Tag: oscillation

Droplet Springs
Prior to reaching terminal velocity, a falling droplet typically oscillates between a prolate shape (like an American football about to be kicked) and an oblate one (like that same football when thrown or carried). As explained by Minute Laboratory, this oscillation behaves very similarly to a mass on a spring. For a spring/mass system, the frequency of oscillation is related to the spring’s stiffness; for the falling droplet, it is instead governed by surface tension. If only high schools had high-speed cameras, this would make a fantastic fluids lab experiment! (Video credit: Minute Laboratory; submitted by Pascal W.)
FYFD is sporting a new look and new about and resources pages for those who want more fluidsy goodness.
February 6, 2013
Leidenfrost Dynamics
When a liquid impacts a solid heated well above the liquid’s boiling point, droplets can form, levitating on a thin film of vapor that helps insulate them from the heat of the solid. This is known as the Leidenfrost effect. Here a very large Leidenfrost droplet is shown from the side in high-speed. A vapor chimney forms beneath the drop, causing the dome in the liquid. When the dome bursts, the droplet momentarily forms a torus before closing. The resulting oscillatory waves in the droplet are spectacular. The same behavior can be viewed from above in this video. (Video credit: D. Soto and R. Thevenin; from an upcoming review by D. Quere)
October 11, 2012
Space Didgeridoo
This week astronaut Don Pettit is playing with acoustic oscillators on the space station. He and Dan Burbank transform some of their vacuum cleaner tubes into didgeridoo-like instruments. By buzzing into the tube, Pettit is creating an acoustic standing wave, and, depending on the geometry at the far end, the wavelength of the standing wave and thus pitch of the sound is shifted.
June 1, 2012
Why Walking with Coffee is Tough
Almost everyone is familiar with the problem of coffee or tea sloshing over the sides of a mug as one walks, but this may be the first time researchers have systematically studied the problem. The results show that the typical frequency of the human stride closely matches the natural frequency for back-and-forth sloshing of a low-viscosity liquid in a cylindrical container the size of a typical coffee mug. Even though our natural side-to-side motion plays a role in coffee sloshing, its effect is small in comparison. A person’s maximum acceleration, which usually happens early on when walking, sets the initial sloshing amplitude, which is subsequently amplified by the stepping frequency. The researchers did find that the time to spill increased substantially if the subject was focused on not spilling the coffee, though it was unclear if this was due to the subject decreasing their acceleration and step frequency, or whether they were actively damping the oscillations with adjustments in the wrist. If you’re a perpetual coffee spiller, there’s still hope: the authors suggest that flexible cups and/or cups with a series of concentric rings–baffles–could help reduce sloshing in spite of our natural tendency to induce it. (Photo credit: dongga/Flickr; Paper: Mayer and Krechetnikov; submitted by @__pj)
May 15, 2012
Sloshing Dynamics
[original media no longer available]
Sloshing refers to the motion of a liquid inside a moving container, for example, in tanker trucks or inside a spacecraft’s fuel tank. The motion of the liquid payload can drastically affect the dynamics of the vehicle carrying it due to the ever shifting center of mass. In the video above, dyed water is being oscillated horizontally to and from the camera. As the frequency of this oscillation changes, the modes of sloshing–the shapes the liquid surface assumes–change dramatically.
November 22, 2011
Faces from Laminar Mixing
These images show the laminar mixing that occurs when a flat plate moves up and down in an otherwise motionless fluid. Each face-like snapshot represents a different point in time. The longer the plate oscillates, the more elaborate the “faces” become. (Photo credit: S. Brunton)
November 21, 2011
Aeroelastic Flutter
Flutter is a rather innocuous term for a potentially dangerous phenomenon that can occur for any flexible structure in a moving flow. Aeroelastic flutter occurs when aerodynamic forces and a structure’s natural modes of vibration get coupled: the surrounding flow causes the object to vibrate, which alters the nature of the aerodynamic forces on the object, which, in turn, feeds into the object’s vibration. In some cases, damping will contain the motion to a limit cycle, but under other conditions, flutter results in an uncontrollable self-exciting oscillation that persists until destruction, as in the famous Tacoma Narrows Bridge collapse.
November 16, 2011
Sound and Harmonics
The vibrations we perceive as sound, whether in air, water, or any other fluid, are tiny pressure waves emanating from a source, transmitting like ripples across a pond, and finally being caught by our ears and translated by our brains. In this video, the mechanisms and mathematics of sound and harmonics are explained. Although we’re most familiar with these concepts in acoustics, the same principles are used when studying other oscillatory motions, including pendulums, mass-spring systems, disturbances in boundary layers, and the vibrations of a diving board. All of these things rely on the same fundamental principles and mathematics.
August 23, 2011
Liquid Settling
Despite the strange shapes of the arms on this container, the fluid inside will always settle to a common height. This is because each interconnected section is open to the outside air. The fluid’s surface has to reach a static equilibrium with the atmosphere–i.e. the surface of the fluid must be at atmospheric pressure–and the pressure at the lowest level in each section must match because the arms are connected. When fluid is added, the height of the columns oscillates some because the momentum of the added fluid carries the column past its equilibrium position, much like a perturbed mass hanging from a spring will oscillate before settling.
January 31, 2011