FYFD

Tag: wakes

  • Hunting By Whisker

    Hunting By Whisker

    Seals and sea lions often hunt fish in waters too dark or turbid to rely on eyesight. Instead, they follow their whiskers, using the turbulence generated by a fish’s wake. The vortices shed by the fish cause the seal’s whiskers to vibrate, giving them sensory information. To better understand what a seal can derive from this, a recent experiment looked at what a thin whisker can pick up from an upstream cylinder.

    As expected, the strength of the whisker’s vibration fell off the farther away the cylinder was. But the researchers found that, if they moved the cylinder quickly — like a fish trying to dart away — the vibration of the whisker was stronger. They also found that the whisker was sensitive to misalignment. If the cylinder was placed ahead and to the side of the whisker, the whisker would still vibrate but would do so around a different equilibrium position. That result implies that a seal can get information both about the fish’s speed and direction, simply from the twitch of its whiskers. (Image credit: seal – K. Luke, illustration – P. Gong et al.; research credit: P. Gong et al.; via APS Physics)

    Illustration of a seal following a fish versus the experiment, a whisker following a cylinder's wake.
    Illustration of a seal following a fish versus the experiment, a whisker following a cylinder’s wake.
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    Walking in the Wake

    Flow visualization is an important tool in fluid dynamics, and scientists have many ways to capture and visualize flow information. But our methods are not the only — or even the best — ways to express a flow. Here, engineers teamed up with architects and artists to explore the flow behind an oscillating cylinder. When free to move forward-and-backward the cylinder’s wake takes on three distinctively forms. The team explored many ways to display the wakes — drawings, 3D-printed sculptures, and more — before ultimately building an art installation that lets visitors walk through the wake to experience it. I love the creativity of these interdisciplinary efforts. To see a similar, yet very different, take on the wake of a cylinder, check out this interpretative dance. (Image and video credit: P. Boersma et al.)

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    Vortex Arms

    A fixed cylinder will shed alternating vortices in its wake, but one allowed to oscillate forward and backward in the flow instead sheds simultaneous vortices. The shape of the wake still depends on the flow’s velocity. At low flow speeds, the two vortices are the same size when they shed. At higher velocities, the two vortices still shed simultaneously, but one will be large while the other is small. The larger vortex moves faster and travels downstream, but the smaller, slower vortex drifts inward. In the next shedding cycle, the small and large vortices switch positions, creating alternating symmetric shedding. (Image and video credit: P. Boersma et al.)

  • An Oasis Among Dunes

    An Oasis Among Dunes

    The Saudi Arabian oasis of Jubbah sits in the bed of an ancient lake. It’s protected from the westerly winds that sculpt the surrounding dunes by the wind shadow of the mountain Jabel Umm Sinman. The long, skinny shape of the settlement reveals the shape of the mountain’s wake! (Image credit: NASA; via NASA Earth Observatory)

  • Sensing Obstacles Through Flow

    Sensing Obstacles Through Flow

    Mosquitoes, bats, and even eels use non-visual means to sense their environments. For mosquitoes, part of their obstacle avoidance comes from the exquisite sensitivity of their antennae, which are able to sense subtle changes in the air flow around them as they approach a wall or the ground. Researchers used this same technique to help a quadcopter avoid crashing by adding air pressure sensors that respond to the changes in the copter’s wake as it approaches the ground. (Image and research credit: T. Nakata et al.; via Science)

  • Dunes Avoid Collisions

    Dunes Avoid Collisions

    The speed at which a dune migrates depends on its size; smaller dunes move faster than larger ones. That speed differential implies that small dunes should frequently collide into and merge with larger dunes, eventually forming one giant dune rather than a field of smaller separate ones. But that’s not what we observe in nature.

    To figure out why dunes aren’t colliding that often, researchers built a dune field of their own in the form of a rotating water tank. Inside the tank, their two artificial dunes can chase one another indefinitely while the researchers observe their interactions. What they found is that the dunes “communicate” with one another through the flow.

    As flow moves over the upstream dune, it generates turbulence in its wake, which the downstream dune then encounters. All that extra turbulence affects how sediment is picked up and transported for the downstream dune, ultimately changing its migration speed. For two dunes of initially equal size and close spacing, these interactions push the downstream dune further away until the separation between the dunes is large enough that they both migrate at the same speed. Even between dunes of unequal sizes, the researchers found that these repulsive interactions force the dunes away from collision and into migration at the same speed. (Image credit: dune field – G. Montani, others – K. Bacik et al.; research credit: K. Bacik et al.; via Cosmos; submitted by Kam-Yung Soh)

  • The Microscopic Ocean

    The Microscopic Ocean

    When you’re the size of plankton, water may as well be molasses. Viscosity rules at these scales, and swimming plankton leave distinctive wakes that are slow to dissipate. Fish that feed on plankton use these trails to find their prey. But this microscopic world is changing as the ocean warms.

    At higher temperatures, water is less viscous, and plankton wakes don’t last as long. To make matters worse for hungry fish, warmer waters have led to an explosion in a species of faster plankton, capable of moving hundreds of body lengths a second. This species is far more difficult to catch, which may explain some of the collapses we’re observing in populations of fish like cod and haddock. (Video and image credit: BBC Earth Lab)