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Search results for: “waves”

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    Kelvin Wakes

    Whether you’re watching ducks cruise by on a pond or a boat making its way across the ocean, you’ve probably noticed a distinctive V-shaped wake. This shape is known as a Kelvin wake, and it forms because waves in water don’t all move at the same speed. Instead, the speed a wave travels at depends on its wavelength; smaller wavelengths travel slower than larger ones, a phenomenon known as dispersion. The characteristic shape of a Kelvin wake is the result of many waves of different wavelength (and therefore speed) added together. (Video and image credit: Minute Physics)

  • Acidic Sea Spray

    Acidic Sea Spray

    As waves crash and break, they generate a spray of droplets — known as aerosols — that make their way into the atmosphere. Researchers investigated the chemistry of these aerosol droplets by generating spray in a wave tank filled with ocean water. They found that aerosol droplets are far more acidic than the ocean they come from, and the smaller the droplet, the more acidic it is. This acidification happens in a matter of minutes, as acidic gases interact with the spray. Their findings will be critical for accurately modeling the climate connections between our oceans and atmosphere. (Image credit: Elle; research credit: K. Angle et al.; via OceanBites; submitted by Kam-Yung Soh)

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    Coastal Erosion

    The same dynamic forces that make coastlines fascinating create perennial headaches for engineers trying to maintain coastlines against erosion. This Practical Engineering video discusses some of the challenges of coastal erosion and how engineers counter them.

    In a completely undeveloped coastline, waves and storms erode the shoreline while rivers and currents replenish sand through sedimentation. Manmade structures tend to strengthen erosion processes while disrupting the sedimentation that would normally counter it. Beach nourishment — where sand gets dredged up and deposited on a beach — is an engineered attempt to replace natural sedimentation.

    Dunes, mangrove forests, and wetlands are all nature’s way of protecting and maintaining coastlines. We engineers are still learning how to both utilize and protect shorelines. (Image and video credit: Practical Engineering)

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    Breaking Bubbles

    What do a nineteenth-century war ship, a sardine-hunting shark, and a viral bottle trick have in common? Cavitation! The phenomenon of cavitation occurs when a fluid is accelerated such that its local pressure drops below the vapor pressure. As a result, bubbles form and then violently collapse, creating shock waves that can damage nearby surfaces or stun prey. Dianna explains — and reveals some cool historical context that was new to me! — in the video above. (Image and video credit: Physics Girl)

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    Slow Mo Pulse Jet Engine

    Pulse jet engines rely on their shape to maintain combustion without moving parts. The pressure waves that travel through the engine pump fresh oxygen into the combustion chamber and then ignite it with exhaust remaining from the last cycle. In this Slow Mo Guys video, we get to see that process in action. It’s a pretty neat view of combustion in a working engine, but these guys are definitely not going to win any awards for safety measures. Seriously, don’t try this at home! (Image and video credit: The Slow Mo Guys)

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    Seismic Events Reveal Ocean Temperatures

    Decades ago, researchers proposed sending sound waves through the ocean to measure its temperature. Although the technique worked, it ran into noise pollution issues, but now it’s back, using naturally-occurring seismic events as the sound source.

    When fault lines shift, they generate seismic waves that travel through the ocean as sound. When they reach a land mass, the waves get converted back into seismic energy that’s then picked up by a receiver. Knowing the distance from the source to the receiver and the time necessary for the wave to travel, scientists can then determine the average temperature of the water based on the speed of sound.

    The technique can track temperature changes down to thousandths of a degree. Based on more than a decade of seismic data from the Indian Ocean, researchers found almost double the temperature increase measured by a different sensor network. (Image and video credit: Science; research credit: W. Wu et al.; submitted by Kam-Yung Soh)

  • Jets Beneath Leidenfrost Drops

    Jets Beneath Leidenfrost Drops

    When a droplet impacts, it’s not unusual for converging ripples to form an upward jet, like the one seen here. But under the right circumstances, jets can form downward, too. This study looks at the ultrafast jets that can form beneath an impacting Leidenfrost drop.

    These Leidenfrost drops are striking a surface much hotter than their boiling point, so a large vapor cavity forms quickly beneath them. Using x-ray imaging, the researchers were able to capture the dynamics of this cavity’s formation and collapse (Image 2). The field of view in the animation shows only a portion of the drop’s cavity, so Image 3 may help you orient relative to the drop at large.

    Initially, we see the center of the droplet hitting the surface, followed by the fast growth of a vapor cavity. Rippling capillary waves converge on top of the cavity, creating a pinch-off. From there, a bubble rises up while a fast jet shoots downward. (Image credit: water jet – A. Min, others – S. Lee et al.; research credit: S. Lee et al.)

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    “The Unseen Sea”

    San Francisco’s picturesque fogs form “The Unseen Sea” in Simon Christen’s timelapse. Viewed at the right speed, the motion of clouds becomes remarkably ocean-like, with standing waves and surges against the hillside like waves crashing on a beach. Clouds in air don’t have the same surface tension effects as water waves in air, but, for the most part, the physics of their motion is the same, which is why they look so alike. (Image and video credit: S. Christen)

  • Dead Water

    Dead Water

    In the days before motorized propulsion, sailors would sometimes find themselves slowed nearly to a stop by what they called ‘dead water‘. As discovered in laboratory experiments over a century ago by Vagn Walfrid Ekman, the dead water phenomenon occurs where a layer of fresh water exists over saltier water. The ship’s motion generates internal waves in the salty layer, which in turn causes substantial additional drag on the boat. In a related phenomenon, named for Ekman, the internal waves generated by a boat’s initial acceleration cause its speed to fluctuate.

    While these phenomena have little effect on today’s shipping, they can be relevant for swimmers in areas like harbors and fjords where fresh water meets the sea. And their effects were undoubtedly substantial for much of history. There is even speculation that dead water might have caused the defeat of Mark Antony and Cleopatra’s superior navy at the hands of Octavian’s smaller ships in the Battle of Actium. (Image credit: M. Blum; research credit: J. Fourdrinoy et al.; via Hakai Magazine; submitted by Kam-Yung Soh)

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    Coalescing Drops

    This year’s Nikon Small World in Motion competition was won by fluid dynamics! The first place video shows droplets on a superhydrophobic surface coalescing. The droplets are a mixture of water and ethanol. Their initial merger creates a ripple of waves that’s followed by a ghostly vortex ring that jets into the interior. Previous research on coalescence during impact shows jets driven by surface tension but the jet here doesn’t appear to be confined to the surface. (Image and video credit: K. Rabbi and X. Yan; via Nature; submitted by Kam-Yung Soh)

    Droplets on a superhydrophobic surface coalescing.

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