FYFD

Month: August 2015

  • Shock Waves in Flight

    Shock Waves in Flight

    Schlieren optical systems have been used to visualize shock waves in labs for more than a century, but the technique did not translate well to photographing shock structures outside the lab. But now NASA’s Armstrong Research Center and Ames Research Center have developed a method that allows them to capture highly-detailed images of the shock waves around airplanes while they are flying. This is incredible stuff. Be sure to check out the high-resolution versions on this page, along with more description of the coordination necessary to pull off the photos.

    The light and dark lines you see emanating from the airplane are places with strong density gradients. The dark lines are mostly shock waves, with the strongest shock waves appearing black due to the large change in air density. Many of the light streaks are expansion fans, areas where the density and pressure drop as air speeds up.

    The goal of this research is to better understand shock wave structures around supersonic planes in order to reduce the noise supersonic aircraft cause when flying overhead. As you can see in the photos, the shock waves at the nose and tail of the aircraft persist far away from the aircraft; these are what cause the twin sonic boom heard when the plane flies by. (Photo credit: NASA; via J. Hertzberg)

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    “The Chase”

    Sometimes it takes timelapse photography to truly appreciate the dynamic behavior of our atmosphere. In “The Chase” Mike Olbinski, whose work we’ve featured previously, has captured some of the most incredible and stunning weather timelapse footage I have ever seen. Despite watching it repeatedly, I continue to be awed to the point that I have no words. Seriously, just watch it. Be amazed by the drama of our sky. (Video credit: M. Olbinski)

  • Vapor Cones

    Vapor Cones

    Vapor cones typically appear around aircraft flying in the transonic regime–near, but still below, the speed of sound. Air moving over the vehicle accelerates and decelerates as it moves around different parts of the plane; if it didn’t, the plane couldn’t generate lift and wouldn’t fly. When the local flow accelerates past the speed of sound, the accompanying drop in pressure and temperature can be enough to for conditions to fall below the dew point, causing the condensation we see. At the back of the airplane, a shock wave decelerates the airflow back to subsonic speeds and raises local conditions back above the dew point, thereby truncating the cone. (Image credit: C. Caine)

  • Fire Tornadoes

    Fire tornadoes, despite their name, are more closely related to dust devils or waterspouts than to true tornadoes. Though rarely documented, they are relatively common, especially in wildfires. The heat of the fire creates an updraft of warm, rising air that leaves behind a low-pressure region. Air from outside is drawn toward this low-pressure area, gets heated, and rises. As the outside air gets pulled in, any vorticity or rotation it had gets intensified via conservation of angular momentum–the same way a spinning ice skater speeds up when she pulls her arms in. The result is the tightly-spinning vortex at the heart of a fire tornado. (Video credit: C. Fleur; via NatGeo)

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    Bubbles and Hurricanes

    You may think of soap bubbles as a childhood plaything, but there’s a lot to be learned from them. In her newest video, Dianna of Physics Girl explores some of the fascinating research scientists use soap bubbles for and how you can recreate some of their experiments at home. Scientists have used bubbles to explore how atmospheric vortices behave, how to tie knots in fluids, how grass waves in the wind, and even how explosive detonations occur. Just modeling bubbles and foams can be incredibly complex. It turns out the humble bubble has quite a lot to teach us. (Video credit: Physics Girl/PBS Digital Studios)

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    Turbulent Ink

    Turbulence is found throughout our lives, but rarely is it as startlingly beautiful as in this Slow Mo Guys video. Here they show high-speed videos of ink being injected into water. The resulting plumes are turbulent from the very start, with innumerable folds and eddies billowing outward as the plume expands. The large difference in length scales–from the millimeter-sized curls to the meter-sized length of the plume–is one of the classic characteristics of turbulence and part of what makes turbulent flows so difficult to model computationally. Energy in these flows is generated at the large scales, but it’s dissipated at the very smallest scales through viscosity. This means that to properly model a turbulent flow, you have to capture the largest scales, the smallest scales, and everything in between in order to represent this energy cascade from large to small. It’s a problem that engineers, mathematicians, meteorologists, and physicists have struggled with for more than a century. But, here, at least, we can all just sit back and enjoy the beauty. (Video credit: The Slow Mo Guys)

  • Controlling Droplet Bounce

    Controlling Droplet Bounce

    Water repellent, or hydrophobic, surfaces are common in nature, including lotus leaves, many insects, and even some geckos. These hydrophobic surfaces typically gain their water-repelling ability from extremely tiny nanoscale structures in the form of tiny hairs or specially textured surfaces. But, while the nanoscale structures impart superhydrophobicity, researchers have found that larger macroscale structures can improve water-repellent characteristics by reducing a drop’s time of contact with the surface. A smaller contact time means less chance of contamination on self-cleaning surfaces. It’s also helpful in preventing water from freezing on contact to cold surfaces – valuable, for example, in protecting airplane wings’ leading edges from icing over. This combination of nanoscale and macroscale, water-repelling structures can be found in nature, too, such as on the wings of butterflies, which must quickly shed water in order to fly. (Image credits: K. Hounsell et al.; A. Gauthier et al., source video)

  • Flow Around a Delta Wing

    Flow Around a Delta Wing

    Colorful streaks of dye wrap like ribbons along the leading edge of a delta wing. At an angle of attack, this triangular wing forms a set of vortices that run along its edge, providing much of the low pressure–and therefore lift–on the upper surface of the wing. In contrast, the red streaks of dye in the middle of the wing demonstrate clean, laminar flow. Highly swept delta wings are popular for aircraft traveling at supersonic speeds, but they can also work well subsonically, as shown here. For more incredible and beautiful examples of flow visualizations by Henri Werlé, check out his 1974 film Courants et couleurs. (Photo credit: H. Werlé; via eFluids)

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    Leaping Mobulas

    Mobula rays engage in some pretty incredible aerial acrobatics. This species of ray, second only to manta rays in size, can jump up to 2 meters into the air. Large groups of mobula rays will engage in this behavior, including both males and females, but it remains unclear to scientists exactly what purpose the jumping serves. It may be a form of communication, which might explain the rays’ apparent preference for belly flopping. By striking the water surface with as much of their body as possible simultaneously, the rays generate both a large splash and a concussive clap that carries through the water. (Video credit: BBC; via J. Hertzberg)

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    Tides

    Most of us think we understand why Earth’s oceans have tides, but it turns out that there are some misconceptions in the common explanation. Yes, it’s true that the moon’s gravity pulls on water in the ocean, but it equally pulls on everything else, too, and we don’t levitate at high tide! In reality, it’s the distribution of tidal forces across the enormity of the ocean that causes the ocean to bulge along the Earth-moon line and create high and low tides. Lakes, puddles, and humans experience tides, too, but we’re so small that the tidal forces we experience are too tiny to be noticeable. For the full explanation, I encourage you to watch PBS Space Time’s video. Don’t let the 15 minute run-time deter you; the tidal explanation is contained within the first 9 minutes. (Video credit: PBS Space Time; via It’s Okay To Be Smart)