Week 4- Eggy Rings and Bending Wings
Itās week 4 and First Team=Best Team is back in the wind tunnel, getting a disproportionate amount of wind tunnel time in (if the tunnel is open, we might as well use it right?). We hadnāt made our wing yet, so we decided to use one of Dr. Doigās pre-made wings to make sure we knew how to set up the sting assembly and the smoke machine. We didnāt actually end up using the sting, but thatās a minor detail that I can discuss later. Below is a picture from that lab period.
Besides the fact that it looks like a still from a Michael Bay film, there isnāt anything particularly notable about this shot. I suppose itās just proof that we actually know how to set the sting and pitch assembly up, sans wiring.
Triumphant in having set up the sting, we now set to our original plan of making our own NACA 4412 wing out of materials we had laying around the DBF lab. Luckily we found a wing already made out of EPP in the shape of what looks like a 4412. It had a chord of just over 6 inches, which is a little bit shorter than we wanted but we figured it was worth the time savings. All that was left to do was to cover it in Microlite to take away any surface roughness so that our VGs were working with as clean of air as possible.
(insert picture of covered wing here)
I didnāt actually grab a picture of the wing by itself, oops. Thereās still a lot of other pictures and videos that show the wing pretty well.
The next step was to design some VGs. We found this nice website that very helpfully laid out how we should go about choosing a size for our VGs. Upon doing a boundary layer height calculation, we found that if we made our VGs 80% of the size of the boundary layer, they would be on the order of 10^-5 m tall, which is pretty difficult to measure, so we decided to make them 1.5 mm tall, since that was about 1% of our chord, an arbitrary number we found a lot of actual planes using for the size of their VGs (of course thereās quite the difference in Reynolds numbers between a 777 and some dinky foam wing in a wind tunnel). We made the length of the VGs 5% of the chord, per the recommendation of the website, so they ended up about 8mm long. Then, using this handy dandy equation:
We calculated a vortex radius (and thus vortex generator spacing) of like half a meter. Well, that clearly wasnāt right given our total span couldnāt have been much longer than .5m, so where did all the error come from? First of all, I blame whoever wrote this website because they chose variable names that are literally the exact same as the rest of the aerospace industry uses to describe wings. So in this calculation I was listing the planform area of the wing and the span of the wing, which, if youāll take a gander at that handy āVariable Meaningā section, was not correct. Plugging the proper values into the equation spit out a spacing of 1 VG per 1 cm, which would have been about 50 VGs on our wing. We decided that was excessive, and decided to bump that spacing to 1 per inch (2.54 cm for you commies out there), and that would yield about 20 VGs on our wing, a much more reasonable number.
We didnāt jam the laser-cut VGs into the wing just yet, because we only had one wing and we wanted to test out the smooth wing first. I grabbed some video that shows roughly what we were trying to capture. The first is this profile shot that shows the flow maybe kinda sorta detaching toward the trailing edge? Either way, thereās not much to actually compare this smoke to, because we ran out of time before we were able to attach the VGs. We also had some, uh, wing flexibility issues. (excuse the rotation on the gif)
The picture below gives a pretty good idea what I mean by wing flexibility.
So if you canāt tell, that wing is actually bending incredibly strongly toward the glass, and not because itās trying to escape the test section. It appears that when crafting the wing, we didnāt take into account that it would actually be experiencing lift (thatās a hell of a thing to forget, right?). And that that lift would be strong enough to bend the wing. Because of this, I think that we werenāt able to reliably detach the flow by changing the angle of attack. The solution to this bending problem would be to jam a carbon rod in the wing and affix it to the roof so it stays upright. One of the other reasons I included this picture is because it shows very clearly the effects that the wingtip vortex has on the flow, as shown by the smoke splitting off upwards (I thought it looked super cool in person).
As a bonus for sitting through my posts, hereās a neat thing that I saw when I was making some hardboiled eggs. Aerodynamics? No. But fluid mechanics is close enough right?
To start, I pinky promise I didnāt arrange the eggs that way to begin with, they did that themselves. I think the pot was just centered enough that there was hotter water in the middle than the outside, and this made a convection current that pushed the eggs to the outside of the pot. This would have had to happen while the water was still heating up of course, since water is a constant temperature when boiling. The eggs were no less delicious.
