Surface tension is the result of an imbalance between intermolecular forces near an interface. Imagine a water molecule far from the surface; it is surrounded on all sides by other water molecules and feels each of those pulling on it. Since all the nearby molecules are water, the tugs from every direction balance and there is no net force. Now imagine that water molecule near the air interface. Instead of being influenced on all sides by water, our molecule now feels water in some directions and air molecules in another. The water molecules tug harder on it than air, leaving a net force that pulls along the interface. This is surface tension, and, for a liquid-gas interface, it behaves somewhat like an elastic sheet. Surface tension is even strong enough to let a jet of soap solution bounce repeatedly off a soap film. Each bounce deforms the interface, like a trampoline dimpling when someone jumps on it, but surface tension keeps the interface taut enough for the jet to skip off without breaking it. (Image credit: C. Kalelkar and S. Phansalkar, source)
Search results for: “jet”
Flag Flapping
Everyone has watched a flag flutter in the breeze, but you may not have given much thought to it. One of the earliest scientists to consider the problem was Lord Rayleigh, who wrote an aside on the mathematics of an infinite flag flapping in a paper on jets (pdf). Today researchers consider the problem in terms of fluid-solid interaction; in other words, to study a fluttering flag, you must consider both the properties of the flag – its flexibility, length, elasticity, and so on – and the properties of the fluid – air speed, viscosity, etc. The combination of these factors governs the complicated shapes taken on by a flag. The image above is a composite of several photos of a string (a 1-d flag) flapping in a flow that moves from left to right. By combining photos, the image highlights the envelope of shapes the flag takes and demonstrates at a glance just how far the flag flutters in either direction along its length. (Image credit: C. Eloy)
Galapagos Week: Pistol Shrimp
One of the most striking things about snorkeling in the Galapagos was how loud it was underwater. There were hardly any boats nearby, but every time my ears dipped below the surface, I could hear a constant cacophony of sound. Some it came from waves against the sand, some of it was the sound of parrotfish nibbling on coral, but a lot of it was likely the work of a culprit I couldn’t see hidden in the sand: the pistol shrimp.
These small crustaceans hunt with an oversized claw capable of snapping shut at around 100 kph. When the two halves of the claw come together, they push out a high-speed jet of water. High velocity means low pressure – a low enough pressure, in fact, to drop nearby water below its vapor pressure, causing bubbles to form and expand. These cavitation bubbles collapse quickly under the hydrostatic pressure of the surrounding water, creating a distinctive pop that makes the pistol shrimp one of the loudest sea creatures around. (Image credit: BBC Earth Unplugged, source; research credit: M. Versluis et al.)
All week we’re celebrating the Galapagos Islands here on FYFD. Check out previous posts in the series here.
Paintball Collisions
In their latest video, the Slow Mo Guys collide paintballs in mid-air, creating some pretty great paint splashes. The high-speed video does a nice job of revealing some of the typical stages a splash goes through. Initially, the paint spreads in a liquid sheet. As it expands and (necessarily) thins, holes form and grow, driving the paint into string-like ligaments. These ligaments are also stretching and eventually break up into an spray of droplets, much like the jet dripping from your faucet does. If you’d like to see some more awesome high-speed liquid collisions, check out what happens when a solid projectile hits a falling drop and then look at when a laser pulse hits a droplet. (Image and video credit: The Slow Mo Guys; submitted by Omar M.)
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.)
Chains of Salps
Salps are small, jellyfish-like marine invertebrates that swim by ejecting a pulsatile jet. They are unusual creatures whose lives have two major stages: one in which salps swim individually and one in which they link together and swim in large chains. In the chain, salps don’t synchronize their jetting; each salp jets with its own phase and frequency. A new study suggests that, in spite of this lack of synchronicity, the salp chain’s swimming reduces the animals’ drag. There are several factors that contribute to this result. One is that drag is generally lower on a body moving at constant speed compared to one moving in bursts. When linked together and firing randomly, all the individual jets tend to average out into one continuous swimming speed. There’s even a benefit to being out of sync: previous work showed that synchronized jets lose some of their thrust when they are too close together. Salps avoid that loss by keeping to their own beat. (Image and research credit: K. Sutherland and D. Weihs, source; via Gizmodo)
Squishy Impacts
How spheres impact water has been studied for more than a century. The typical impact for a rigid sphere creates a cavity like the one on the upper left – relatively narrow and prone to pinching off at its skinny waist. If the sphere is elastic –squishy – instead, the cavity ends up looking much different. This is shown in the upper right image, taken with an elastic ball and otherwise identical conditions to the upper left image. The elastic ball deforms; it flattens as it hits the surface, creating a wider cavity. If you watch the animations in the bottom row, you can see the sphere oscillating after impact. Those changes in shape form a second cavity inside the first one. It’s this smaller second cavity that pinches off and sends a liquid jet back up to the collapsing splash curtain.
From the top image, we can also see that the elastic sphere slows down more quickly after impact. This makes sense because part of its kinetic energy at impact has gone into the sphere’s shape changes and their interaction with the surrounding water.
If you’d like to see more splashy stuff, be sure to check out my webcast with a couple of this paper’s authors. (Image credits: top row – C. Mabey; bottom row – R. Hurd et al., source; research credit: R. Hurd et al.)
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)
Reconnecting
Vortices are a common feature of many flows. Here we see a helical vortex tube spinning in a swirling flow. The vortex itself is visible thanks to air trapped in its low-pressure core. As the vortex spins, two sections of it come together. This results in what’s known as vortex reconnection: the vortex lines break apart and rejoin in a new configuration – as a small independent vortex ring and a shorter section of helical vortex. Events like this are common but usually hard to observe directly. They’ve been previously visualized using vortex knots and have even been sighted in the quantum vortices of superfluid helium. (Image credit: S. Skripkin, source; research credit: S. Alekseenko et al., pdf)
Breaking Up
Liquid sheets break down in a process known as atomization. Above are top and side views of a liquid sheet created by two identical liquid jets impacting head-on. The jets themselves are off-screen to the left. Their collision generates a thin sheet of liquid that flows from left to right. In the center of the images, the sheet has begun to flap and undulate, shedding large droplets from its edges as it does. At the far end of the sheet, much finer droplets are sprayed out from the center as the sheet collapses completely. This is an example of an instability in a fluid. Initially, any disturbance in the liquid sheet is extremely tiny, but circumstances in the flow are such that those disturbances gather energy and grow larger, creating the large undulations. Those undulations are unstable as well and kick off a fresh set of disturbances that grow until the flow completely breaks down. (Image credit: N. Bremond et al., pdf)