#flowviz

Week 10: dead week. Contrary to popular belief it’s not because there’s nothing to do, but because you actually wish you were dead. We had one last test to get done during dead week, and it was the full scale smoke-car setup. Because we couldn’t do much during the day, on Monday we decided to play around with the smoke in the maintenance road between the aero labs to see what the best setup would be for the test later that night.

We tried deploying the smoke closer to the wall:

Closer to the ground:

And we tried shining the sheet laser through it to see if we got any interesting flow viz during the testing/verify if the laser would work later that night

Because it was so bright and windy during the day, we actually didn’t get any more useful information about how to set up the smoke machine and laser that we wouldn’t have already done. We did, however, set up the mounting brackets for the lasers that would allow them to be taped to a window (which, sadly, did not end up as the final setup).

Once we had gathered all we thought we needed from testing different setups, we put the smoke machine and lasers away in the props lab and worked on the presentation some more.

When night fell, it came time to test. The winds were much calmer and the light was much...darker, but it still wasn’t perfect. This is what it looked like once we had all the lasers and smoke machine set up

It was still a little bright, so we shut down the lights in the props lab and covered up the walkway lights closest to the test.

We also tried different methods of lighting up the smoke, like the Ryobi spotlights, since it was somewhat difficult to see the lasers from every viewing angle (an explanation for that can be found below). The spotlight wasn’t much better, so we decided to stick with the laser sheet.

Who knew a CR-V could be menacing?

The smoke was hanging around much better than it was during the day, but it still needed a little assistance. Thus Chase was designated as the smoke-scooper, wherein he used a box to redirect some of the smoke into the ground as it exited the machine to bleed off some of the momentum. It worked pretty well, to my surprise.

Once the smoke and lasers were set up, then came the admittedly pretty fun task of driving through the smoke attempting to not be blinded by the laser sheet. I found that the faster I drove, the better Dr. Doig said the flow viz was, so I tried to speed up as quickly as possible through the smoke without peeling out. This was the best run we had that night, slowed down a little as the car passed through it, with an annoying unexplainable ten seconds of black screen at the end of it. I think I was too hasty when I exported the video to YouTube, and didn’t make sure I had cut it all the way.

The smoke looks like these awesome green flames, credit due to cameraman Dr. Doig. The vortex due to the RVSM is admittedly pretty difficult to distinguish from the rest of the vehicle, but if I squint really closely at the right area, I can convince myself there’s a little bit of movement in the smoke before the rest of it churns up.

With the final videos taken, all that was left was to embed them in the presentation and prepare for Wednesday.

Presenting on Wednesday went smoothly and I was especially excited to see what other groups had come up with. Things that I found especially cool included the active flow control wing and the shock tube project. I think the active flow control in particular is something that will absolutely be widely used in the future in some form of implementation or another, the benefits are way too good to look over.

If you were wondering why the smoke and lasers only looked good from a specific angle, I have these sources for you:

https://www.wyatt.com/library/theory/multi-angle-light-scattering-theory.html

https://www.azom.com/article.aspx?ArticleID=9821

A quick rundown:

source: azom.com

When you shine a light (laser) through a medium, you’ll get two types of scattering: isotropic and anisotropic. As the names suggest, isotropic is more even scattering, and anisotropic is uneven. Anisotropic scattering occurs when the light impinges on particles that are larger than it’s wavelength. The particles create zones of constructive and destructive interference that manifest themselves in certain “islands”, if you will, of local maximum intensity. The schematic below provides a simple overview of this.

Cursory observation of the Deconvolution and Scattering Pattern charts show pretty clearly that the intensity of the light will gather at certain deflection angles. You’ll notice that the most intense of these deflection angles are mostly small angles, however. As you move further away from the center, or focus, of the laser, it gets progressively less intense. This means that looking at the medium the laser passes through from a small angle means you’ll get a fairly sharp image due to the relatively high intensity of scattered light. However, if you are looking, say, normal to the medium, you won’t see much of anything. This explains why the laser sheet only looked good through the smoke when you were on the opposite side from the lasers, because you were able to capture a smaller deflection angle from the laser.

Course Reflection:

a) If you were to pick one thing that you feel like you understand pretty well about aerodynamics now that you didn't at the start of 307, what is it and why?

I feel I understand the actual mechanism of things like turbulators, trip strips, and vortex generators. Previously I didn’t exactly understand how roughness influenced the boundary layer, but now I know that surface roughness and vortex generators can influence tiny vortices into the flow, localized within the boundary layer, and that is how the boundary layer becomes energized.

b) If you were to pick one thing about aerodynamics that still confuses you, what is it and why?

I still don’t fully understand how you can aim a dye needle directly at a model car mirror, in the freestream and still have it push right off of the car. I guess that means I don’t understand streaklines. We got good enough results by moving the dye needle closer, but that shouldn’t have been necessary. Is there a theoretical spot that the streaklines around the car will direct the dye flow into the mirror? I think there should be but we certainly didn’t find it.

c) What was your 307 highlight?

