FYFD

Year: 2021

  • Featured Video Play Icon

    “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.)

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    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)

  • Microjets and Needle-Free Injection

    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.)

  • Whiffling Geese

    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)

  • Sea Sponge Hydrodynamics

    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)

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    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)

    A red-tailed hawk hovers on the wind while hunting.
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    Fish Versus Bird

    You’ve seen birds catch fish, but have you ever seen a fish that catches birds? In this video, giant trevally fish hunt fledgling terns — including those in flight! To do so, the fish must correctly assess the bird’s speed and trajectory across the water interface, a feat reminiscent of the archer fish’s aim. They also need the power and control to leap from the water and catch the birds in their mouth without relying on the suction technique so many fish use underwater. (Image and video credit: BBC Earth, from “Blue Planet II”)

  • Leidenfrost Without the Heat

    Leidenfrost Without the Heat

    Leidenfrost drops slide almost frictionlessly on a layer of their own vapor, generated by extremely hot surfaces nearby. But in this experiment researchers recreated many of the classic behaviors of a levitating Leidenfrost drop without the added heat. Instead, they supersaturated water droplets with carbon dioxide to create “fizzy droplets” that slide and self-propel along superhydrophobic surfaces.

    Initially, the drops don’t levitate. It takes a little while for the carbon dioxide layer to build up beneath them, as seen by the slowly appearing interference fringes in the second image. But once the layer forms, the drops behave like conventional Leidenfrost drops until their carbon dioxide is depleted. They’re even able to self-propel on a racheted surface (third image)! (Image and research credit: D. Panchanathan et al.; via Physics World; submitted by Kam-Yung Soh)

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    Shattering With Resonance

    Resonance is a phenomenon that is both familiar and somewhat mysterious. It takes place when a system is excited near its natural frequency. In this case, we’re seeing a mechanical resonance that’s driven by sound waves near the glass’s natural frequency. Once excited, the glass vibrates by flexing side-to-side along one axis and then again in a perpendicular direction. Eventually, the amplitude of this flexing is large enough to break the glass. When the glass is filled with water, its flexing instead generates a cloud of tiny droplets in a process known as vibration-induced atomization. The inverse problem — an empty glass resonating within a pool of liquid — is also an extremely cool problem. (Image and video credit: The Slow Mo Guys)

  • The Froghopper’s Incredible Suction

    The Froghopper’s Incredible Suction

    The tiny froghopper feeds on the sap in xylem, a feat that requires overcoming more than a megapascal of negative pressure. Plants, as you may recall, transport water and nutrients from their roots to their leaves through negative pressure, essentially pulling on the water as if it were a rope. So drinking that sap is not as simple as making a hole and waiting for sap to flow. Instead, froghoppers must generate even more suction than the plant. Some scientists have been so skeptical that such a feat is even possible that they’ve disputed whether plants are truly at such high negative pressures.

    But a new study shows that froghoppers can, indeed, generate immense suction – up to nearly 1.5 megapascals. (By comparison, humans generate less than a tenth of that suction, even on a stubborn milkshake.) The researchers used two complementary methods to prove the insects’ ability. First, they studied the anatomy of the pumplike structure in the froghoppers’ heads, where the suction is generated, and determined the insects’ sucking potential from a simple calculation of force divided by area. Then, they observed feeding froghoppers in a chamber where they could measure their metabolic rates through carbon dioxide output. As the froghoppers fed, their metabolic rates spiked to 50 – 85% higher than when at rest. Only when the xylem tensions exceeded the theoretical biomechanical limits for froghopper suction did the tiny insects seem to stop feeding. (Image and research credit: E. Bergman et al.; via Science News; submitted by Kam-Yung Soh)