On Earth, it’s easy for the effects of surface tension and capillary action to get masked by gravity’s effects. This makes microgravity experiments, like those performed with drop towers or onboard the ISS, excellent proving grounds for exploring fluid dynamics unhindered by gravity. The video above looks at how colliding jets of liquid water behave in microgravity. At low flow rates, opposed jets form droplets that bounce off one another. Increasing the flow rate first causes the droplets to coalesce and then makes the jets themselves coalesce. Similar effects are seen in obliquely positioned jets. Perhaps the most interesting clip, though, is at the end. It shows two jets separated by a very small angle. Under Earth gravity, the jets bounce off one another before breaking up. (The jets are likely separated by a thin film of air that gets entrained along the water surface.) In microgravity, though, the jets display much greater waviness and break down much quicker. This seems to indicate a significant gravitational effect to the Plateau-Rayleigh instability that governs the jet’s breakup into droplets. (Video credit: F. Sunol and R. Gonzalez-Cinca)
Year: 2014
Droplets Surfing
The Leidenfrost effect can make water droplets skitter across a hot griddle or briefly protect a hand dunked in liquid nitrogen. When a liquid is exposed to a solid surface much, much hotter than its boiling point, the contact vaporizes part of the liquid, and, in the case of a droplet, forms a thin lubricating layer of vapor that the liquid drop can skate around on. Researchers have found that releasing these Leidenfrost droplets on textured surfaces creates self-propelling drops by directing the flow of vapor. In this video, one team demonstrates some of the neat tracks they’ve built for their drops. (Video credit: D. Soto et al.)
Advantages of Schooling
Many fish swim in close proximity to one another in large schools, causing scientists to wonder if this behavior is motivated primarily by defense against predators or whether fish derive some hydrodynamic advantages from schooling. Examining the fluid dynamics of an entire school of fish is rather impractical, so researchers approximate two neighboring swimmers using flapping hydrofoils. The images above show flow visualizations of the wakes of these two mechanical swimmers. When the two hydrofoils flap in-phase with one another (top image), one oscillation period produces a complicated pattern of many vortices zig-zagging behind the foils. This configuration produces more efficient propulsion than a single hydrofoil, meaning that more of the energy in the wake is used to produce thrust. The cost, however, is reduced thrust overall. The bottom image shows the wake pattern for hydrofoils flapping out-of-phase. This behavior enhanced thrust without reducing propulsive efficiency. The results suggest that schooling fish might choose different swimming strategies depending on the situation. (Image credits: P. Dewey et al.)
The March of Drops
I love science with a sense of humor. This video features a series of clips showing the behavior of droplets on what appears to be a superhydrophobic surface. In particular, there are some excellent examples of drops bouncing on an incline and droplets rebounding after impact. For droplets with enough momentum, impact flattens them like a pancake, with the rim sometimes forming a halo of droplets. If the momentum is high enough, these droplets can escape as satellite drops, but other times the rebound of the drop off the superhydrophobic surface is forceful enough to overcome the instability and draw the entire drop back off the surface. (Video credit: C. Antonini et al.)
Putting Out Wildfires Using Explosives
Wildfires damage millions of acres of land per year in the United States alone. Using explosives to put out an uncontrolled wildfire sounds a bit crazy, but it’s actually not that far-fetched. The animations above are taken from high-speed footage of a propane fire interacting with a blast wave. The first animation shows what the human eye would see, and the second is a shadowgraph video, a technique which highlights differences in density and makes the flame’s convection and the blast wave itself visible. At close range, the shock wave from the explosion and the high-speed gas behind it push the flames away from their fuel source, stopping combustion almost immediately. For a flame farther away from the blast, the shock wave introduces turbulent disturbances that can destabilize the flame. Much work remains to be done before the technique could be scaled from the laboratory to the field, but it is an exciting concept. You can read more about the work here. (Research credit: G. Doig/UNSW Australia; original videos: here and here; submitted by @CraigOverend)
Oily Foams
It is common in many industries to use oil as a defoamer to break up existing foams or prevent foams from forming. But with the right surfactants–additives that change the foam’s surface tension–it’s possible to make aqueous foams that are actually stabilized by the presence of oil. This video explores some of the ways that oil can interact with these kinds of foam, beginning with capillary action, which draws the oil up into the junctions between foam films. For more, see Piroird and Lorenceau. (Video credit and submission: K. Piroird)
Supercell Timelapse
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)
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)
Shrinking Red Spot
Observations show Jupiter’s iconic Great Red Spot is shrinking, most recently at a rate of more than 900 km a year. As it gets smaller, the storm is also changing shape and becoming more circular. Scientists don’t yet have an explanation for the shrinkage or its recent acceleration, but this is unsurprising given the rich complexity of the storm. For example, the source of the Red Spot’s longevity–it may be more than 300 years old–is still an open topic of research. Some of the most recent observations show smaller eddies feeding into the storm; the current hypothesis is that these eddies may be increasing the Red Spot’s dissipation and accelerating its breakup. (Photo credit: NASA/ESA; h/t to io9)
Separating Flow
Flow separation occurs when a fluid is unable to flow smoothly around an object. In the case of the photo above, fog is being used to visualize flow around an airfoil at a large negative angle of attack. The incoming flow stagnates at a point on top of the airfoil, and streamlines on either side of that point split to move around the airfoil. Those on top are accelerated to high velocity, generating smooth, low-pressure flow over the aft section of the upper surface. On the other side of the stagnation point, however, the fog is trying to flow around the curve of the leading edge but the local pressure gradient is increasing, which slows the flow. Ultimately, it separates from the airfoil, creating a large region of recirculating, turbulent flow. When this effect occurs on the upper surface of a wing at a high (positive) angle of attack, it is called stall and causes a dramatic loss in lift. (Photo credit: Wikimedia/Smart Blade GmbH)
