FYFD

Tag: shockwave

  • Cavitating Inside a Tube

    Cavitating Inside a Tube

    Cavitation – the formation and collapse of low-pressure bubbles in a liquid – can be highly destructive, shattering containers, stunning prey, and damaging machinery. Inside an enclosure, cavitation can happen repeatedly. Above, a spark is used to generate an initial cavitation bubble, which expands on the right side of the screen. After its maximum expansion, the bubble collapses, forming jets on either end that collide as the bubble shrinks. Shock waves form during the collapse, too, although in this case, they are not visible.

    Those shock waves travel to either end of the tube, where they reflect. The reflected waves behave differently; they are now expansion waves rather than shock waves. Their passage causes lower pressure. The two expansion waves meet one another toward the left end of the tube, in the area where a cloud of secondary cavitation bubbles form after the first bubble collapses. Pressure waves continue to reflect back and forth in the tube, causing the leftover clouds of tiny bubbles to expand and contract. (Image credit: C. Ji et al., source)

  • Rocket Launch Systems

    Rocket Launch Systems

    If you’ve ever watched a rocket launch, you’ve probably noticed the billowing clouds around the launch pad during lift-off. What you’re seeing is not actually the rocket’s exhaust but the result of a launch pad and vehicle protection system known in NASA parlance as the Sound Suppression Water System. Exhaust gases from a rocket typically exit at a pressure higher than the ambient atmosphere, which generates shock waves and lots of turbulent mixing between the exhaust and the air. Put differently, launch ignition is incredibly loud, loud enough to cause structural damage to the launchpad and, via reflection, the vehicle and its contents.

    To mitigate this problem, launch operators use a massive water injection system that pours about 3.5 times as much water as rocket propellant per second. This significantly reduces the noise levels on the launchpad and vehicle and also helps protect the infrastructure from heat damage. The exact physical processes involved – details of the interaction of acoustic noise and turbulence with water droplets – are still murky because this problem is incredibly difficult to study experimentally or in simulation. But, at these high water flow rates, there’s enough water to significantly affect the temperature and size of the rocket’s jet exhaust. Effectively, energy that would have gone into gas motion and acoustic vibration is instead expended on moving and heating water droplets. In the case of the Space Shuttle, this reduced noise levels in the payload bay to 142 dB – about as loud as standing on the deck of an aircraft carrier. (Image credits: NASA, 1, 2; research credit: M. Kandula; original question from Megan H.)

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    Burning a Rocket Underwater

    In a recent video, Warped Perception filmed a model rocket engine firing underwater. Firstly, it’s no surprise that the engine would still operate underwater (after its wax waterproofing). The solid propellant inside the engine is a mixture of fuel and oxidizer, so it has all the oxygen it needs. Fluid dynamically speaking, though, this high-speed footage is just gorgeous.

    Ignition starts at about 3:22 with some cavitation as the exhaust gases start flowing. Notice how that initial bubble dimples the surface when it rises (3:48). At the same time, the expanding exhaust on the right side of the tank is forcing the water level higher on that side, triggering an overflow starting at about 3:55. At this point, the splashes start to obscure the engine somewhat, but that’s okay. Watch that sheet of liquid; it develops a thicker rim edge and starts forming ligaments around 4:10. Thanks to surface tension and the Plateau-Rayleigh instability, those ligaments start breaking into droplets (4:20). A couple seconds later, holes form in the liquid sheet, triggering a larger breakdown. By 4:45, you can see smoke-filled bubbles getting swept along by the splash, and larger holes are nucleating in that sheet.

    The second set of fireworks comes around 5:42, when the parachute ejection charge triggers. That second explosive triggers a big cavitation bubble and shock wave that utterly destroys the tank. If you look closely, you can see the cavitation bubble collapse and rebound as the pressure tries to adjust, but by that point, the tank is already falling. Really spectacular stuff!  (Video and image credit: Warped Perception)

  • Break-Up of the Chelyabinsk Meteor

    Break-Up of the Chelyabinsk Meteor

    In 2013, a meteor about 20-meters in diameter broke up over Chelyabinsk, Russia in a dramatic display that damaged buildings within 100 km and injured more than 1200 people. To better understand the threat presented by such objects, NASA has been conducting 3D, hypersonic simulations like the one shown here. The meteor material is shown in gray and black. Brighter colors like red and yellow indicate the hot, high-pressure shock wave caused when the meteor slams into the atmosphere. Aerodynamic effects quickly erode the meteor, ripping it into pieces that disperse energy explosively in the atmosphere. While you might think the meteor breaking up is good for us, it’s actually the blast waves from its break-up that cause the most damage.  (Image and video credit: NASA, source; via Gizmodo)

