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Search results for: “drag”

  • Measuring Mucus by Dragging Dead Fish

    Measuring Mucus by Dragging Dead Fish

    A fish‘s mucus layer is critical; it protects from pathogens, reduces drag in the water, and, in some cases, protects against predators. But little is known about how mucus could affect terrestrial locomotion in species like the northern snakehead, which can breathe out of the water and move across land. So researchers explored the snakehead’s mucus layer by measuring the force required to drag them (and two other non-terrestrial species) across different surfaces.

    The team tested the same, freshly euthanized fish twice: once with its mucus layer intact and again once the mucus was washed off. Unsurprisingly, the fish’s friction was much lower with its mucus. But they also found that the snakehead was slipperier than either the scaled carp or the scale-free catfish. The biologists suggest that the snakehead could have evolved a slipperier mucus to help it move more easily on land, thereby extending the distance it can cover.

    As a fluid dynamicist, I think fish mucus sounds like a great new playground for the rheologists among us. (Image and research credit: F. Lopez-Chilel and N. Bressman; via PopSci)

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  • Drag Is Greatest Before Submersion

    Drag Is Greatest Before Submersion

    A new study shows that partially submerged objects can experience more drag than fully submerged ones. This unexpected result comes from the excess fluid that piles up ahead of the object, as seen in the image above, where flow is moving from left to right. The experiments used centimeter-sized spheres and showed that the maximum drag on a nearly-submerged sphere could be 300-400% greater than the drag on a fully submerged sphere.

    Even more surprisingly, they found that water-repellent hydrophobic coatings — which are often suggested for drag reduction — actually increased the drag even further on partially submerged spheres. That’s because the water-repelling coating caused an even larger build-up of fluid ahead of the sphere, increasing the pressure on the front side of the sphere and creating even more drag. Spheres with a hydrophilic coating had less water build-up and thus lower drag.

    The study suggests that — at the centimeter-scale — drag physics at the air-water interface may be more complicated than we assume. (Image and research credit: R. Hunt et al.; via Physics World; submitted by Kam-Yung Soh)

  • Drag Reduction for Swimming Shrimp

    Drag Reduction for Swimming Shrimp

    Marsh grass shrimp, despite their small size, are zippy swimmers. They move using a series of closely-spaced legs that stroke asynchronously. Researchers found that the flexibility and stiffness of the legs are critical for the shrimp’s efficiency. During the power stroke, the shrimp’s leg is held stiff, maximizing the force it’s able to transfer to the water. But during the forward-moving recovery stroke, the shrimp bends its legs almost horizontal and presses both legs in the pair together tightly. This action minimizes the area of the leg pair and reduces the drag they cause as they move into position for the next stroke. (Image, video, and research credit: N. Tack et al.; via Ars Technica; submitted by Kam-Yung Soh)

    https://www.youtube.com/watch?v=hWOtF0RXTwk
    A close-up view of the shrimp’s leg as it swims.
  • Measuring Drag

    Measuring Drag

    After a noticeable rise in the prevalence of home runs beginning in 2015, Major League Baseball commissioned a report that found the increase was caused by a small 3% reduction in drag on the league’s baseballs. When such small differences have a big effect on the game, it’s important to be able to measure a baseball’s drag in flight accurately.

    In the past, that measurement has often been done in a wind tunnel, but the mounting mechanisms used there result in drag measurements that are a little higher than what’s seen from video tracking in actual games. Now researchers have developed a new free-flight method for measuring a baseball’s drag. The drag measurements from their new method are lower than those for wind-tunnel-mounted baseballs and in better agreement with video-based methods. The authors’ method should be adaptable to other sports like cricket and tennis, which will hopefully provide new insight into the subtleties of their aerodynamics. (Image credit: T. Park; research credit: L. Smith and A. Sciacchitano; via Ars Technica; submitted by Kam-Yung Soh)

  • Superhydrophobic Drag

    Superhydrophobic Drag

    Using air or bubbles to reduce drag on boats is a popular idea, whether using supercavitation, the Leidenfrost effect, or superhydrophobic coatings. But most of the experiments done thus far use spheres rather than realisitic boat shapes. In this study, the researchers used two model boats — one with a hydrofoil and the other in a conventional motorboat shape — and applied superhydrophobic coatings to different parts of the model to see how superhydrophobicity affected the overall drag.

    Perhaps surprisingly, they found that superhydrophobic coatings can actually increase the drag! The effect was particularly stark for the hydrofoil boat (Image 2), where the surface jets (lower half) caused by the superhydrophobic coating slowed the boat by 30% compared to its unmodified speed (upper half).

    For the speedboat, a superhydrophobic hull made no overall difference in its drag, though it changed how water splashed in its wake. And coating the boat’s propeller was particularly detrimental, resulting in a speed up to three times slower. Overall, the study suggests that superhydrophobic coatings may be useful in some circumstances, but they have to be applied carefully, as they can have negative impacts, too. (Image credits: top – S. Anghan, others and research credit: I. Vakarelski et al.)

  • Superhydrophobic Drag

  • Flexible Filament Reduces Drag

    Flexible Filament Reduces Drag

    Most shapes aren’t streamlined for fluid flow. We call these bulky, often boxy shapes, bluff bodies. Above, we see two examples of a bluff body, a flat plate, in a soap film. On the left, the plate sits perpendicular to the soap film’s top-to-bottom flow. Two large, counter-rotating vortices form behind the plate and a wide wake stretches behind it.

    On the right, we see the same flat plate but now a long, flexible filament is attached to either end. As the flow moves past, it deforms the filament, creating a rounded shape. Researchers found that, under the right conditions, this flexible afterbody could reduce drag on the object by up to 10%. (Image and research credit: S. Gao et al.)

  • Peloton Drag

  • Dragonfly Hovering

  • Titan’s Dragonfly

    Titan’s Dragonfly

    Last week, NASA announced its next New Frontiers mission: a nuclear-powered drone named Dragonfly heading to Titan. This astrobiology mission is set to search our solar system’s second largest moon for signs of life. It’s exciting aerodynamically, as well, since Titan’s thick atmosphere makes it uniquely suited for heavier-than-air flight. Therefore, rather than using wheeled rovers like we have on Mars, Dragonfly is a rotorcraft. It will be capable of traveling up to 8km per flight, which will quickly surpass the fewer than 21km the Curiosity Rover has managed on Mars! 

    Like Earth, Titan has rainfall and open liquid bodies on its surface. I, for one, can’t wait to see the alien vistas Dragonfly sends back as it cruises over methane lakes. (Image and video credit: NASA)

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