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

How does a whip break the sound barrier? (Slow Motion Shockwave formation) - Smarter Every Day 207
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    (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-Mach 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 can 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 whips 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 seems 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 F-117 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 spacial.
    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.
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