My 307 highlight was definitely the UV flow viz of the Tesla. It was the perfect combination of good dye mixing, the dye actually flowing, and getting the needle in the perfect spot.

d) What was your 307 lowlight?

307 lowlight was probably either the force balance lab or the wake survey. The force balance lab because it was incredibly strange how drag numbers were high across the board but we couldn’t seem to figure out why. The necessary numbers were being subtracted from each other, but numbers were still way too high. I am tempted to chalk it up to bad calibration practice, even though it’s a lazy answer.

The wake survey deserves honorable mention, in my opinion, because of the run where we adjusted the angle of attack even though we hadn’t hit the wake yet with the probe position. It was a mad dash to gather the correct data before the end of the lab, and though stressful at the time, it’s good to laugh about it now.

e) For many of you, this'll be the last time you really engage with aerodynamics, since you prefer structures or controls or design or just anything else... for others, this course will have been a springboard to many future aerodynamic adventures. What do you think the future holds for you in aerospace engineering?

There’s a metaphor (simile? analogy?) I heard a while ago. I think it’s from an Asian philosophy like Taoism or Buddhism but I’m not actually sure. There’s all sorts of things in the river, like sand, which goes with the flow no matter what, it has no choice. There are large rocks, which sink to the bottom and move no matter what. Pebbles, however, can stay if they fall in the right spot, but once they get kicked up, they’re along for the ride.

Why the cheesy metaphor? I feel like I’m the pebble. I don’t have a strong desire to do one thing or another in aerospace engineering, because there’s so many things to know and gain mastery on. So, I don’t know what I think the future holds for me in aerospace engineering. Once I get kicked up by graduation, I’m going to follow the opportunities I see to land in the perfect spot to settle down.

With week 9 being an abbreviated week, I didn’t expect there to be much to report, but, as a pleasant surprise, we were able to get some great flow visualization of the Tesla model side mirrors. We got out the fluorescent dye and set up the blacklights over the water channel. It took some minor adjustments, but eventually we figured out the best spot for the dye needle to be to produce optimum flow viz results. In order to avoid the dye catching a streamline that ran up the A-pillar and down the length of the car, we had to place the dye needle a short ways upstream of the mirror. For a little while we were able to get some great shots. More on the failures of the test later on.

Here’s the flow viz captured at 4K auto settings, so the camera AF causes the dye to flash in and out of focus (That’s a little disingenuous, since YouTube only plays back at 1080p):

And here’s the flow viz captured at 1080p@60fps. This one was taken on pro settings so the dye is not terribly in focus. This should allow better analysis of the film in a video editor, however:

There are three things I think really stand out from these videos. The first is that there’s no perfect dilution possible between the dye and water, as the dye is nonpolar, as evidenced by the fact that it must be diluted in mineral or paraffin oil, while water is polar. The end result is that instead of a clean bit of glowing fluid, there are distinct particles of dye that swirl around in the flow. Any shedding vortices or other flow features are still fairly visible, so this isn’t too concerning.

The second thing that stands out to me is the apparent downward component to the wash behind the mirror. Dr. Doig theorizes (you might be able to hear this in one of the videos) that this is because the flow coming off of the A-pillar whips down and drags some of the flow behind the mirror with it. This is a fairly likely explanation, but I would want to know if the turbulence having that downwash effect increases or decreases the drag from the mirror.

The final thing that I notice from these shots is the sheer size of the wake. The main part of the recirculation zone reaches almost a third of the way down the driver’s window, and every so often you can see licks of the dye reach all the way back to the B-pillar. This is about 4 times the characteristic length of the mirror (the thickness). Given that the wake of a car is anywhere from 1-2 characteristic lengths long, this leads me to believe that the mirror has proportionally a larger amount of drag associated with it than the rest of the vehicle. Thus, I think that simple frontal area calculations will not be sufficient to calculate the effect that removing the side mirrors will have on the drag of the car.

Speaking of frontal area calculations, we took rough measurements of the Tesla to figure out what percentage of the frontal area that the mirrors were. Approximating the main chassis as rectangular (a bad assumption) and the mirror as also rectangular (an even worse assumption), we got measurements of 108*75 mm for the chassis and 13*9 mm for the mirror. This comes out to 117 mm^2 for the mirror and 8100 mm^2 for the chassis. That’s about 1.44 percent of the frontal area per mirror. Times two mirrors means that in total,  the RVSMs on a Tesla Model S make up 2.88% percent of the total frontal area. It’s worth noting that as a sedan, the mirrors on the Model S are much smaller than they are on, say, a pickup truck or a semi truck. When we eventually test with the truck and smoke (hopefully on Monday), we should be sure to take measurements of the truck and its mirrors to get a sense of how big the mirrors are on other types of vehicles. Running back around to my theory about the drag on the mirrors, just because the mirrors make up 2.88 percent of the frontal area on the Model S, I predict that if they were to be removed, the drag on the vehicle would be improved by more than that value. However, in the absence of a way to test this prediction, I’ll leave this problem to future scholars who revisit this problem using CFD or even practical testing.

