FYFD

Tag: shockwave

  • Jets from Lasers

    Jets from Lasers

    Laser-induced forward transfer (LIFT) is an industrial printing technique where a laser pulse aimed at a thin layer of ink creates a tiny jet that deposits the ink on a surface. In practice, the technique is plagued with reproducibility issues, in part because it’s difficult to produce only a single cavitation bubble when aiming a laser at the liquid layer. This is what we see above. 

    The laser pulse creates its initial bubble just above the middle of the liquid layer. Shock waves expand from that first bubble and quickly reflect off the liquid surface (top) and wall (bottom). When reflected, the shock waves become rarefaction waves, which reduce the pressure rather than increasing it. This helps trigger the clouds of tiny bubbles we see above and below the main bubble. 

    The effect is worst along the path of the laser pulse because that part of the liquid has been weakened by pre-heating, but impurities and dissolved gases in the liquid layer are also prone to bubble formation, as seen far from the bubble. The trouble with all these unintended bubbles is that they can easily rise to the surface, burst, and cause additional jets of ink that splatter where users don’t intend. (Image and research credit: M. Jalaal et al.; submitted by Maziyar J.)

  • Astrophysical Turbulence

    Astrophysical Turbulence

    Subsonic turbulence – like the random and chaotic motions of air and water in our everyday lives – is something we have only a limited understanding of. Our knowledge of supersonic turbulence, where shock waves and compressibility rule, is even more tenuous. In part this is because, although we can observe snapshots of supersonic turbulence in astronomical settings like the Orion Nebula shown above, we cannot watch it evolve. On these scales, features simply don’t change appreciably on human timescales.

    This has limited scientists to mostly numerical and theoretical studies of supersonic turbulence, but that is starting to change. Researchers are now building experimental set-ups that collide laser-driven plasma jets to generate boundary-free turbulence at Mach 6. Thus far, the observations are consistent with what’s been seen in nature: at low speeds, the turbulence is consistent with Kolmogorov’s theories, with energy cascading from large scales to smaller ones predictably. But as the Mach number increases, the nature of the turbulence shifts, moving toward the large density fluctuations seen in nebulae and other astrophysical realms. (Image credit: F. Battistella; research credit: T. White et al.; see also Nature Astronomy; submitted by Kam-Yung Soh)

  • Seeing Shock Waves

    Seeing Shock Waves

    This week NASA released the first-ever image of shock waves interacting between two supersonic aircraft. It’s a stunning effort, requiring a cutting-edge version of a century-old photographic technique and perfect coordination between three airplanes – the two supersonic Air Force T-38s and the NASA B-200 King Air that captured the image. The T-38s are flying in formation, roughly 30 ft apart, and the interaction of their shock waves is distinctly visible. The otherwise straight lines curve sharply near their intersections.

    Fully capturing this kind of behavior in ground-based tests or in computer simulation is incredibly difficult, and engineers will no doubt be studying and comparing every one of these images with those smaller-scale counterparts. NASA developed this system as part of their ongoing project for commercial supersonic technologies. (Image credit: NASA Armstrong; submitted by multiple readers)

  • Noisy Jets

    Noisy Jets

    One major problem that has plagued supersonic aircraft is their noise. The Concorde – thus far the only supersonic commercial airliner – was plagued with noise complaints that ultimately restricted its usability. Noise reduction is a major area of inquiry in aerospace, and the video below shows one experiment trying to understand the connections between supersonic flow and noise.

    Above you see a supersonic, Mach 1.5 microjet emanating from a nozzle at the top of the image. The jet is hitting a flat plate at the bottom of the image. Just beyond nozzle’s exit, you can see the X-shape of shock waves inside the jet. The position of that X is oscillating up and down.

    In the background, you can see horizontal light and dark lines traveling up and down. Those horizontal lines in the background are acoustic waves. When they hit the bottom plate, they reflect and travel upward until they hit another surface (outside the picture) and reflect back down. As they travel, they interact with the jet, causing those X-shaped shock waves to move up and down. This coupling between flow and acoustic waves makes the jet much louder – up to 140 dB – than it would be otherwise.

    Researchers hope that unraveling the physics of simpler systems like this one will help them quiet more complicated aircraft. (Image and video credit: F. Zigunov et al.)

  • Ricequakes

    Ricequakes

    Rockfill dams, sinkholes, ice shelves, and other geological features often consist of brittle, porous materials that are partially submerged. Over time, pressure and chemical reactions with the fluid around them can cause these structures to collapse, but it can take many, many years. 

    To study the physics behind this, researchers have turned to a new model: puffed rice cereal. Like their counterparts in nature, puffed rice grains contain micropores that slowly soften and get crushed after being wetted. Researchers filled their test container with puffed rice and put it under pressure to give the whole stack a constant stress. Then they injected milk in the bottom section of the container. After an immediate collapse in the wet material (lower left), the remaining grains collapsed slowly in a series of “ricequakes”. 

