As this smoke visualization shows, trees have a significant impact on airflow around them. Flow in the image is from left to right. On the left, the upstream air is traveling in smooth, laminar lines that are quickly disrupted as the flow moves into the trees. After the first shorter trees, flow inside the wooded area has been broken up and slowed. Above the canopy, the smoke streaklines have also slowed and become more turbulent. Understanding how wind and trees interact is important in a variety of applications, including when adding renewable energy options to buildings and when predicting the spread of forest fires. (Image credit: W. Frank et al.)
Category: Research
Sniffing Underwater
You’d be forgiven for thinking that the star-nosed mole looks funny. Its distinctive star-shaped nose is a highly-sensitive organ, but the mole doesn’t just use it for finding its way through the underground tunnels it lives in. These moles can actually sniff underwater. By exhaling a bubble and then re-inspiring it, the moles collect scent particles that they can use to locate food. In experiments, both star-nosed moles and water shrews could use this technique to successfully follow a scent trail, demonstrating exploring and pausing behaviors similar to terrestrial sniffing as they did. To learn more about this impressive mammal, listen to the latest episode of Science Friday, where research Ken Catania describes his work with them. (Image credits: K. Catania; via Science Friday)
How the Jellyfish Stings
Many jellyfish are capable of venomously stinging both their prey and their predators. The stings originate from specialized cells in their tentacles called nematocysts (middle image) that, when activated, rapidly extend a thin tubule that acts like a hypodermic needle to deliver venom into the jellyfish’s victim (bottom image). The tubules can elongate in about 50 ms – about one-sixth of the time needed to blink your eye. This rapid extension is driven by osmotic pressure – pressure generated when water flows across a semi-permeable membrane in response to chemical changes.
Researchers originally thought all of the osmotic pressure resided in the nematocyst’s capsule end from which the tubule expands, but new work indicates that the tubule is instead pulled along by high osmotic pressure along its moving front. That means that disrupting osmosis at the front – by say, wearing a material with no osmotic potential – can slow down the tubule expansion and stop the jellyfish’s sting. (Image credits: jellyfish – A. Kongprepan; nematocyst – D. Brand; tubule expansion – S. Park et al.; research credit: S. Park et al.; submitted by L. Buss)
Growing Droplets on a Trampoline
Droplets on a liquid surface will typically coalesce, thanks to gravity and the low viscosity of the air layer between them and the pool. In certain cases, droplets will partially coalesce, producing smaller and smaller droplets until they finally coalesce completely. Vibrating the liquid surface can help prevent this coalescence but only when droplets are small.
In fact, if the pool is more viscous than the droplets, bouncing can be used to produce droplets of a desired size, as shown above. Because the droplets are less viscous, they deform more than the pool does – behaving somewhat like a bouncy ball hitting a rigid wall. In this system, large droplets are unstable and will undergo partial coalescence until they are small enough to bounce stably. The size of stable drops is determined by the frequency and acceleration of the bouncing bath; by tuning these parameters, researchers can select what size droplets they want to end up with. (Research credit: T. Gilet et al.; images and submission by N. Vandewalle)
Gravity Waves on Mars
It may look like grainy, black and white static from a 20th-century television, but this animation shows what may be the first view of gravity waves seen from the ground on another planet. The animation was stitched together from photos taken by the Mars Curiosity rover’s navigation camera, and it shows a line of clouds approaching the rover’s position.
Gravity waves are common on Earth, appearing where disturbances in a fluid propagate like ripples on a pond. In the atmosphere, this can take the form of stripe-like wave clouds downstream of mountains; internal waves under the ocean are another variety of gravity wave. If these are, in fact, Martian gravity waves, they are likely the result of wind moving up and over topography, much like their Terran counterparts. (Image credit: NASA/JPL-Caltech/York University; research credit: J. Kloos and J. Moores, pdf; via Science; h/t Cocktail Party Physics)
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.)
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.)
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)