Edit: I realized I didn’t talk about the failures we experienced using the fluorescent dye. Unfortunately we didn’t grab any pictures as we had our hands full trying to clean up the mess, but after a very long time using the fluorescent dye without any hiccups, the dye suddenly stopped coming out of the needle. Upon opening the bottle up, it was found that the dye had congealed into a goopy mess. We figured that we had been a little lax on making sure the bottle was shaken up, so we washed the bottle out and tried again, making sure to shake the bottle vigorously this time. Still, nothing came out. We found that again the dye had congealed into a mess around the outgoing tube.

We finally got it to work! Check it out! We used 2 flood lights, 2 go pros, and came in at night to prevent lighting errors and we got a lot of sweet videos and pictures of flow around this football. From this video alone we can see that the boundary layer grows slower and the flow remains attached longer when the ball is spinning vs. when it is not. Of course, the way the ball is mounted prevents 100% accuracy due to interactions with the big wood block, but from this angle, the wake behind the ball is visibly smaller when the ball begins to rotate. This small wake also causes more interaction with the brick, but we can compare the relative size to the 3.5“ long edge of the wood block to get a reasonable measurement.

Day 10- Mounting Balls & Excitement Mounting

Pictured above is the final draft(lol) of the mounting/spinning rig that spins our football to visualize flows. We are still having some issues with the ball being centered and it can shake violently upon starting the motor, but no issues occur during constant speed. We have gone through a couple threaded pieces that actually mount the ball trying a couple of different centering techniques based on the geometry. Our final try was our best and over an hour of centering and we have come to the conclusion that when thrown, a normal football wouldn’t spin perfectly (usually, ok Tom Brady) especially after kicking and it will suffice for the amount of accuracy we can obtain. Overall, we are excited to test tonight...

Welp...the laser that we were planning on using is no longer working. The next opportunity to capture plane laser data would be on Friday afternoon. So, John and I will be capturing footage with many flood lights and hopefully obtain something useful! I am kinda sad that the laser isn’t going to work, but hopeful since our rig is finally working the way we had envisioned, we will see something worth seeing. It is what it is and we will do our best to get something useful!

I will be skipping part of Wind Ensemble tonight to test so I hope I didn’t overschedule myself for no reason. Having my own project that fulfills such open ended requirements has made me more invested in the success of the results and methods. I have found myself more invested in the project because it is something I helped( :-/ ) come up with and design. Sometimes the dynamic nature of the curriculum in the Aero department doesn’t always work out on the first iteration (cough AERO299 cough), but this lab has been amazing so far and I have learned so much because the problems that arise are my own making the solutions even more my own. Learn by doing at its best, thanks Cal Poly.

It’s 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 hadn’t made our wing yet, so we decided to use one of Dr. Doig’s pre-made wings to make sure we knew how to set up the sting assembly and the smoke machine. We didn’t actually end up using the sting, but that’s 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 isn’t anything particularly notable about this shot. I suppose it’s 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 didn’t actually grab a picture of the wing by itself, oops. There’s 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 there’s 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 wasn’t right given our total span couldn’t 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 you’ll take a gander at that handy “Variable Meaning” 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 didn’t 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, there’s 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 can’t tell, that wing is actually bending incredibly strongly toward the glass, and not because it’s trying to escape the test section. It appears that when crafting the wing, we didn’t take into account that it would actually be experiencing lift (that’s 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 weren’t 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, here’s a neat thing that I saw when I was making some hardboiled eggs. Aerodynamics? No. But fluid mechanics is close enough right?

We decided to go with the roof rack for our flow viz and finished up the last of our videos. Here’s a quick peak.

It looks like the flow is still separating with the roof rack, and the wake actually appears to be quite a bit larger, so there’s probably more drag. Also, we’re seeing the same Von Karmon vortices from the cylinder lab! This shouldn’t be a surprise since the racks are cylinders, but I imagine this will likely mean the racks are noisy.

With our report turned in, we began playing around with some ideas for flow viz. my group was interested in the water channel, because we’ve seen some really cool videos taken with the dye in the water channel, and they look a lot better than some of the videos we’ve taken of smoke in the tunnel.

We decided to use the Tesla model and spent most of the period trying our different amounts of dye and figuring out the optimal distance to put the model at, etc. Here’s one of the videos we took.

It definitely looks pretty cool if nothing else. It’s got separated flow behind the car, which we expected, considering the blunt rear of the car, but it looks like separation occurs at the top of the hood, which we didn’t expect.