Heard Island, a remote patch of rock in the southwestern Indian Ocean, peeks its head above the marine cloud layer. The volcanic island disrupts the atmosphere enough to generate a von Karman vortex street, a line of alternating vortices shedding from either side of the island. Usually these vortices would march in a straight line downstream from their source. But here strong winds from the south have blown a bunch of its vortices northward, creating an unusual kink in the island’s wake. (Image credit: J. Schmaltz/LANCE EOSDIS Rapid Response; via NASA Earth Observatory)
Month: June 2016
Visualizing Smell
Every day we’re surrounded by an invisible world of smells. Like the fluorescein dye in the animation above, these odors drift and swirl in the background flow. What you may not have stopped to consider when you smell the roses, though, is how the very act of sniffing changes the scent. When you inhale, filaments of the odor are drawn into your nose, and, likewise, when you exhale, your breathe mixes with the scent and sends it swirling outward in turbulent eddies. To see more about the science of scent, check out PBS News Hour’s full video below. (Video credit: PBS News Hour; GIF via skunkbear)
Paper Marbling
Fluid dynamics and art have gone hand-in-hand for centuries. In this video, artist Garip Ay demonstrates one of the coolest fluids-based art techniques: paper marbling. In this technique, artists float ink or paints on a liquid surface, manipulate the colors as desired–in this case to recreate Van Gogh’s “Starry Night”–and then float a piece of paper atop the surface to transfer the image. Multiple cultures around the world developed marbling techniques, dating all the way back to the Middle Ages. Ay is an expert in ebru, a Turkish form of the art. For more of Ay’s art, check out his website and YouTube channel. (Video credit: G. Ay; via Gizmodo)
Bubble Tricks
[original media no longer available]
Everyone remembers playing with soap bubbles as a child, but most of us probably never became as adept with them as magician Denis Lock. In this video, Lock shows off some of the clever things one can do with surface tension and thin films. My favorite demo starts at 1:25, when he constructs a spinning vortex inside a bubble. He starts with one big bubble and adds a smaller, smoke-filled one beneath it. Then, using a straw, he blows off-center into the large bubble. This sets up some vorticity inside the bubble. When he breaks the film between the two bubbles, the smoke mixes into the already-swirling air in the larger bubble. Then he pokes a hole in the top of the bubble. Air starts rushing out the deflating bubble. As the air flows toward the center of the bubble, it spins faster because of the conservation of angular momentum and a miniature vortex takes shape. (Video credit: D. Lock/Tonight at the London Palladium/ via J. Hertzberg)
Reader Question: Shower Curtains
Reader thansy asks:
Why do the bottoms of shower curtains drift in toward the water coming from the shower head?
We all know that moment. You’re minding your own business, scrubbing away, and all of a sudden, the shower curtain billows up and grabs you. Scientists have debated the cause of this behavior for years. Some argued that the curtain billowed due to hot air rising from the shower. Others claimed the fast-moving spray caused lift that pulled the curtain up. But fifteen years ago, one scientist tackled the problem computationally. He performed a numerical simulation of a shower head spraying into a bath and found that this spray of droplets creates a weak horizontal vortex in the shower.
This shower vortex has a low-pressure core at the middle, which is thought to provide the suction that causes the shower curtain to billow. The scientist, David Schmidt, was awarded the 2001 Ig Nobel Prize for his work. (Image credits: N. Paix, D. Schmidt; research credit: D. Schmidt)
Swimming at Microscale
Tiny organisms live in a world dominated by viscosity. There’s no coasting or gliding. If a microorganism stops swimming, friction will bring it to a halt in less than the space of a hydrogen atom! To make matters worse, simply flapping an appendage forward and backward will get them nowhere. As we’ve seen before, these highly viscous laminar flows are reversible, meaning that a backward power stroke is simply undone by a mirrored forward recovery stroke. Instead, microorganisms like the paramecium swimming above are covered in tiny hairlike cilia which beat asymmetrically. They extend to their full length during the power stroke, but they stay bent during the forward recovery stroke. That asymmetry guarantees that they move more fluid backward than forward, thereby letting the paramecium make progress. (Image credit: C. Baroud, source)
Shelf Cloud
Sydney, Australia was treated to a spectacular meteorological show over the weekend when an impressive shelf cloud swept over the city. These timelapses show the dramatic leading edge of the incoming thunderstorm. Notice how the cloud streams upward along the shelf. The storm is driven by this updraft of warm moist air, which rises until it is capped by the troposphere. At this point, the air spreads, creating an anvil-like shape, and cools. The moisture drawn up at the storm’s front will condense, freeze, and fall as rain or hail. When the updraft weakens, the storm will be dominated by the downdraft of the falling precipitation and eventually peter out. (Image credit: W. Reed and H. Vann, source; via J. Hertzberg)
Drawing Up Dew
Desert plants have evolved to efficiently collect and capture whatever water they can. Each leaf of the moss Syntrichia caninervis ends in a hairlike fiber called an awn (seen in white in the top image). Tiny as they are, awns are vital to the moss’s water collection, correlating to more than 20% of their dew collection. Extremely tiny grooves on the surface of the awn provide nucleation sites where dew condensed from fog collects. Once a droplet forms on the awn, it grows larger as more fog condenses (middle image). When the droplet grows large enough, the conical shape of the awn will cause surface tension to draw the droplets along the awn and toward the leaf (bottom image).
(Credits: Syntrichia caninervis moss image – M. Lüth; videos and research – Z. Pan et al., Supplementary Videos 3 and 4; h/t to T. Truscott)
Why Fishing with Dynamite is So Harmful
In some countries, there are still people using dynamite to catch fish. This practice is incredibly destructive, not just to adult fish but to the entire marine ecosystem. A blast wave traveling through air loses some its energy to the compression of the gas. Water, on the other hand, is incompressible, so the blast wave’s energy just keeps going, expanding its destructive radius. Many fish contain swim bladders, gas-filled organs the fish use to regulate their depth. When a shock wave passes through the fish, the gas in the swim bladder will expand and contract violently, much like the balloons shown underwater in the animation below. This typically ruptures the swim bladder and surrounding tissues.
Fish without swim bladders will often hemorrhage after being struck by a blast wave. The sudden changes in pressure create bubbles in the dissolved gases collected in their gills. Those bubbles tear apart the fish’s blood vessels.
Blasting is effective but entirely indiscriminate. It kills adults and juveniles of all species, not just the ones a fisherman can sell. Simultaneously, it destroys the slow-growing coral reefs that are key habitats for these populations. It’s an incredibly short-sighted practice that guarantees there will be no fish to catch in years to come. (Video credit: National Geographic; image credit: M. Rober, source; research credit: K. Dunlap, pdf)
Microscale Rockets
Shown above are a trio of microscale rockets, each about 10 microns in length. These tiny rockets are roughly cylindrical in shape, with a narrower diameter at the front than the back. Like their space-faring brethren, these microrockets are chemically propelled. They draw in fuel from their surroundings, which reacts with the catalysts coating the interior of the microrocket to produce gases. Those gases bubble out the back end of the microrocket, creating thrust capable of propelling the rockets more than 1000 body lengths/second. Researchers have already demonstrated that these tiny rockets can haul cargo along with them. Scientists hope one day to use these self-propelled microrockets to help deliver drugs or isolate cancer cells. (Image credit: J. Li et al., source)
