It’s time for some more fluidsy fun around the Internet! Here are some fun links I’ve come across since our last round-up.
- NPR reviews how dolphins and others play with vortex rings.
- Lawrence Berkeley National Laboratory/UC Berkeley offer some insight into simulating bubbles popping. (Hint: it requires supercomputers.)
- FlowViz shares some awesome accidental Rayleigh-Taylor instabilities you can replicate at home.
- PhysicsBuzz brings us a podcast on tornado physics.
- Reader Cedric Vella sent in his fluids-featuring trailer.
- io9 pointed out some great cymatics footage that shows off how granular materials and vibration creates beautiful patterns.
- And finally: what happens when you drop hot charcoal into liquid oxygen? The Periodic Table of Videos shows us, in high speed! (via Flow Visualization)
(Photo credit: L. L. A. Adams et al., multi-fluid double emulsions)
Sometimes I come across cool links and stories about fluid dynamics that don’t quite fit into a typical FYFD post, but I’d like to start sharing those semi-regularly with round-up posts. Here’s some fun stuff I’ve seen lately:
- We’ve talked before about penguins and fluid dynamics, but this IOP talk from Helen Czerski is a great take on how they use fluids to escape predators.
- Over at FlowViz, Richard has a great video demonstrating deep versus shallow water waves side-by-side.
- io9 shows off the vibrational modes of mercury as well as explaining why birds can be good at flying or swimming but not both.
- Giant paint explosions (specifically here) may be fun to watch but the dangers are not for squeamish viewers.
- Dylan G sent us this 512K real-time fluid particle simulation that’s a fun distraction.
- xckd’s Randall Munroe answers all kinds of physics questions at his What If? blog, but occasionally there are some fluids ones. Two of my favorites consider a hypersonic steak and what happens if the glass is literally half full.
- Finally, for the cyclists and triathletes among you, the recent news that bicycle manufacturer Specialized has built its own wind tunnel has generated lots of discussion of aerodynamics in sport. Mark Cote (@MITAeroBike) and Chris Yu (@chrisyuinc) of Specialized’s aero R&D have been answering questions on Slowtwitch, Twitter, and in live chats.
And, yes, that last Specialized video chat includes an FYFD shout-out about 49 minutes in. :)
(Photo credit: Specialized)
There’s something wonderfully serene about watching water droplets skate their way across the surface of a pool. Here the pool of water is being vibrated at a frequency just below the Faraday instability - meaning that no standing waves form on the surface. Instead, the bounce is just enough to create a thin layer of air between the droplet and the pool to prevent coalescence. With each bounce, gravity’s effect on the water tries to drain the air away, but each rebound lets more air rush in to hold the droplet up. Eventually, gravity wins and the droplets coalesce into the pool. In high-speed that process is mesmerizing, too. (Video credit: K. Welch)
Mixing immiscible liquids—like oil and water—is tough. The best one can usually do is create an emulsion, in which droplets of one fluid are suspended in another. The series of images above shows a double emulsion consisting of oil and water that’s been formed by bouncing the compound droplet on a vibrating bath. The vibration of the liquid surface keeps the droplet from coalescing with the bath and the deformation provides mixing. The top row shows the initial impact while the bottom row of images shows the droplet after many bounces. As time goes on, the layer of oil around the compound drop becomes a cluster of tiny droplets contained within the water portion of the drop. (Photo credit: D. Terwagne et al.)
We saw previously how vibrating a falling stream of water and filming it with a matching camera frame rate appears to “freeze” the falling liquid. This video shows the same illusion, now with a 24 Hz sine wave, which the falling water mimics. Vibrating the speaker that drives the water stream slightly slower or slightly faster than the camera frame rate makes the water appear to slowly fall or rise relative to its “frozen” wave state. This is a beat effect caused by the slight difference in frequency between the water and the camera. (Video credit: brusspup; via BoingBoing; submitted by many readers)
This high-speed video shows a liquid crystal fluid vibrating on a tuning fork. As the surface moves, tiny jets shoot upward, sometimes with sufficient energy that the fluid column is stretched beyond surface tension’s ability to keep it intact, resulting in droplet ejection. The jets and surface waves create a mesmerizing pattern of fluid motion. (Video credit: J. Savage)
Aeroelastic flutter occurs when fluid mechanical forces and structural forces get coupled together, one feeding the other. Usually, we think of it as a destructive mechanism, but, for hummingbirds, it’s part of courtship. When a male hummingbird looks to attract a mate, he’ll climb and dive, flaring his tail feathers one or more times. As he does so, air flow over the feathers causes them to vibrate and produce noise. Researchers studied such tail feathers in a wind tunnel, finding a variety of vibrational behaviors, including a tendency for constructive interference—in other words two feathers vibrating in proximity is much louder than either individually. For more, check out the original Science article or the write-up at phys.org. (Video credit: C. Clark et al.)
One of the interesting challenges in fluid dynamics is the coupling of aerodynamic forces with structural forces. This could be the result of external flow, as with aeroelastic flutter on aircraft or architecture, or internal flow, as with the video above. Here researchers blow air through compliant cylindrical shells—think of a straw made of an elastic solid like latex—and observe the vibrations that result. Depending on the flow rate and material properties, different vibrational modes can be activated. The first mode behaves much like a garden hose that’s not being held; it vibrates wildly back-and-forth. The second mode wobbles the mouth of the shell open and closed, whereas the third mode forms three “flaps” that vibrate inward and outward. Each of these modes behaves very differently, and, for practical applications, it’s important for engineers to be able to predict, control, and account for these kinds of structural behaviors under aerodynamic loading. (Video credit: P. Zimoch et al.)
About a year ago, we featured a video in which a fluid droplet bouncing on a vibrating pool demonstrated some aspects of the wave-particle duality fundamental to quantum mechanics. Work on this system continues and this new video focuses on studying some of the statistics of such a bouncing droplet—called a walker in the video—when it is confined to a circular corral. Using strobe lighting and capturing one frame per bounce, the vertical motion of these droplets is filtered out and the walking motion and the surface waves that guide it are captured. When the droplet is allowed to walk for an extended time, its path appears complicated and seemingly random, but it is possible to build a statistical picture and a probability density field that describe where the walker is most likely to be, much the way one describes the likelihood of locating a quantum particle. Parallels between the physical macroscale system and quantum-mechanical theory are drawn. (Video credit: D. Harris and J. Bush; submission by D. Harris)
When a fluid is vibrated, instabilities can form along its surface. With a sufficient amplitude, voids form inside the fluid and their collapse leads to a jet that shoots out from the fluid. A very different process leads to air cavities forming in a vibrated granular medium, but the jets produced are remarkably similar, as seen in this video. (Video credit: M. Sandtke et al.)
