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Search results for: “waves”

  • Alligators Water Dancing

    Alligators Water Dancing

    Amorous alligators call to mates with a behavior known as water dancing. Their audible bellows are accompanied by infrasonic sound–vibrations below the 20 Hz limit of human hearing. These vibrations from their lungs excite Faraday waves in the water near the alligator’s back and make the surface explode in a dance of jets and atomized droplets. I’ve seen similar results in other instances of vibration, but this may be the only example of this I’ve seen in the wild. Researchers studying the phenomenon noted that the frequency of sound the alligators emit corresponds to a wavelength equal to the spacing of the raised scales, or scutes, on the alligators’ backs. They hypothesize that the shape of the scutes helps males create the display.  (Image credit: N. Marven, source; research credit: P. Moriarty and R. Holt; h/t to io9)

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    The Kelvin-Helmholtz Instability

    The Kelvin-Helmholtz instability is a pattern frequently found in nature. It has a distinctive shape, like a series of breaking ocean waves that curl over on themselves to create a string of vortices. The instability shows up when there is a velocity difference between two fluid layers. The unequal shear between the two layers magnifies any disturbance to their interface, which manifests in the fractal, overturning whorls seen in the numerical simulation above. You can find the Kelvin-Helmholtz instability in the lab, in the sky, in the ocean, on Jupiter and Mars–even on the sun! For more information on the methods used to create the simulation above, check out the full paper. (Video and research credit: K. Schaal et al.)

  • Recreating Hurricanes

    Recreating Hurricanes

    Hurricane-related winds and storm surge cause massive damage every year. Understanding and being able to predict the impact of these storms on coastal structures can help save lives and properties. Until recently the most ferocious of hurricanes–category 5 storms that feature winds above 250 kph (150 mph)–could not be recreated in a laboratory scale. Now the University of Miami’s SUSTAIN (SUrge-STructure-Atmosphere INteraction) facility can produce category-5 equivalent winds, waves, and surge in a controlled environment. The massive test section measures 18 m x 6 m x 2 m and can be filled with over 140,000 liters of saltwater. The acrylic walls of the facility let researchers use optical flow diagnostics like particle image velocimetry (PIV) to measure flow anywhere in the test section. Some of their planned studies include experiments on how oil spills behave in storms and how strong aquaculture nets must be to maintain their catch through a storm. It will also be used to study interactions between buildings and storm surge. For more, check out their website or this video from the Weather Channel. (Image credits: Gort Photography, AFP/K. Sheridan, AP Photo/W. Lee; SUSTAIN Laboratory)

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    Underwater Explosions

    As dangerous as explosions are in air, they are even more destructive in water. Because air is a compressible fluid, some part of an explosion’s energy is directed into air compression. Water, on the other hand, is incompressible, which makes it an excellent conductor of shock waves. In the video above we see some simple underwater explosions using water bottles filled with dry ice or liquid nitrogen. The explosions pulsate after detonation due to the interplay between the expanding gases and the surrounding water. When the gases expand too quickly, the water pressure is able to compress the gases back down. When the water pushes too far, the gases re-expand and the cycle repeats until the explosion’s energy is expended. This pulsating change in pressure is part of what makes underwater explosions so dangerous, especially to humans. Note in the video how the balloons ripple and distort due to the changing pressure. Those same changes in pressure can cause major internal damage to people. (Video credit: The Backyard Scientist; submitted by logicalamaze)

  • Jumps in Stratified Flows

    Jumps in Stratified Flows

    One of the factors that complicates geophysical flows is that both the atmosphere and the ocean are stratified fluids with many stacked layers of differing densities. These variations in density can generate instabilities, trap rising or sinking fluids, and transmit waves. The animations above show flow over two ridges with dye visualization (top), velocity (middle), and contours of density (bottom). The upstream influence of the left ridge creates a smooth, focused flow that quickly becomes turbulent after the crest. The jet rebounds as a turbulent hydraulic jump before slowing again upstream of the second ridge. Like the first ridge, the second ridge also generates a hydraulic jump on the lee side. Clearly both stratification and the local topography play a big role in how air moves over and between the ridges. If prevailing winds favor these kinds of flows, it can help generate local microclimates. (Image credit and submission: K. Winters, source videos)

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    How Loud Can Sound Get?

    Sound and acoustics often intersect with fluid dynamics. Most of the sounds we experience are pressure waves traveling through air. In this video, Joe of It’s Okay To Be Smart takes a closer look at sound: what it is; how we measure it; and just how loud a sound can get. For air at sea level, the loudest possible sound is 194 dB. Add any more energy and it distorts the pressure wave from what we recognize as sound into what’s known as a shock wave. (Video credit: It’s Okay To Be Smart/PBS Digital Studios)

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    Extinguishing Fires With Sound

    Engineering students from George Mason University have built a fire extinguisher that uses sound to put out flames. Since sound waves are mechanical pressure waves, they can move the air surrounding a burning material. Through trial and error the students found the high-frequency sound had little effect, but at frequencies between 30-60 Hz the sound waves could jostle enough oxygen away from the flame to extinguish the fire. They’re hoping the solution is scalable and can be applied to larger fires. For other wild ideas for chemical-less fire extinguishers, check out how researchers put out fires with explosions.  (Video credit: George Mason University; submitted by @isanaht)

  • The Dance of the Droplets

    The Dance of the Droplets

    Milk and juice vibrating on a speaker can put on a veritable fireworks display of fluid dynamics. Vibrating a fluid can cause small standing waves, called Faraday waves, on the surface of the fluid. Add more energy and the instabilities grow nonlinearly, quickly leading to tiny ligaments and jets of liquid shooting upward. With sufficiently high energy, the jets shoot beyond the point where surface tension can hold the liquid together, resulting in a spray of droplets. (Image credit: vurt runner, source video; h/t to @jchawner)

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    Dead Water

    Sailors have long known about the “dead water” phenomenon, which can bring ships to a near-standstill, but it was only within the last century that an explanation for the behavior was found. The underlying cause is a stratification of fluids of different densities. As seen in the video above, when a boat moves by exerting a constant force, such as with propellers, it generates an internal wave along the interface between two density layers in the water. As the wave grows in amplitude, it speeds up, chasing and eventually breaking against the boat. The energy that drives the internal wave’s growth comes from the energy the boat expends for propulsion; the larger and closer the wave gets, the slower the boat goes because its energy is sapped by the wave. In the ocean, particularly near sources of freshwater run-off, like melting glaciers, the water can be extremely stratified, with many layers of different salinity and density. The end of the video simulates this with a three-fluid demonstration in which the boat’s motion generates internal waves across multiple density interfaces. (Video credit: M. Mercier et al.)

  • Rowing Water Striders

    Rowing Water Striders

    Water strider insects are light enough that their weight can be supported by surface tension. For some time, they were thought to propel themselves by using their long middle legs to generate capillary waves–ripples– that pushed them forward, but juvenile water striders are too small for this technique to work. Instead researchers found that water striders move by using their middle legs like oars. The leg motion creates vortices about 4 mm below the water surface, and this water moving backward propels the insect forward. In the photos above, the scientists visualized the flow by sprinkling thymol blue on the water and letting the striders move freely. You can learn more about the work here or in this Science Friday episode. (Photo credits: J. Bush et al.)

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