Nature includes many animals that are so-called fliers: flying squirrels, flying snakes, and draco lizards, to name a few. These animals aren’t true fliers like birds, bats, or insects, though. Instead, they are expert gliders, able to produce enough lift to control their descent and land safely at a distance far greater than a normal leap could carry them. Like the flying squirrel, the draco lizard extends a thin membrane that acts as its wings. The additional area provides enough lift that the lizards can glide as far as 60 m (200 ft) while only losing 10 m (33 ft) in altitude. That’s an impressive glide ratio – about 3 times better than the Northern flying squirrel and twice as good as a wingsuit. (Video credit: BBC/Planet Earth II)
Tag: fluid dynamics
How We Sweat
Sweat plays a critical role in controlling body temperature for humans. Most of the sweat glands on our bodies are eccrine sweat glands, which pump out a mixture of water and electrolytes in response to temperature changes or emotional stimuli. Beneath the surface, these glands consist of three major areas, the tightly bunched secretory coil, where the cells that produce sweat are located; a long dermal duct that transports sweat to the skin surface; and the upper coiled duct just below the pore where sweat exits. Eccrine glands can produce an impressive amount of pressure – about 70 kN/m^2, equivalent to 70% of sea-level atmospheric pressure – to help drive sweat up and out onto the skin. Flow from pores is not steady; like many other biological processes, sweat flow is pulsatile. (Image credit: Timelapse Vision Inc., source; Z. Sonner et al.; submitted by Marc A.)
Avoiding Coalescence
Droplets hitting a liquid surface don’t always coalesce. Above you can see a tiny droplet bounce and skate along the surface of a larger, vibrating drop. The smaller droplet doesn’t coalesce because a tiny layer of air sits between it and the vibrating drop. To actually contact and coalesce, the droplet has to sit still long enough for that air layer to get squeezed out. Instead, the vibration of the larger drop bounces it upwards, refreshing the air layer and scooting the droplet along until it falls off the vibrating drop. (Image credit: C. Kalelkar and S. Phansalkar, source)
Reducing Drag with Bubbles
Large ships experience a great deal of drag due to friction between their hull and the water. One method shipbuilders are considering to combat this drag is the use of bubbles, which have been found to reduce drag by up to 40%. The physical mechanism behind this drag reduction is not yet understood, but a recent study suggests that bubble size and bubble coalescence play an important role.
Researchers introduced surfactants into bubbly boundary layers and found that the reductions in drag evaporated as soon as the surfactants spread. Adding only 6 parts per million of the surfactant decreased average bubble size from 1 mm to 0.1 mm and helped prevent the bubbles from growing via coalescence. The implications are that bubble-induced drag reduction could be extremely sensitive to water conditions. (Image credit: G. Kiss; research credit: R. Verschoof et al.)
Hawaii’s Lava
Sometimes the best way to appreciate a flow is standing still. In “Hawaii – The Pace of Formation” filmmakers explore how the Big Island is constantly changing, from fresh lava flows to towering waterfalls. Much of the footage presented is timelapse, which gives viewers a different perspective on familiar subjects; it highlights the similarities between clouds and the ocean, and it reminds us that a lava flow and the syrup flowing down a stack of pancakes have a lot in common. To me, this is one of the most beautiful parts of fluid dynamics: physics of flows on different length-scales and time-scales – even in different fluids – are still very much the same. (Video credit: A. Mendez et al.)
Cavity Collapse
One of the most iconic images in fluid dynamics is that of a drop impacting a liquid. When a drop hits a pool, it creates a crater, or cavity. That cavity expands and then collapses to form a jet that rebounds above the pool’s surface. If the jet is fast enough, it will eject one or more droplets before it falls back into the pool. Faster droplets, like the one that formed the cavity and jet shown above, actually create slower and fatter jets. In this regime, the complicated interplay of surface tension and gravity effects results in a jet velocity that is independent of impact speed and the liquid’s viscosity. Understanding this jet and splash dynamics is important for many industrial applications, including ink-jet printing. (Image credit: G. Michon et al.)
Water Skiing Beetles
Waterlily beetles employ an unusual method of getting around: they skim across the water surface. The beetles are mostly covered in tiny hairs that help make their body hydrophobic (water-repellent) – a common adaptation for insects that spend their time sitting on the water’s surface – but the beetles also have hydrophilic claws on their legs that help anchor them to the water’s surface. When they need to move quickly, the beetles lean upright and start flapping their wings, creating thrust that helps push them along the interface. Between water’s viscosity and drag from the waves the insect generates, it has to expend a lot of energy for this method of travel – more than these insects do flying in air – but researchers suspect that staying at the surface could remain beneficial for the beetles because it’s easier to locate their floating food sources this way. (Image credit: H. Mukundarajan et al., source; via New Scientist)
Simulating Thunderstorms
With today’s supercomputing power, it’s possible to simulate entire thunderstorms to study how and why some of them can spawn deadly tornadoes. The animation above comes from a computer simulation of a supercell thunderstorm. The simulation uses initial conditions from a 2011 storm that produced an EF-5 tornado – the highest category of tornado, based on its wind speeds. To see more of the simulation, check out the video below. One thing that might surprise you is just how enormous the towering supercell clouds are compared to the tornado produced in the simulation. Often what we can see of a storm from the ground is only the tiniest part of what goes into producing it. (Image credit: L. Orf et al., source; GIF via @popsci; video credit: UWSSEC)
Fanning the Flame
A fan’s blade passes through the hot air rising above a flame in this iconic image by high-speed photography pioneer Harold Edgerton. This photo uses an optical technique known as schlieren photography that makes density differences in transparent media like air visible. Because of its lower density, the hot plume of air above the flame rises. When the fan blade swings past, it sheds a vortex off its tip and the rising air from the flame gets pulled into the vortex to make it visible. To the left, a ghostly counter-rotating vortex sits on the opposite side of the fan blade. (Photo credit: H. Edgerton and K. Vandiver)
When the Mediterranean Flooded
Around 6 million years ago, the African and Eurasian plates moved together, cutting the Mediterranean Sea off from the Atlantic. Without an influx of water from the Atlantic, evaporation began removing more water from the Mediterranean than rivers could replace. The sea dried out almost completely over the course of a couple thousand years.
About 5.3 million years ago, the Straits of Gibraltar reopened, creating a massive flood into the Mediterranean known as the Zanclean Flood. Water rushed down the straits and into the Mediterranean at speeds as high as 40 m/s (90 mph). At its peak, the Zanclean Flood is estimated to have reached rates 1000 times greater than the volumetric flow rate of the Amazon River.
A similar breach flood occurred in the Black Sea within the past 10,000 years when the Bosporus became unblocked. That flood likely had a devastating impact on Neolithic societies in the area and may be the inspiration for the floods described in the Epic of Gilgamesh and the Bible. (Image credit: BBC, source)
