The storm chasing group Basehunters captured this stunning timelapse of a supercell thunderstorm forming in Wyoming. This class of storm is characterized by the presence of a mesocyclone, seen here as a large, rotating cloud. These rotating features form when horizontal wind shear is redirected upright by an updraft. This requires a strong updraft, which is often formed by a capping inversion, where a layer of warm air traps colder air beneath it. Supercells can be very dangerous in their own right, releasing torrential rains and large hail, but they are also capable of spawning violent tornadoes. (Video credit: Basehunters; via Bad Astronomy; submitted by jshoer)
Videos
River Paths
As a follow-up to this recent post about river meander, check out this video from Numberphile about some of the mathematics behind the path of rivers. A river’s course is typically much longer than the direct distance between its origin and outlet; the ratio of these two distances is the river’s sinuosity. The fluid dynamics of a river’s bend tend to create stronger bends, but, once a bend reaches an extreme point, it will often be cut off, thereby straightening the river’s path. A model of unconstrained rivers suggests that, on average, the sinuosity of rivers should be about pi. As noted in the video, it would be very interesting to see how this theory holds up next to real rivers. But, given the way humans have fixed the course of rivers to prevent flooding, their current sinuosity is probably far from natural or unconstrained. (Video credit: Numberphile; research credit: H. Stølum; submitted by haxpaxmax)
Melting Ice Sheets From Below
A new study of ice sheets in West Antarctica has made major news this week with the announcement that the ice melt in this region is unstoppable and may raise sea levels by more than 1.2 meters. Part of what makes the ice sheet so unstable is the local topography, shown schematically in the animation above. The land on which the glacier sits lies well below sea level, and the grounding line marks where the ice, sea, and land meet. Part of the glacier projects outward as a sheet, with seawater between it and the land; this is not unusual, but it can encourage melting if the water under the ice sheet is warmer. A major problem for this region, though, is that the slope of the underlying land tilts downward. This means that, as warmer water begins circulating under the ice sheet, it causes the grounding line to retreat and expose a greater volume for warm water to fill beneath the ice. More warm water melts more ice and the process continues unabated. (Video credit: NASA/JPL; h/t to jtotheizzoe, jshoer)
Double-Diffusive Convection
Convection can be driven several mechanisms, including temperature and concentration differences. The video above shows convection between a a layer of sucrose solution and a layer of saline solution. Initially, the lighter sucrose layer sits over the denser salt water. After the interface is perturbed, the differences in concentration – and thus in density – between the fluids causes diffusion both upward and downward in the form of fingers. This instability behavior is analogous to salt-fingering, which occurs in the ocean when a layer of warm, salty water lies over a layer of cooler, less saline water. In the ocean, these temperature and salinity differences help drive ocean circulation as well as the mixing that occurs between different depths. (Video credit: William Jewell College)
Forming a Vortex
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Vortex rings show up remarkably often in nature. In addition to being the playthings of dolphins, whales, scuba divers, humans, and swimmers, vortex rings appear in volcanic outbursts and spore-spreading peat mosses. Vortex rings even occur in blood flow through the human left ventricle in the heart. In each of these cases, the vortex ring is formed by impulsively accelerating fluid through a narrow opening, like the dolphin’s blowhole. The fluid at the edge of injected jet is slowed by friction with the quiescent surrounding fluid. The fluid at the edge of the jet then slips around the sides and into the wake of the faster-moving fluid, where it’s accelerated through the middle of the forming vortex ring. This spinning from the inside-out and back-in persists as long as the vortex is intact, and is part of what keeps the ring from dissipating. (Video credit: SeaWorld; submitted by John C.)
Why Ketchup is Hard to Pour
Oobleck gets a lot of attention for its non-intuitive viscous behaviors, but there are actually many non-Newtonian fluids we experience on a daily basis. Ketchup is an excellent example. Unlike oobleck, ketchup is a shear-thinning fluid, meaning that its viscosity decreases once it’s deformed. This is why it pours everywhere when you finally get it moving. Check out this great TED-Ed video for why exactly that’s the case. In the end, like many non-Newtonian fluids, the oddness of ketchup’s behavior comes down to the fact that it is a colloidal fluid, meaning that it consists of microscopic bits of a substance dispersed throughout another substance. This is also how blood, egg whites, and other non-Newtonian fluids get their properties. (Video credit: G. Zaidan/TED-Ed; via io9)
Pointed Drops
When water droplets sit on a cold substrate, they freeze into a shape with a pointed tip. At first glance, this behavior seems very odd since surface tension usually acts to prevent such sharp protrusions. The shape is, however, a result of water’s expansion as it freezes. The droplet freezes from the substrate upward, with a concave shape to the solidification front. The angle of the point does not depend on the substrate temperature or the wetting angle between the water and surface. Instead, it turns out that this concave front shape and water’s expansion are the key factors that determine the pointed cusp’s angle, and that the final geometry of the cusp is essentially universal. (Video credit: M. Nauenberg; additional research credit: A. Marin et al.)
May the Fourth Be With You
It only seems appropriate to share this little bit of schlieren photography today. May the Fourth be with you all. (Video credit: M. Hargather and J. Miller)
Vibrating on a Subwoofer
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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)
Granular Jet
Sometimes the similarity between fluid flow and granular flows is quite striking. This video shows a stream of sand falling down a tube and impacting a rod. (Note: the view is rotated 90 degrees counter-clockwise, so down points to the right.) As the sand strikes the rod, it’s deflected into a conical sheet, very much like a water bell. There are even ripple-like instabilities that form in the granular sheet, though they move differently than in a liquid due to the sand’s lack of surface tension. (Video credit: S. Nagel et al.)