This footage of last year’s eruption of Eyjafjallajökull in Iceland shows shockwaves emanating from the mouth of the volcano as hot ash and gases explode from underground.
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Shock Waves
Flow visualization really can be considered a form of art. Though we fluid mechanicians are looking for physics, we’re quite aware of the beauty of what we study. The clips in this video mostly show transient shockwave behavior, including lots of shock reflection and even a few instabilities. It’s unclear what the speeds are, aside from faster than sound; the medium is air.
Waves on Cornstarch
A thin layer of the non-Newtonian fluid oobleck on a vibrating surface (in this case, a speaker) is a great way to show off nonlinear standing waves known as Faraday waves. The waves form because, under these circumstances, the flat surface of the air/oobleck interface has actually become unstable.
Seeing Shock Waves with Schlieren
Schlieren photography is actually a pretty commonly used system in high-speed experimental aerodynamics. A typical schlieren system will shine a collimated light source on the target (a wind tunnel test section or, above, a candle), bounce that light off a mirror, block half the light with a knife-edge at the focal point, and then record the subsequent images with a camera (high-speed or otherwise). The density of air is closely related to its index of refraction, so light that hits air of a different density will be bent more or less than a neighboring ray. This uneven bending of the light rays due to density gradients is what causes the light and dark areas on the schlieren images. Since the density of air changes drastically across a shock wave, the schlieren system is perfect for visualizing shock waves and has, in fact, been used for that purpose since 1864!
Shock Waves in Space
Shock waves are not just an earthbound phenomenon. They can be found in space as well. In this photo, gas (colored yellow) ejected from a dying star hits clouds of gas and dust (colored blue), creating shock waves. #
Shock Waves From a Gun
Often fluid motion is invisible to the human eye. Researchers use techniques like schlieren photography to make changes in fluid density apparent. In this high-speed schlieren photo, an AK-47 is being fired. The spherical shock wave centered on the gun’s muzzle is due to the explosive discharge of gases used to fire the bullet. At the left of the frame, the bullet also causes a shock wave, this time a conical one, as it travels supersonically out of the gun.
Photo Source; Credit: Gary Settles, Penn State Gas Dynamics Lab
Inside an Ear
Our ears, like those of many other animals, convert mechanical signals to electrical ones, through a Rube-Goldberg-esque series of transformations. External sound waves make their way down the soft tube of the ear canal, which funnels them to a thin-walled cone, the eardrum, that’s about half as large as a dime. Here, the vibrating air pushes against the cone’s membrane, and those vibrations travel onward through a linked trio of small bones that amplify the vibration’s amplitude.
The last of these bones presses against an even smaller, oval-shaped membrane. As the bone moves, it shakes the membrane, sending waves through the liquid on its other side. Those waves travel down the spirals of the tiny, pea-sized cochlea, named for a snail shell’s shape. As the waves move through the liquid, they bend bundles of hair-like strands back and forth, like tall grass waving in a breeze. The bending triggers a chemical that binds to nerves at the base of the bundles, sending an electrical signal through the nerve and into the brain.
But the hair-like bundles, known as stereocilia, are also able to amplify incoming vibrations. In this case, the bundles in the outer portion of the cochlea expend energy to bend more than the incoming vibrations naturally make them move. This bending amplifies the fluid motion that gets transmitted to stereocilia further down the line; it’s those bundles that will make the final conversion to an electrical signal the brain receives. (Image credit: B. Kachar; research credit: Y. Thipmaungprom et al.; via APS)
Scanning electron microscope view of the stereocilia “hair bundles” inside a frog’s inner ear. Fediverse Reactions
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Scrubbing Bubbles
Cleaning produce helps fruits and vegetables last longer and reduces the chances for foodborne illness. But it can be a difficult feat with soft, delicate foods like tomatoes, berries, or greens. Current methods often combine ultrasonic cleaning and chemicals like chlorine. Instead, researchers are looking to boost the cleaning power of bubbles themselves by giving them an acoustic pick-me-up.
Stop-and-go. A bubble slides along an inclined surface in a pronounced stop-and-go motion when vibrated near its frequency for translational resonance. The team combined a bubble-filled bath with sound at low (sub-cavitation) frequencies. They found that driving sound waves at the right frequency could vibrate the bubbles in a way that made them slide in a stop-and-go motion along inclined surfaces. This swaying significantly boosted their cleaning power; getting surfaces 90% cleaner than non-resonating bubbles did. (Image credit: S. Hok/Cornell University; video and research credit: Y. Lin et al.; via Gizmodo)
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Turbulence and Bioluminescence
If you’ve ever seen crashing waves glowing blue, you’ve been treated to bioluminescence. Although many creatures can bioluminesce, tiny dinoflagellates–a type of marine phytoplankton–are one of the easiest to spot. These microscopic organisms create a flash of light in response to viscous stresses. Their response to flow-induced stresses is so robust that they can be used to visualize stress fields.
In a new study, researchers explored how turbulence affects the dinoflagellate’s luminescence. They mathematically modeled the dinoflagellate as an elastic dumbbell that emitted light based on its extent and rate of deformation. Then they explored how this model dinoflagellate behaved in different types of turbulent flows. They found that the fluctuations and intermittency of turbulent flows both encouraged the radiant displays. (Image credit: T. McKinnon; research credit: P. Kumar and J. Picardo)
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Making Sound Visible
Sound is not something we can typically see, though there are ways to visualize it, including cymatics and special acoustic cameras. This video pursues a different tactic: using schlieren photography and stroboscopic lighting to show how sound waves reflect and deflect. It’s no easy feat, but one worth enjoying–especially when others have already done the hard part for you! (Video and image credit: All Things Physics; submitted by David J.)
