AK-47 Underwater at 27,450 frames per second (Part 2) - Smarter Every Day 97

Hey, it's me, Destin. Welcome back to Smarter Every Day! So, I've been learning a lot about guns underwater, which is pretty cool. I mean, in the first video, I learned all about what's happening back here in the action. But the problem is, because of limitations in my setup, I didn't get to see what happened when the bullet exits and goes into the water.

Now, I really want to see that, so I had to think about it for a while and I had an aha moment. You see, instead of building an aquarium that was keeping the water in with the gun, I built this to keep the water out. But the problem is, even though I built all this cool stuff and got my hands on one of the best high-speed cameras on the market, I didn't have enough hands to run it all. So, I invited some friends to help me. I was hoping to get the widest view possible that we can get inside that mirror.

So, we got to be as close to the mirror as possible. I think, sadly, the river—I think so. What kind of lens do you think you're gonna use?

Lens? So you're gonna be in the middle? So we should have it when sitting. Wait, the problem with doing an 8k underwater is that I can't trigger the camera. So that's the slow-mo guys to come help me.

So, what are you gonna run? Run in the fence. It would be 1610 today, which goes up to 80 thousand frames a second at 720p. I can't count that high. Definitely more than he's in me.

Alright, just to show you how cold it is, or do this: that’s about 40 degrees—it’s cold! So, the camera’s gonna look into the top mirror. It’s going to bounce down, and then basically, it's a periscope underwater. A bit of a gun here.

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See it there with a bat?

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Alright, it has been very consistent. We're getting about five to six feet of bullet down.

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You okay? So what did we just learn? You can see that there's gas that comes out right here—it's a little gas bubble. The reason that's happening is because the piston is venting just when it gets pet.

Talking freezing, the bullet goes down the barrel. It passes this gas block and pushes gasses against this piston. Now, you can see it starts to move the bolt, but watch what happens right there. It opens up this little gas port—a bubble right there during the shot.

So, if we take this off, you can see that the piston vents after it goes only about a quarter of an inch back. If you think about it, that’s cool because this very short pressure impulse is enough to overcome all the springs and the friction in the weapon and cycle it simply due to inertial forces.

Okay, so now we're gonna do over-the-shoulder?

Yeah, yeah, yeah! Over the shoulder stuff!

Alright!

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Four, five, six, one, three, two, one! Let’s play!

Oh!

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Yeah, I know that was gonna be cold, but not that cool. Let me try to explain what I think is going on here. So, we got this oscillation that you can see after the shot and the bubbles—that's awesome! I didn't understand why the bubble would start back up after it collapsed. But here's what's going on.

There's an equation called the Rayleigh-Plesset equation that describes everything a bubble does underwater. It's too hard to solve by hand. You have these computers to figure it out. But this is basically what's going on: at the initiation point, you have a super high pressure inside of a bubble, and it begins to impart momentum to the fluid around it.

So, it begins to grow. At some point, it passes the point where the pressure inside the bubble is equal to the pressure outside the bubble, and it continues to grow until eventually, the water stops it. Now, at this point, you have low pressure inside and high pressure outside, so it begins to collapse again.

Because of fluid momentum, it goes again beyond that equilibrium point and begins to compress. So, it gets a super high pressure on the inside again, and boom! Another shockwave in the process starts all over again. This oscillation occurs until you dissipate all the kinetic energy in the system.

Now, at this point of tidal closure, you have the highest pressure. At this point, sometimes something can occur called sonoluminescence. Sonoluminescence occurs when you get a flash of light when a cavitation bubble is collapsing. Now, as much as I want to believe this is sonoluminescence, I'm pretty sure it's just a really cool reflection from the sunlight above the pool. But it’s still really interesting that occurs at the point of collapse.

I wonder why that's happening. Okay, there's something else we need to talk about. This is my favorite show.

Originally, I thought that the first gases out of the barrel were from where the bullet exits and the expanding gases from the cartridge float around it. But you see that black color traveling down the link to the bubble—that's the burnt gunpowder being released from the barrel behind the bullet.

So if you follow that powder down the bubble, it should line up with that bullet. Yeah, there it is! So what's the first white cloud then? If you have a flowing liquid and you speed it up, the pressure of that fluid drops.

Now, it seems a little bit backwards from how it should be, but this is what happens: it’s called the Bernoulli principle, where flow is high, pressure is low. So let's look at this phase diagram for water. The water we were in was about four degrees Celsius and about one atmosphere.

If we drop the pressure of the water below a certain point, the water turns to vapor. The inside of the barrel was full of water before we shot, right? And so the bullet pushed it out at a very high speed. So, where flow is high, pressure's low, cavitation is happening in the barrel on the front side of the bullet.

I'm still trying to wrap my head around this, but there it is! You can actually see it in the video. Once the bullet punches out of this cavitation cloud, something else is happening.

You'll notice that the bubble on the right looks like a cloud, and the bubble on the left looks more like glass. Andrew, David, and Daisy took some awesome shadow graphs of bullets in flight, which showed the shock wave on the front of the bullet. The area behind the shock wave is lower in pressure; it turns to vapor.

Because this low-pressure region has a smoother flow boundary, it looks more like glass instead of the fuzzy cloud look caused by the turbulent flow coming out of the barrel. So there you have it: you're not just shooting a bullet out of this gun—you’re shooting three different things.

Now that we understand the physics behind cavitation, you can clearly see the effects of each of these three components in the high-speed video. Oh yeah, and we also understand bubble bounce now too, don’t we?

So a huge thank you to Gavin and Dan, the man from the slow-mo guys. They came all the way to Alabama to help me shoot this video. That's a pretty big deal. So, we redid a video on their channel—something I did a while back: pistols underwater, only we use the B 16:10. It's awesome! Go check it out on their channel—it’s totally worth your time!

On my channel, the intent here was to make an awesome video that you enjoyed, and you also learn something and perhaps earn your subscription. So, if you think I got close, we got a little chemistry going on. Check out part three of this video—Russian frogmen guns! Yeah, they exist!

So I got my hands on some of those. I still have the Ninja Scope 3000 or whatever I’m gonna call that thing. Kind of see where this is going.

Anyway, I'm Destin. You get smarter every day. Have a good one!

So we actually selected this pool for a reason. Jimmy Neutron lives in the middle of nowhere, so if the bullet got away from us, everybody downrange would be safe. Please be smart—don’t try this!

To check out how he kissed the glass—huh? What? What did you just do? Get away from our computer! Get away from a computer!

Alright, we got the bullets. There is no damage, but you can definitely see the grooves from the rifling and barrel.

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