How does a whip break the sound barrier? (Slow Motion Shockwave formation) - Smarter Every Day 207

(Whooshing) (Smacking) - What's up, I'm Destin, this is Smarter Every Day. This is the tip of a bull whip and that crack you hear is this breaking the sound barrier. My question is why or how? Like, if you think about it, your arm's never leaving your body and something's going faster than the speed of sound in just a few hundred milliseconds and over several feet. That's a big deal.

OK, now would be an excellent time for me to explain April, April Choi. So April is an engineer first and foremost. I think all this whip business is just a reason for you to explore—

• Fluid dynamics?

• Fluid dynamics. (Cackling) I really do. So April on the internet, you may have seen her, Guinness Book of World Records whip stuff, she's good with whips. But what's really interesting about April is your brain.

• There's something in fluid dynamics known as the no-slip boundary condition. That means air molecules that are right next to this fluffy part stay next to that fluffy part.

• [Destin] And so, it's pulling that air with it.

• It depends whether or not you're using a Lagrangian framework, which centers on here, or an Eulerian framework that centers on the overall mesh.

• That's what I was thinking. I was wondering if it was a Lagrangian or Eulerian framework, but I wasn't going to say anything.

The first thing we did was create a new tip for the bull whip and attach it to the whip and after that, we set up the camera system. The way we're getting this shot is using the schlieren technique, and this is what took us so long to coordinate. Basically, we have a point-light source right here and that light is coming out, it's spreading out, it's hitting this mirror, this parabolic mirror, and as it comes back, what it's doing is it's converging to this point right here. You can see there's the light coming through at the focal point. And then we've got red and green gels right there and (whip cracks). (Laughing) That's scary. Go watch Derek's video on schlieren, it's better than this one. We're just gonna show you how a whip breaks the sound barrier. That is unnerving. After everything was set up, we literally got crackin'. (Whip cracks) OK, that triggered. Let's see what it did.

We learned two major things in my buddy's garage. First a question, though. What point in the whip extension do you think the crack happens? Growing up, I used to play Castlevania a lot, so for me, it made sense that the crack of the whip would happen at full extension of the whip 'cause that's what you want to do with the bad guys, right? You want to keep them as far away from you as possible. When we set up a high-speed camera expecting the whip to crack like it does in Castlevania, the shockwave would always enter the field of view before the whip did. (Thundering) Therefore, it was clear that the crack was happening before the whip was fully extended. So it's cracking way back there.

• It's cracking before we think.

• I learned something. I didn't know that.

• Yeah. I didn't know that either.

It actually happens as the whip unrolls, not at the end like I thought. And in order to visualize what's happening, we switched from the overhand strike to the sidearm strike. What you're about to see here are two engineers that have researched this stuff and were totally blown away because the experiment worked and we're starting to see things for the first time that we totally didn't expect. OK. We are getting somewhere. (Laughing) (Whooshing) (Thundering) (Snapping) The second thing that we learned in the garage is there may be a mechanism that's causing it to accelerate just before breaking the sound barrier. (Whip cracks) Those strands right there are not in tension. You see that?

• Yeah, they're just, it's chaos.

• [Destin] And then there's this moment—

• [April] Where they all come together.

• [Destin] Where they all come together, and when it starts to pull, that's when the initial shockwave starts.

• [April] So it's the collapse.

• [Destin] The collapse is when it happens.

• [April] And the drag coefficient's going down.

The fact that we're seeing a new mechanism is a really big deal. So obviously, we have to take this more seriously. We just figured out how whips work. We should totally publish this.

• Yeah.

Whip shockwaves have been studied from the experimental perspective in Germany in 1998 as well as the theoretical perspective at the University of Arizona in 2003. The Ernst-Mott Institute paper, by the way, freakin' amazing. There's a dude in it that looks like a moose wearing bells and a clown suit. (Laughing) I don't know what's happening. It's actually a great paper. You should totally read it. They talk about wishing they had a faster high-speed camera so they could see what happens at the tip. Also, the paper at the University of Arizona, they try to measure with math the entire length of the whip as it unrolls. They try to describe that movement.

But what if we could design one experiment that would do all of this? Like all of it at the same time. It would measure the three-dimensional position of the wave as it goes down the whip. It could also measure the tip velocity as it goes supersonic. What if we could do that? And that's exactly what we're about to do. Under the guidance of my doctoral advisor, Dr. Kavan Hazeli at the University of Alabama in Huntsville, we've assembled the team and we're about to figure this junk out.

We designed the experiment and gathered together in what's called the atom lab. It uses an array of cameras to track anything with reflective tape on it. The way it works is essentially this. You have a camera and you have a little infrared light around it, right? If you have a piece of reflective tape out there and you shine the light from the infrared camera onto the reflective tape, it bounces back to the camera and it shows up as a really bright dot. So we simply put reflective tape around the whip every 250 millimeters down the length of the whip. We also put reflectors on her arm so we could better understand the mechanical input to the whip.

