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 ah-ha 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. It’s gonna work.
– Sickeningly good. (Destin) I was hoping to get the widest view possible, that we can get inside that mirror. – So, we’re gonna have to be as close to the mirror as possible with the wide angle otherwise we’ll see the rig won’t we.
– I think so. (Destin) What kind of lens do you think you’re gonna use?
– Lens… So you’re gonna be in the middle.
– So we should have a lens, shouldn’t we? [laughs]
– We should have a lens. [laugh] – The problem with doing an AK under water is that I can’t trigger the camera, so that’s the Slow Mo Guys who’ve come to help me. So what are you gonna run?
– Run the Phantom v1610 today which goes up to 18,000 frames per second at 720p. – I can’t count that high.
– Definitely more than.. you’ve got fingers. – Can you zip me?
– Unless you’re from Alabama.
– Alright just to show you how cold it is when we’re doing this, that’s about 40 degrees. And it is cooold. So the camera’s gonna look into the top mirror, it’s gonna bounce down and then basically it’s a periscope under water. I’ve got the gun here… You can see it there, with the phantom. [gun cocking] [gunshot] [gunshot]
[bubbles] [gun clearing] [slowed down gunshot] [multiple bangs] Alright, it has been very consistent. We’re getting about 5-6 feet of bullet travel. [gunshot] [slowed down gunshot] [slowed down gunshot] [bangs] [gun clearing]
OK. So, what did we just learn? You can see that there’s gas that comes out right here. It’s a little gas bubble. And the reason that’s happening is because it’s.. the piston is venting just when it gets past… [laugh] I can’t talk I’m freezing. As the bullet goes down the barrel it passes this gas block and pushes gas 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. That’s why there’s 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 1/4 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. [gunshot] [gunshot] [water spray] Oh gee.. it’s cold.
[laughter] Alright, so… So now we’re gonna do over the shoulder? – Yeah, yeah. Over the shoulder shot.
– Alright. (Destin) All clear to load?
– Clear to load. – Clear to fire? – Clear to fire.
– 3.. 2.. 1.. [gunshot] (Destin) Weapon’s clear. – Ohh. – Did you get it?
– I got it. I’ll go check it. [slowed down gunshot] [multiple bangs] [slowed down gunshot] [music] [slowed down gunshot] [bangs] [underwater gunshot] Yeah. I know that was gonna be cool, but not that cool. Let me try to explain what I think’s going on here. So we got this oscillation that you can see after the shot in 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 under water. It’s too hard to solve by hand, you’ve gotta use 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, and 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 shock wave and the process starts all over again. This oscillation occurs until you dissipate all the kinetic energy in the system. Now at this point of tightest 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 it occurs at the point of collapse. I wonder why that’s happening. OK there’s something else we need to talk about. This is my favourite shot. Originally I thought that the first gases out of the barrel were from where the bullet exits and the expanding gases from the cartridge flowed around it, but look again. Do you see that black colour trailing down the length of 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 and pressure’s low. So let’s look at this phase diagram for water. The water we were in was about 4 degrees celcius and about 1 atmosphere. If we drop the pressure of the water below a certain point the water turns to vapour. 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 Davidhazy took some awesome shatter graphs of bullets in flight which show the shock wave on the front of the bullet. The area behind the shock wave is lower in pressure. It turns to vapour. 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 re-did a video on their channel, something I did a while back, pistols under water, only we used a v1610. 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 3 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 we’re gonna call that thing. You kinda see where this is going. Anyway, I’m Destin, you’re getting 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 down range would be safe. Please be smart, don’t try this. So check out how he cuts the glass. [scratching]
(Destin) What? What did he just do?! [laughs]
(Destin) Get away from my computer, get away from my computer! Alright we got the bullets. There was no damage but you can definitely see the grooves from the rifling in the barrel. [ Captions by Andrew Jackson ]
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Please contact Destin if you can help.