A soap bubble bursts when its surface tension is broken, and, although from our perspective, the bubble bursts instantly, the process is actually directional. The bubble disintegrates from the point of contact outward. See it in high-speed video here or see more photos here. (Photo credit: Richard Heeks) #
Tag: fluid dynamics
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.
Simulating Turbulence
Turbulent flows are complicated to simulate because of their many scales. The largest eddies in a flow, where energy is generated, can be of the order of meters, while the smallest scales, where energy is dissipated, are of the order of fractions of a millimeter. In Direct Numerical Simulation (DNS), the exact equations governing the flow are solved at all of those scales for every time step–requiring hundreds or thousands of computational hours on supercomputers to solve even a small domain’s worth of flow, as on the airplane wing in the video. Large Eddy Simulation (LES) is another technique that is less computationally expensive; it calculates the larger scales exactly and models the smaller ones. The video shows just how complicated the flow field can look. The red-orange curls seen in much of the flow are hairpin vortices, named for their shape, and commonly found in turbulent boundary layers.
Feynman: The Universe in a Glass of Wine
Some wisdom for you this Friday from the incomparable Richard Feynman:
A poet I think it is who once said the whole universe is in a glass of wine. I don’t think we’ll ever know in what sense he meant that for the poets don’t write to be understood. But it is true that if you look at a glass of wine closely enough, you’ll see the entire universe.
There are the things of physics: the twisting liquid, the reflections in the glass, and our imagination adds the atoms. It evaporates, depending on the wind and weather. The glass is a distillation of the earth’s rocks and in its composition, as we’ve seen, the secret of the universe’s age and the evolution of the stars. What strange array of chemicals are in a wine? How did they come to be? There are the ferments, the enzymes, the substrates and the products, and there in wine was found great generalization: all life is fermentation. Nor can you discover the chemistry of wine without discovering, as did Pasteur, the cause of much disease. How vivid is the claret, pressing its existence into the consciousness that watches it?
And if our small minds for some convenience divides this glass of wine, this universe, into parts: to physics, biology, geology, astronomy, psychology and all, remember that nature doesn’t know it. So we should put it all back together and not forget at last what it’s for. Let it give us one final pleasure more: drink it up and forget about it all.
(submitted by @jerrodh)
How Coffee Rings Form
Coffee rings (an ubiquitous feature of academia) are formed by the deposition of particles as the liquid evaporates. When a coffee drop evaporates, capillary action draws the coffee particles toward the edges of the drop, where they congregate into a ring. Research now suggests that this is due to the spherical nature of the particles. Ellipsoidal particles, in contrast, clump together and result in a uniform stain once their carrier liquid evaporates. The effect seems to be due to the particles’ effects on surface tension; the ellipsoidal particles deform the surface of the droplet as it evaporates such that they are not pulled to the edges. Adding a surfactant, like soap, that decreases surface tension caused the ellipsoidal particles to form rings just as the spherical particles do. (submitted by Neil K) #
Underwater Cloaking
Researchers have suggested that it may be possible to cloak submerged objects as they move through a fluid using layers of mesh and micro-pumps. By redirecting the fluid so that it enters and leaves the mesh surrounding the object in the same speed and direction that it entered, it is theoretically possible to have zero drag and no wake. So far researchers have only simulated this set-up computationally using a sphere with 10 layers of mesh. It’s also unfortunately limited in size and speed: a vehicle 1 cm across could only remain wake-free at speeds below 1 cm/s. (Photo credit: Michael J Rinaldi) #
The Dance of Jets and Droplets
Placing a prism upside down in a bath of silicone oil creates a trapped bubble of air inside the prism. When oscillated above a critical amplitude, the corners of the prism, the oil, and the air perform an intricate dance of bubbles, singularities, jets, and droplets. Read more in the research paper. #
Spiky Ferrofluid
Ferrofluids consist of ferromagnetic nanoparticles suspended in a fluid. When subjected to strong magnetic fields, they develop a distinctive peak-and-valley formation due to the normal-field instability. The shape is a result of minimizing the magnetic energy of the fluid. Both gravity and surface tension resist the formation of these peaks. Ferrofluids, in addition to appearing in art exhibits, can be used as liquid seals, MRI contrast agents, and loudspeaker cooling fluids. (Photo credit: Maurizio Mucciola)
The Spinning Underwater Vortex
Vortex rings are a topic we’ve covered before with dolphins, whales, humans, volcanoes and even moss, but this video is particularly fun thanks to the addition of a bottle cap. By sticking the bottle cap next to the ring, these swimmers are able to demonstrate the forceful spinning of the fluid near the vortex. This spinning is what helps the vortex hold its shape over distances much larger than its diameter. As you can also see, though, sticking a bottle cap in the ring causes it to break up faster than it would otherwise! (submitted by Kris S)
Glorious Coronal Mass Ejection
In early June, NASA’s Solar Dynamics Observatory recorded a stunning coronal mass ejection, in which larger than usual quantities of cool (relatively speaking) plasma erupted from the surface of the sun and rained back down along magnetic field lines. Plasma is an ionized gas-like state of matter subject to the same laws that govern more familiar fluids like water or air, with the additional caveat that, being electrically conductive, plasmas also obey Maxwell’s equations. #
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