The image from one camera would essentially be an array of white dots at 500 frames per second, but if you coupled this data with the data from other cameras, you can triangulate each individual segment of the whip at 500 frames per second giving you true three-dimensional data. OK, here we go. This is the kind of data we can now get from a whip strike.

• [Man] Three, two, one, go. (Whip cracks)

The footage you're now watching is 5,000 frames per second. You can see that the Vicon cameras up on the wall are taking data at 500 hertz, which means they're flashing every 10 frames on the high-speed camera here. You'll notice that the whip unrolls normally, very similar to how the paper from the University of Arizona described it mathematically.

Let's make a few observations here. First, there seems to be a wave that moves down on the whip. As the hand moves forward and then stops, it transfers momentum into the whip itself. Then, one segment of the whip, as it unrolls and straightens out, seems to transfer all of its momentum into the next segment, and then the next segment and so on and so forth. As indicated by this red line moving along the bottom here, you can see the velocity of that straightening out of the whip moves forward. We can then look at the atom lab data and measure the input momentum in three dimensions and use that information as a tool to help us build a model.

So the whip's coming up towards the mirror. (Muffled mumbling) That's awesome. Is that awesome?

• Yes.

Another thing to look at is what's happening on the top of the whip. The velocity is speeding up. Most researchers think this has to do with the conservation of momentum. The whip is tapered so each smaller section on the way down has to speed up to maintain the same amount of momentum. This is the exact reason we took so much time up front to measure the mass and dimensional properties of the whip all the way down.

This is where it gets most interesting for me. If you look closely at the atom lab data, you'll notice that right at the tip of the whip, the markers seem to disappear right when the whip accelerates. This is because the trackers lose the position of the whip markers when they're traveling their fastest. Even if the atom lab didn't lose the track, you can tell that the frame rate of the atom lab isn't sufficient to determine the acceleration through the most interesting part of the wave, which, of course, is the shock formation. This is exactly why we set up the schlieren camera. The atom lab gets all of the wave kinematics on the macro scale, and then the phantom can record the tip velocity and actually capture the formation of the shockwave. (Light guitar music) (Whooshing)

I'm not gonna explain any of our preliminary conclusions, but at this point, we're doing two types of analysis, obviously how that wave propagates, but also the tip velocity of the whip. If you watch closely, it looks like the tip's getting pulled along behind that shockwave. This is super complicated and we're still analyzing this. (Whooshing) (Smacking)

What we do know is that the popper isn't necessary. Dr. Kanistras really wanted us to visualize the end of the whip with only a knot on it and just look at how happy he was when his hypothesis was proven correct.

• There you go. There you go. There you go.

So it's like this. For the first time in history, we have true X-Y-Z data from the handle all the way to the tip of the whip and we can straight up write an equation for whip dynamics as a function of mass of the whip, length of the whip, mechanical input, maybe even aerodynamic drag. I know this sounds crazy, but I'm already changing my habits in everyday life because I understand whip dynamics better.

Have you ever done this? You're in your car, you reach for your charging cable and you pull it towards you real quick and it whips you really hard? That hurts like a mother. The reason that happens is whip mechanics. I cannot be the only person in the world that's ever done that. You don't want to just pull it quickly because that conservation momentum builds up and you get lashed in the face.

So bull whip was probably the first manmade invention to break the speed of sound. But my favorite manmade invention to break the speed of sound was the SR-71. This is not an SR-71, this is the A-12, the predecessor to the SR-71. There are 13 of these built. I'm now going to simulate running to the back at the speed that this aircraft can fly. Ready, watch. That was fast, wasn't it? OK? (Laughing) I'll go back and do it slower and tell you about the aircraft on the way.

OK, we're back at the front. So, I want to tell you about Audible. Audible is sponsoring this video. There's a book called Skunk Works. You can get a free audiobook of your choice by going to audible.com/smarter or texting the word smarter to 500500 to get any audiobook of your choice. In this case, your choice is Skunk Works. I've already made your choice for you. You have to listen to this book. It's about the development of the SR-71 and the F1-17 stealth fighter. I'm sorry, I just passed the hot naughty bits. Look at this.

So think about the shockwave. As you're going Mach 3.3, which this could do, think about what happened. The shockwave would go right there and it'd spread out. But you had to get air inside the cowling there. It's amazing. Anyway. Go to audible.com/smarter, download Skunk Works, listen to it with your ear holes. You're gonna love it. This thing would heat up in flight. They had to make it out of titanium. All kinds of cool stuff in the book. I just want you to go to audible.com/smarter, download Skunk Works, or text the word smarter to 500500. You're gonna learn stuff, it's gonna make you smarter and you're gonna know more about breaking the sound barrier.

Um, I have two blasters and if I fire the one that you're thinking about right now, feel free to subscribe. Or not, whatever. Ready? (Sirens blare) (Chuckling) They're on the same thing. That's a, they cycle...

(Sirens blare) See, but now they're not. The gap in my data is spatial. I'm not gonna get any of that. And the gap in your data is temporal.

• Right, yes, you're right.

• [Destin] But with our powers combined, we're gonna track a whip.

• [Man] Right, the fact that my (mumbling) is accurate position.