Month: September 2021

Hydrodynamic Spin Lattices
Droplets bouncing on a fluid bath display some strikingly quantum-like behaviors thanks to the interactions between a drop and its guiding surface wave. Here, researchers use submerged wells beneath the drop to confine each droplet into a space where it bounces in a clockwise or anticlockwise trajectory.
With an array of these wells, the droplets form a lattice. Each drop remains in its well, but its wave travels beyond and interacts with nearby wells. Through this interaction, the researchers found that lattices tended to synchronize, similar to the way groups of fireflies will synchronize their flashing. This sort of behavior is also observed in quantum systems, and the researchers hope that further studying their bouncing droplets will give insight into quantum spin systems and their behaviors. (Image and research credit: P. Saenz et al.; via Nature; submitted by Kam-Yung Soh)
September 16, 2021
Breaking Up Is(n’t) Hard to Do
Engineers often need to break a liquid jet up into droplets. To do so quickly, they surround the jet with a ring of fast-moving air in a set-up known as a coaxial jet. Shear between the gas and liquid creates instabilities that quickly distort the jet’s initial cylinder into sheets and ligaments. Those formations then undergo their own instabilities to break up into drops. The method is, as you can see in the high-speed images above, quite effective, though the breakup mechanism itself is tough to quantify. (Image credit: G. Ricard et al.)
September 15, 2021
Keeping Cool in the Cretaceous
I love that fluid dynamics can bring new insights to other subjects, like this study on how heavily-armored ankylosaurs avoided heat stroke. Scans of ankylosaur skulls show a complicated, twisty nasal cavity that researchers likened to a child’s crazy straw. Using numerical simulations, they showed that the airflow through these passages acts like a heat exchanger. As air gets drawn into its body, it warms up from exposure to blood vessels lining the nasal cavity; that means that, simultaneously, the hot blood is getting cooled. Those blood vessels lead up to the animal’s brain, indicating that these twisted cavities essentially serve as air-conditioning for the sauropod’s brain! (Image and video credit: Scientific American; research credit: J. Bourke et al.; via J. Ouellette)
September 14, 2021
Flying Out of the Water
Flying fish and diving birds often navigate the interface between water and air in their flight, but few studies have actually looked at the effects of this transition on lift. In this work, researchers measured forces on a small, fixed wing as it egresses from water into air at a constant velocity.
The tests showed that exit velocity had a large effect on lift generation. At low speeds, an exiting wing experienced a strong, positive lift spike as soon as the leading edge broke the surface. But that lift changed to strongly negative as the wing continued out of the water. At higher speeds, the wings had no lift reversal but also reached lower peak lift coefficients. The team studied the effects of angle of attack and starting depth as well, concluding that any vehicles intended to navigate the water-air transition will need robust control systems prepared to deal with fast-changing forces. (Image credit: fish – J. Cobb, wing – W. Weisler et al.; research credit: W. Weisler et al.)
September 13, 2021
“Kármán Vortex Street”
Although engineers often consider fluid mechanics through the lens of mathematics, that’s far from the only way to understand fluid physics. Today’s video is an alternative interpretation of a classic flow — the flow around a cylinder — created in a collaboration between dancers and engineers. The result is what they call a “physics-constrained dance improvisation” that shows how the flow changes as its speed increases. I love this concept! It highlights the visual and qualitative differences between flow states and maintains space for artistic creativity. Be sure to watch the full video! (Video and submission credit: J. Capecelatro et al.)
September 10, 2021
Moths in Flight
As student engineers, we often use fixed-wing aircraft to build our intuition for flight, but nature has so many other incredible examples to offer. Here we see high-speed video of seven different moth species taking off, and understanding fixed-wing flight won’t help you here at all! These moths have small, rough, and incredibly flexible wings — all characteristics an aircraft designer typically avoids. Yet these insects are agile, fast, and capable fliers at a scale that continues to thwart engineers. Some of the earliest pioneers of flight watched birds for inspiration; for small crafts, there’s no better teacher than insects. (Image and video credit: A. Smith/AntLab)
September 9, 2021
Microjets and Needle-Free Injection
Some people don’t mind needles, and others absolutely detest them. But to replace needles with needle-free injections, we have to understand how high-speed microjets pass through skin. Given skin’s opacity, that’s tough, so researchers are instead using droplets as a model. If we can understand the dynamics of a microjet passing through different kinds of droplets, getting jets of medicine into arms becomes easier.
Researchers found that jets passed completely through a droplet if they impacted above a critical velocity. For Newtonian droplets, the jet creates a cavity and shoots straight through because the inertia of the impact outweighs the countering force of surface tension. But with viscoelastic drops, the jet goes through, slows down, and gets sucked back into the droplet. In this case, the combination of surface tension and viscoelasticity can, eventually, overpower the jet’s inertia. (Image, research, and submission credit: M. Quetzeri-Santiago et al.)
September 8, 2021
Whiffling Geese
This wild photograph shows a goose flying upside down with its head turned 180 degrees in a behavior known as whiffling. In this orientation, the bird’s typical lift characteristics are reversed, but as you can see in the video below, this doesn’t exactly make them fall out of the sky. I suspect the geese compensate by changing their angle of attack (unless descending rapidly is their goal). There are numerous theories as to why the birds whiffle, including escaping hunters by using an erratic flight path or just showing off to the other geese. Maybe they’re just out to have a little fun! (Image credit: V. Cornelissen; video credit: Flightartists Project; via Colossal; submitted by jpshoer)

September 7, 2021
Sea Sponge Hydrodynamics
The Venus’s flower basket is a sea sponge that lives at depths of 100-1000 meters. Its intricate latticework skeleton has long fascinated engineers for its structural mechanics, but a new study shows that the sponge’s shape benefits it hydrodynamically as well.
The sea sponge’s skeleton is predominantly cylindrical, with tiny gaps that allow water to flow through it and helical ridges alongside its outer surface to strengthen it against the deep-sea currents surrounding it. Through detailed numerical simulations, researchers found that both of these features — the holes and the ridges — serve fluid mechanical purposes for the sponge. The porous holes of the sea sponge drastically reduce flow in the sponge’s wake (third image), which provides major drag reduction for the sea sponge. That drag reduction makes it easier for the sponge to stay rooted to the ocean floor.
The helical ridges, on the other hand, create low-speed vortices within the sea-sponge’s body cavity (second image). Such vortices increase the time water spends inside the sponge, likely helping it to filter-feed more efficiently. The additional vorticity comes at the cost of slightly increased drag but not enough to outweigh the savings from its porosity. (Image and research credit: G. Falcucci et al.; via Nature; submitted by Kam-Yung Soh)
September 6, 2021
Hovering Hawk
Birds have a level of control in flight that would make any engineer jealous. This 2021 Audubon Photography Award winning video by Bill Bryant shows off the skills of a red-tailed hawk. On this occasion, the hawk is using strong winds coming off the Rocky Mountains to hover in place. Notice how active his wings and tail are in adjusting to the changes in the wind while his head is perfectly still. With his head still, the hawk can scan the ground for mice and other prey. It’s absolutely incredible to see how effortlessly the hawk is accounting for unsteadiness in the wind here! (Video and image credit: B. Bryant; via Audubon)
September 3, 2021