start using it. I imagine there will be some debugging at the end of all the construction.
Next, we must determine the spring coefficient of our springs.
In order to conduct science you need measurements. For our Wind Tunnel tests investigating fluid mechanics and aeronautics we need to measure lift and drag. There are a lot of ways to measure lift and drag, but with all basement science we need to ask, “What’s available?” As usual, my what’s available is wood. This time I have a box of old balsa hobby wood. This wood was my grandfathers and was used to make little glider airplanes.
In the box are long thin sticks I thought would be useful. My idea was to hold the test specimen in the wind tunnel with these balsa sticks, then, the force on the stick could be measured. I knew I could not buy digital force meters for cost reasons, so I needed something else to respond to the lift and drag…Springs! Cheap, available, and easily characterized, springs would do the trick.With few springs in the house, I bought a bag of assorted small springs from Home Depot.
If we just attached the springs to the stick there would be no stability, so I chose to go with levers. Usually, the best mechanism is the one that performs the task in the simplest way and it’s hard to get more simple than the lever. With two connected levers one could measure lift and the other drag. Using wood glue I stacked a few sticks to make two thicker ones, drilled some holes, and here is what I got:
I have talked some about knowing which parts of construction need to be accurate. This lever needs to be very accurate. You need to know as accurately as possible the distances between the rotation points and specimen and spring attachments. This is a great reason to get a digital micrometer, and they are fairly cheap. As for bolts, the 3 inch long screw anchor is from an old Xbox and the rest are the smallest bolts I could scrounge up.
Looking back, it always seems like an easier task than it actually was. That's probably because you have to think of all the ideas for the first time. In manufacturing you can be making the same part a million times, so you get pretty fast at it. In our basement science, we are designing and making individual parts, which can be difficult if you try and do it right.
Our next task will be to make a box of some sort to provide an anchor for the anchor screw and springs.
Here are two pictures I took while at the Space Center in Huntsville AL. I always am inspired by the Apollo program.
Ever seen those commercials with cars and smoke lines going across their body? Those cars must be really fast if smoke can go over their roofs! Anyways, they were in a wind tunnel. Wind tunnels are built to determine how objects behave as they pass through the air, only this time the objects are still and the air moves around them. Wind tunnels can be used to study a host of very interesting questions about how fluids (air is considered a fluid) work and the way things we build interacts with it. Investigating my wing designs through a wind tunnel provides the possibility to create developments in aircraft designs and promises the ability for me to learn more about fluid mechanics.
Originally, I bought a great and beautiful piece of poplar wood from Home Depot's 85% off cart. It was 1"x9" and about 6 feet long. I bought it not knowing what I was going to do with it, I just thought it was wonderful. I don't remember what came first deciding to build a wind tunnel, or finding the sheet of Plexiglas. Well, I found a sheet of Plexiglas about 3 foot square in my basement and determined to build a wind tunnel! From the get-go, I understood this would not be a University Research Lab quality, size, or speed, but just enough to take some basic, low accuracy measurements which could roughly characterize performance of my wing or aircraft designs. Therefore, I did a little online reading: http://www.grc.nasa.gov/WWW/k-12/WindTunnel/build.html and got started.
It would be best for the wind tunnel to be as large as possible and be round, but I am limited in shape, height, width, and length to my piece of poplar. With a middle section of Plexiglas on three sides, I calculated how long the wind tunnel could be given my length of poplar. It turned out to be about 3 feet. The bottom entirely poplar and the two sides and top about a foot of poplar then a foot of Plexiglas then another foot of poplar with some overlap between wood and Plexiglas for attaching the two. I measured and cut the poplar. Then using a router, cleared out space for the over lap of Plexiglas and wood. I cut the Plexiglas using a hand saw, and wow I can't stand inhaling the Plexiglas dust. I assembled the sides and top using epoxy to attach the Plexiglas to the wood. With each side and top assembled, I decided to make the wind tunnel wider than taller, creating joints where the sides but up against the edge of the top and bottom. I pre-drilled four holes at each joint and used 2 inch wood screws to attach all four parts.
The biggest difficult came in the joint between the top and two sides. I wanted the Plexiglas of the top to hug the sides at the joint to reduce leaks, but in doing so, made it too wide. Therefore, when I screwed it together the epoxy gave way on top as the Plexiglas bowed inward.Bummer.
What to do. I had already applied caulking to all the joints and wanted to be clever and find a solution outside of disassembling the entire thing. I took out my trusty electric drill and drilled holes along the edge of the Plexiglas where it was too wide. Then using wire cutters, cut the remaining material between each hole, eliminating the problem and not damaging anything caulking couldn't fix! I had never caulked anything before this day, but found it pretty easy and messy.
I finished the day(s) telling people I had a Wind Tunnel, and yes, I literally had a wind tunnel, merely the tunnel. Exciting and possessing a wind tunnel, bravely imagine what one day may be tested, and boldly think what interesting things may be learned!
Here is the created Tunnel, sorry not the best picture.
|Epoxy after Plexiglas bowed inward|
I call myself Ben Washington. I have initiated this blog to record my personal executions into science. I call this Basement Science, since my experiments and investigations are conducted in my free time and in my home.
This is not a new or unique phenomenon, but inspiration comes out of my own genuine interest and following the example of many who have come before me. For instance, John Harrison was a carpenter's son and self taught in mechanics. Through is own independent study and work developed the first accurate seaworthy clock, solving a centuries old problem of determining longitude at sea. The story is a little more complicated, but that is the gist. http://www.amazon.com/Longitude-Genius-Greatest-Scientific-Problem/dp/0140258795
I believe we may still follow his example. Let us seek answers, build our own devices, and create solutions even in our own homes. I call this Basement Science.