This high speed video shows schlieren photography of a bottle rocket’s exhaust. The supersonic CO2 leaving the nozzle is underexpanded, meaning its pressure is still higher than the ambient atmosphere. As a result, a series of diamond-shaped shock waves and expansion fans appear in the exhaust jet. Each shock and expansion changes the pressure of the exhaust until it ultimately reaches the same pressure as the ambient air. This distinctive pattern, also known as Mach diamonds or shock diamonds, often occurs in wake of rockets. (Video credit: P. Peterson and P. Taylor)
Videos
Diving Peregrines
Few animals can compete with a peregrine falcon for pure speed. There is evidence that, when diving, the falcon can reach speeds upward of 200 mph (320 kph). That the birds can achieve this by pulling their wings back into a low-drag profile is impressive, but the control they exert to do so is even more astounding. The placement and acuity of a falcon’s eyes would require tilting its head roughly 40 degrees if diving straight down on its prey. Such asymmetry increases their drag by more than 50% and creates a torque that yaws the bird. Instead, as seen in the video above, the falcon keeps its head straight and flies in a spiral-like dive, allowing it to maintain sight contact with its target and maximizing its speed despite the extended dive. (Video credit: BBC; research credit: V. A. Tucker)
Cracks in Sea Ice
Arctic sea ice often appears as a single extensive sheet when, in reality, it is made up of many smaller sections of ice shifting and grinding against one another under the influence of winds and ocean currents. This can cause cracks–known as leads–to open up between sections of the ice. This animation, constructed from infrared satellite images, shows the growth of several cracks, leading to extensive break-up of the ice sheet from late-January through March. The fracturing was driven by a high-pressure system that parked over the region, bringing warmer temperatures and southwesterly winds that fueled the Beaufort Gyre, a large-scale, wind-driven, clockwise circulation in the sea that helped pull the ice apart. For more, see NASA EO’s explanation. (Video credit: NASA Earth Observatory)
Skittering Droplets
Water splattered onto a a hot skillet will skitter and skip across the surface on a thin layer of vapor due to the Leidenfrost effect. The partial vaporization of the droplet provides a low-friction cushion for the droplet to glide on and acts as an insulating layer that delays the vaporization of the rest of the droplet. Modernist Cuisine shows us how serene this common and sometimes explosive effect looks at 3,000 frames per second. (On the topic of cooking, you can use the Leidenfrost effect to see if your skillet is hot enough when making pancakes. If a few droplets of water skitter across the pan before sizzling away, then your pan is ready for batter!) (Video credit: Modernist Cuisine; submitted by Eban B.)
Gravity’s Effect on Bursting Bubbles
In a gravitational field, the pressure in a fluid increases with depth. You can consider it due to the weight of the fluid above. Outside of scuba diving or hiking at altitude, this effect is not one typically given much thought. But what effect can it have at a smaller scale? This video shows the collapse and rebound of three initially spherical cavitation bubbles inside a liquid. Each bubble is created in a different gravitational field – one in microgravity, one in normal gravity, and one at 1.8x Earth gravity. The bubble in microgravity remains axisymmetric and spherical, but the two bubbles recorded in gravitational fields develop jets during rebound. Even at a scale of only a few millimeters, gravity causes an imbalance in pressure across the bubble that creates asymmetry. (Video credit: D. Obreschkow et al.)
Saturn’s Great White Spot
We’ve touched a couple times on Saturnian storms, but this NASA video gives a great overview of the Great White Spot, a storm that appeared in late 2010. Gauging the fluid dynamics of gas giants like Saturn and Jupiter is difficult, in large part because we can see only the outermost portion of the atmosphere. Numerous theories and models have been suggested to explain features and dynamics that we observe, but much of the overall behavior remains a subject of debate among planetary scientists. (Video credit: NASA Goddard)
Mixing the Southern Ocean
Motion in the ocean is driven by many factors, including temperature, salinity, geography, and atmospheric interactions. While global currents dictate much of the large-scale motion, it’s sometimes the smaller scales that impact the climate. This visualization shows numerically simulated data from the Southern Ocean over the course of a year. The eddies that swirl off from the main currents are responsible for much of the mixing that occurs between areas of different temperature, which ultimately impacts large-scale temperature distributions, in this case affecting the flux of heat toward Antarctica. (Video credit: I. Rosso, A. Klocker, A. Hogg, S. Ramsden; submitted by S. Ramsden)
Mixing While Laminar
Although turbulent flows are known for their mixing efficiency, in manufacturing there can often be a need to mix laminar fluid streams without the increased shear stress of a turbulent flow. This can be particularly important for polymeric liquids, where too much shear stress could damage the polymer chains. One possibility is using a static mixer, such as the one demonstrated in this video, which, when placed in pipe flow, will deflect the pipe’s contents in such a way as to produce efficient mixing over a short distance. Here two streams of high-viscosity epoxy are mixed through such a static mixer, hardened, and then ground to show the mixing at each level of the static mixer. (Video credit: Sulzer)
“Frozen” Water Stream
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)
Inside a Blender
The fluid dynamics of a commercial-quality blender amount to a lot more than just stirring. Here high-speed video shows how the blender’s moving blades create a suction effect that pulls contents down through the middle of the blender, then flings them outward. This motion creates large shear stresses, which help break up the food, as well as turbulence that can mix it. But if you watch carefully, you’ll also see tiny bubbles spinning off the blades. These bubbles, formed by the pressure drop of fluid accelerated over the arms of the blades, are cavitation bubbles. When they collapse, or implode, they create localized shock waves that further break up the blender’s contents. This same effect is responsible for damage to boat propellers and lets you destroy glass bottles. (Video credit: ChefSteps; via Wired; submitted by jshoer)