  • Cavitating

    Cavitating

    Cavitation happens when the local pressure in a liquid drops below its vapor pressure. A low-pressure bubble forms, typically very briefly, when this occurs. These bubbles are spherical unless they form near a surface. In that case, the bubbles take on a flatter, oblong shape. As they collapse, the bubbles form a jet, like the one seen inside the bubble above. The jet extends through the bubble and stretches into a funnel shaped protrusion on the bubble’s far side. Eventually, the whole shape becomes unstable and breaks into many smaller bubbles. Shock waves can be generated in the collapse, too; often the jet generates at least two in addition to the ones created when the bubble reaches its minimum size. This is part of why cavitation can be so destructive near a surface. (Image credit: L. Crum)

  • Stellar Bow Shock

    Stellar Bow Shock

    This Hubble image shows a young star in the Orion Nebula and the curved bow shock arcing around it. Despite its age, the star LL Orionis is energetic, producing a stellar wind that exceeds our sun’s. When that wind collided with the flow in the Orion Nebula, it formed this bow shock that is about a half a light-year wide. We don’t often think about fluid dynamics applying in space, but if we consider a lengthscale that is large enough, even space contains enough matter to behave like a fluid. LL Orionis’s bow shock is in many ways comparable to ones we see form around re-entering spacecraft. (Image credit: NASA/Hubble, via APOD; submitted by jshoer)

  • Bottle Rocket Shock Diamonds

    Bottle Rocket Shock Diamonds

    Mach diamonds or shock diamonds can often be seen in the exhaust of rocket engines. Here they’re shown in high-speed video of a bottle rocket’s launch. The rocket’s exhaust exits at a pressure that is higher than the surrounding atmosphere, which causes the exhaust to bulge outward and forms two expansion fans, seen in pink, to lower the pressure. The pressure actually drops too low, however, causing shock waves, seen in turquoise, to form in order to raise the exhaust’s pressure. This back-and-forth between shock waves and expansion fans continues, forming the diamond shapes we see. Each subsequent set gets weaker as the exhaust closes in on the right pressure, and ultimately the series of diamonds fades into turbulence. (Image credit: P. Peterson and P. Taylor, source)

  • Shocks on a Wing

    Shocks on a Wing

    Commercial airliners fly in what is known as the transonic regime at Mach numbers between 0.8 and 1.0. While the airplane itself never exceeds the speed of sound, that doesn’t mean that there aren’t localized regions where air flows over the airplane at speeds above Mach 1. In fact, it’s actually possible sometimes to see shock waves on the top of airliner’s wings with nothing more than your eyes. The animations above show shock waves sitting about 50-60% of the way down the wing’s chord on a Boeing 737 (top) and Airbus A-320 (bottom). The shock wave looks like an unsteady visual aberration sitting a little ways forward of the wing’s control surfaces.

    The wings themselves are shaped so that these little shock waves are relatively stationary and remain upstream of the flaps pilots use for control. Otherwise, the sharp pressure change across a shock wave sitting over a control surface could make moving that surface difficult. This was one of the challenges pilots first trying to break the sound barrier faced. (Image credits: R. Corman, source; agermannamedhans, source)

  • The Sound of a Balloon Popping

    The Sound of a Balloon Popping

    The pop of an overfilled balloon is enough to make anyone jump, but you’ve probably never seen it like this. The photo above uses an optical technique known as schlieren photography that reveals changes in density of a transparent gas like air. The shredded rubber of the balloon is still visible in black, and around the balloon there’s an expanding spherical shock wave. It’s the sudden release of energy when the balloon ruptures and the gas inside begins to expand that causes the shock wave. Notice, though, that the gas from the balloon is still clearly visible and balloon-shaped–much like a water balloon that’s just popped. From that clear delineation, I would say that this balloon was filled with a different gas than air–otherwise the density shouldn’t be different enough to make the interior gas distinguishable.  (Image credit: G. Settles)

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    Underwater Explosions in Slow Mo

    The Slow Mo Guys bring their high-speed skills to underwater explosions in this new video. The physics of such explosions is very neat (but also incredibly destructive). When the fuse ignites, a blast wave travels outward in a sphere, creating a bubble filled with gas. Eventually, the pressure of the surrounding water is too great for the bubble to expand against. When its expansion slows, that much larger pressure from the surrounding water starts to crush the bubble back down. Decreasing the volume of the bubble raises its pressure and its temperature again, and this often reignites any leftover fuel and oxidizer left in the bubble. The secondary shock bubble will re-expand, kicking off another round of expansion and collapse. (Video credit: The Slow Mo Guys; submitted by potato-with-a-moustache)