    As the micropores compacted, the cereal let out audible cracks that corresponded with the motion of a crushing wavefront (lower right). The time between ricequakes increased linearly and depended on pore size. The relationship was so consistent, researchers found, that they could predict how long the puffed rice stack had been wet simply by listening to the time between crackles! Experiments like these offer scientists an exciting chance to understand geological physics that would otherwise take up to millions of years to observe. (Image and research credit: I. Einav and F. Guillard; via Physics World; submitted by Kam-Yung Soh)

  • Exploding Meteors

    Exploding Meteors

    During the recent Perseid shower, photographer Petr Horálek caught an awesome timelapse of an exploding meteor and the vortex ring it created. This is a type of persistent train left when meteors pass through the upper atmosphere. The exact physics are not well understood because such events are difficult to observe; catching them at all is basically just happenstance. But one interpretation is that we’re seeing trails of plasma left by the ionization of parts of the meteor. When the meteor hits the upper atmosphere, there’s an extremely strong hypersonic shock wave. The jump in temperature across that shock wave is enough to pull atoms apart, creating a plasma. The train left by this meteor’s demise was faintly visible even an hour after the fireball. (Image credit: P. Horálek, video version; via APOD; submitted by Andrea S.)

  • Shock Waves in the Solar Wind

    Shock Waves in the Solar Wind

    The empty space of our solar system is not truly empty, as we’ve discussed previously. For one, there’s a fast-moving flux of charged particles – the solar wind – that flows constantly from the Sun. Sometimes these solar wind particles encounter their interstellar equivalents – charged ions from outside our solar system – and exchange energy.

    One predicted mechanism for this energy swap is a solar wind shock wave, which occurs when a faster-moving clump of charged particles plows into a slower-moving one. Scientists hypothesized in the mid 1990s that far from the Sun, solar wind shock waves would lose their energy by passing it to these interstellar ions, in a process known as pickup. Data from the New Horizons spacecraft has finally provided evidence for this theory.

    In October 2015, instruments on the spacecraft recorded a shock wave when the speed of solar wind ions nearby jumped from 380 km/s to 440 km/s. Comparing the energies of solar and interstellar ions before and after the event, researchers found that interstellar pickup ions became 30% more energetic while solar ions lost 85% of their energy. It’s an important confirmation of theoretical predictions and should help us better understand high-energy particle physics at the edges of our solar system. (Image credit: NASA; research credit: E. Zirnstein et al., via J. Ouellette)

  • Heating from Cavitation

    Heating from Cavitation

    When cavitation bubbles collapse, they can produce temperatures well over 2,000 Kelvin. Since cavitation near a surface can be so destructive, researchers have long wondered whether the high temperatures inside the bubble can be transmitted to nearby surfaces. A new set of numerical simulations provides some insight into that process. The researchers found that collapsing cavitation bubbles raised nearby wall temperatures in two ways: bubbles that were further away sent shock waves that heated the material, and nearby bubbles could contact the surface itself as they collapsed.

    Heat transfer requires time, however; this is part of why quickly dunking your hand in liquid nitrogen and pulling it out likely won’t damage you. (Still, we don’t recommend it.) The cavitation bubbles could only transmit these high temperatures for less than 1 microsecond, which means that most materials won’t actually heat up to their melting temperature. The researchers did conclude, however, that softer materials exposed to frequent bubble collapses could show localized melting under the barrage. (Image credit: L. Krum; research credit: S. Beig et al.)

  • Meteoroids

    Meteoroids

    Meteoroids are debris from earlier eras in our solar system. They can be leftovers from planets that never formed or remains of ancient collisions. When these bits rock and metal enter our atmosphere, they become meteors. Since they travel at speeds of several kilometers per second, they create incredibly strong shock waves off their bow once they’re in the atmosphere. These shock waves are so strong that they rip the air molecules apart and create a hot plasma that can scorch the outside of the meteor. That plasma also glows, which is why meteors look like a streak of light from the ground. Any remains that make it to the ground are known as meteorites, and they have some pretty awesome features. Check out the full Brain Scoop episode below to learn some of the typical (and not so typical!) characteristics of meteorites. (Image and video credit: The Brain Scoop/Field Museum)

  • Space Shuttle Sonic Booms

    Space Shuttle Sonic Booms

    The Space Shuttle had a famous double sonic boom when passing overhead during re-entry. This schlieren flow visualization of a model shuttle at Mach 3 reveals the source of the sound: the fore and aft shock waves on the vehicle. The nose of the shuttle generates the strongest shock wave since it is the first part of the vehicle the flow interacts with. This initial shock wave turns the flow outward and around the shuttle. The second boom comes from the back of the shuttle and serves to turn the flow back in to fill the wake behind the shuttle. (The actual shock wave would look a little different than this one because there’s no sting holding the shuttle like there is with the model.) The other major shock wave comes from the shuttle’s wings, but, at least for this Mach number, the wing shock wave merges with the bow shock, making the two indistinguishable. (Image credit: G. Settles, source)