Fuck Yeah Fluid Dynamics

Celebrating the physics of all that flows. Ask a question, submit a post idea or send an email. You can also follow FYFD on Twitter and Google+. FYFD is written by Nicole Sharp, PhD.

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Posts tagged "vibration"

Paint seems to dance and leap when vibrated on a speaker. Propelled upward, the liquid stretches into thin sheets and thicker ligaments until surface tension can no longer hold the the fluid together and droplets erupt from the fountain. Often paints are shear-thinning, non-Newtonian fluids, meaning that their ability to resist deformation decreases as they are deformed. This behavior allows them to flow freely off a brush but then remain without running after application. In the context of vibration, though, shear-thinning properties cause the paint to jump and leap more readily. For more images, see photographer Linden Gledhill’s website. (Photo credit: L. Gledhill; submitted by pinfire)

Vibrating a liquid droplet produces some awesome behavior. The video above shows a water droplet vibrating on a subwoofer at real-time speeds. The behavior and shape of the droplet shifts with the frequency of vibration, which we hear as a change in pitch. To see more clearly the shapes a particular frequency induces, check out this high-speed video of vibrating water droplets. For a given driving frequency, the droplet’s shape, or mode, is distinct and consistent. For a droplet vibrating to a song, though, there is more than one frequency driving its motion. In this case, the droplet’s shape is a superposition of the individual modes, which is just a way of saying adding the shapes together. So frequency determines the droplet’s shape. The vibration amplitude, or audible volume, affects how energetic the drop’s motion is. And the fluid’s surface tension and viscosity act as dampers to the system, controlling how quickly the drop can change shape as well as how well it holds together. (Video credit: A. Read) 

Many systems can exhibit unstable behaviors when perturbed. The classic example is a ball sitting on top of a hill; if you move the ball at all, it will fall down the hill due to gravity. There is no way to perturb the ball in such a way that it will return to the top of the hill; this makes the top of the hill an unstable point. In many dynamical systems, a very small perturbation may not be as obviously unstable as the ball atop the hill, especially at first. Often a perturbation will have a very small effect initially, but it can grow exponentially with time. That is the case in this video. Here a tank of fluid is being vibrated vertically with a constant amplitude. At first, the sloshing effect on the fluid interface is very small. But the vibration frequency sits in the unstable region of the parameter space, and the perturbation, which began as a small sloshing, grows very quickly. In a real system (as opposed to a mathematical one), this kind of unstable or unbounded growth very quickly leads to destruction. (Video credit: S. Srinivas)

Much as I try to keep from getting repetitious, this was just too neat to pass up. This new music video for The Glitch Mob’s “Becoming Harmonious” is built around the standing Faraday waves that form on a water-filled subwoofer. The vibration patterns, along with judicious use of strobe lighting, produce some fantastic and kaleidoscopic effects. (Video credit: The Glitch Mob/Susi Sie; submitted by @krekr)

The recently released music video for Jack White’s “High Ball Stepper” is a fantastic marriage of science and art. The audio is paired with visuals based around vibration effects using both granular materials and fluids. There are many examples of Faraday waves, the rippling patterns formed when a fluid interface becomes unstable under vibration. There are also cymatic patterns and even finger-like protrusions formed by when shear-thickening non-Newtonian fluids get agitated. (Video credit: J. White, B. Swank and J. Cathcart; submitted by Mike and Marius)

Soil liquefaction is a rather unsettling process in which apparently solid ground begins moving in a fluid-like way after agitation. It occurs in loose sediments when the spaces between individual particles become nearly saturated with water. This can happen, for example, after heavy rains or in a place with inadequate drainage. Such cases are typically very localized, though, and require some significant agitation of the surface, like pressing with heavy machinery or jumping in a single spot. Soil liquefaction becomes a greater danger, however, in an earthquake. Even in a dry area, the earth’s shaking can force groundwater up into the surface sediment and vibrate the soil sufficiently to liquify it, causing whole buildings to sink or tip and wreaking havoc on manmade infrastructure. (Video credit: jokulhlaups)

Over the past few years, researchers have been exploring the dynamics of droplets bouncing on a vibrating fluid. These systems display many behaviors associated with quantum mechanics, including wave-particle duality, single-slit and double-slit diffraction, and tunneling. A new paper examines the system mathematically, showing that the droplets obey many of the same mathematics as quantum systems. In fact, the droplet-wave system behaves as a macroscopic analog of 2D quantum behaviors. The implications are intriguing, especially for teaching. Now students of quantum mechanics can experiment with a simple apparatus to understand some of the non-intuitive aspects of quantum behavior. For more, see the paper on arxiv. (Image credit: D. Harris and J. Bush; research credit: R. Brady and R. Anderson)

Paint is probably the Internet’s second favorite non-Newtonian fluid to vibrate on a speaker—after oobleck, of course. And the Slow Mo Guys' take on it does not disappoint: it's bursting (literally?) with great fluid dynamics. It all starts at 1:53 when the less dense green paint starts dimpling due to the Faraday instability. Notice how the dimples and jets of fluid are all roughly equally spaced. When the vibration surpasses the green paint’s critical amplitude, jets sprout all over, ejecting droplets as they bounce. At 3:15, watch as a tiny yellow jet collapses into a cavity before the cavity’s collapse and the vibration combine to propel a jet much further outward. The macro shots are brilliant as well; watch for ligaments of paint breaking into droplets due to the surface-tension-driven Plateau-Rayleigh instability. (Video credit: The Slow Mo Guys)

Opposing ultrasonic speakers can be used to trap and levitate droplets against gravity using acoustic pressure. Changes to field strength can do things like bring separate objects together or flatten droplets. The squished shape of the droplet is the result of a balance between acoustic pressure trying to flatten the drop and surface tension, which tries to pull the drop into a sphere. If the acoustic field strength changes with a frequency that is a harmonic of the drop’s resonant frequency, the drop will oscillate in a star-like shape dependent on the harmonic. The video above demonstrates this for many harmonic frequencies. It also shows how alterations to the drop’s surface tension (by adding water at 2:19) can trigger the instability. Finally, if the field strength is increased even further, the drop’s behavior becomes chaotic as the acoustic pressure overwhelms surface tension’s ability to hold the drop together. Like all of this week’s videos, this video is a submission to the 2103 Gallery of Fluid Motion. (Video credit: W. Ran and S. Fredericks)


This is the last week that my IndieGoGo project is open for donations. All money above and beyond what is needed for the conference will go toward FYFD-produced videos. Also, donors can get some awesome FYFD stickers.

As a reminder, those looking for more fluids—in video, textbook, or other form—can always check out my resources page. And if you know about great links that aren’t on there, let me know so that I can add them. On to the round-up!

I had a lot of fun earlier this week giving a talk for the Texas A&M Applied Mathematics Undergraduate Seminar series. I didn’t get a chance to record it, but the slides are up here if anyone is interested. 
(Photo credit: M. Klimas)