FYFD

Month: June 2017

  • How Cycling Position Affects Aerodynamics

    How Cycling Position Affects Aerodynamics

    New FYFD video! How much does a rider’s position on the bike affect the drag they experience? To find out I teamed up with folks from the University of Colorado at Boulder and at SimScale to explore this topic using high-speed video, flow visualization, and computational fluid dynamics. 

    Check out the full video below, and if you need some more cycling science before the Tour de France gets rolling, you can find some of my previous cycling-related posts here. (Image and video credit: N. Sharp; CFD simulation – A. Arafat)

    ETA: Please note that the video contained in this post was sponsored by SimScale.

  • Watching Radiation

    Watching Radiation

    We’re used to radiation being invisible. With a Geiger counter, it gets turned into audible clicks. What you see above, though, is radiation’s effects made visible in a cloud chamber. In the center hangs a chunk of radioactive uranium, spitting out alpha and beta particles. The chamber also has a reservoir of alcohol and a floor cooled to -40 degrees Celsius. This generates a supersaturated cloud of alcohol vapor. When the uranium spits out a particle, it zips through the vapor, colliding with atoms and ionizing them. Those now-charged ions serve as nuclei for the vapor, which condenses into droplets that reveal the path of the particle. The characteristics of the trails are distinct to the type of decay particle that created them. In fact, both the positron and muon were first discovered in cloud chambers! (Image credit: Cloudylabs, source)

  • Glacial Remains

    Glacial Remains

    The high walls of this alpine canyon were cut by flowing glacial ice. This type of amphitheater-shaped valley is known as a cirque. The photo shows one of the Chicago Lakes on Mount Evans in the Colorado Rockies. The glacier that once sat here carved the steep walls you see in the background but also hollowed out a series of depressions like the ones shown in the figure below. When temperatures warmed and the glacier melted, it left behind a series of three small lakes, or tarns, like the one in the photo above. Cirques are found throughout the mountain ranges of the world. (Image credit: Mt. Evans – J. Shoer; cirque formation – DooFi)

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  • Tendrils of Fog

    Tendrils of Fog

    Fog snakes its way from the ocean into the Strait of Juan de Fuca in this animation constructed from satellite imagery. The strait lies between Vancouver Island and the Olympic Peninsula in the Pacific Northwest. Fogs like this form when skies are clearer and heat from the surface is able to escape upward. The surface air then cools and condenses into fog. Steady winds pushed fog into the strait over the course of about 9 hours. There’s a remarkable level of detail in the satellite images, taken by the new GOES-16 satellite that launched in late 2016. Notice the ragged wave front as the fog stretches eastward and the shock-wave-like lines behind it in the strait. Both result from interactions between the fog cloud and the shape of the land masses it’s encountered. (Image credit: NASA Earth Observatory)

  • Cavitating

    Cavitating

    Cavitation happens when the local pressure in a liquid drops below its vapor pressure. A low-pressure bubble forms, typically very briefly, when this occurs. These bubbles are spherical unless they form near a surface. In that case, the bubbles take on a flatter, oblong shape. As they collapse, the bubbles form a jet, like the one seen inside the bubble above. The jet extends through the bubble and stretches into a funnel shaped protrusion on the bubble’s far side. Eventually, the whole shape becomes unstable and breaks into many smaller bubbles. Shock waves can be generated in the collapse, too; often the jet generates at least two in addition to the ones created when the bubble reaches its minimum size. This is part of why cavitation can be so destructive near a surface. (Image credit: L. Crum)

  • Featured Video Play Icon

    “Galaxy Gates”

    Viewing fluids through a macro lens makes for an incredible playground. In “Galaxy Gates”, Thomas Blanchard and the artists of Oilhack explore a colorful and dynamic landscape of paint, oil, and glitter. The nucleation of holes and the breakdown of sheets to filaments and droplets plays a major role in the visuals. The surface layer is constantly peeling away to reveal what’s going on underneath. In many cases this initial motion settles into a field of oil-rimmed droplets floating like planets against a colorful galactic backdrop. Watch carefully in the second half of the video, and you can even catch a few instances of a stretched ligament of fluid breaking into a string of satellite drops, like at 1:51. Check out some of Blanchard’s previous work here and here. (Video credit: Oilhack and T. Blanchard; GIFs and h/t to Colossal)

     
  • Flow in a Turbine

    Flow in a Turbine

    Fluid flows are complex, complicated, and ever-changing. Researchers use many techniques to visualize parts of a flow, which can help make what’s happening clearer. One technique, shown above, uses oil and dye to visualize flow at the surface. The vertical, black, airfoil-shaped pieces are stators, stationary parts within a turbine that help direct flow. After painting the stator mount surface with a uniform layer of oil, the model can be placed in a wind tunnel (or turbine) and exposed to flow. Air moving around the stators drags some of the oil with it, creating the darker and lighter streaks seen here. Notice how the lines of oil turn sharply around the front of the stator and bunch up near its widest point. Those crowded flow lines tell researchers that the air moves quickly around this corner. (Image credit: D. Klaubert et al., source)

  • Venturi Splashes

    Venturi Splashes

    Diving can generate some remarkable splashes. Here researchers explore the splashes from a wedge-shaped impactor. At high speeds, they found that the splash sheet pushed out by the wedge curls back on itself and accelerates sharply downward to “slap” the water surface (top). Studying the air flow around the splash sheet reveals some of the dynamics driving the slap (bottom). The splash sheet quickly develops a kink that grows as the sheet expands. This creates a constriction that accelerates flow on the underside of the sheet. That higher velocity flow means a low pressure inside the constriction, which pulls the thin sheet down rapidly, making it slap the surface. For more, check out the full video. (Image and research credit: T. Xiao et al., source)

  • Featured Video Play Icon

    Shadows of Flow

    In the latest Veritasium video, Derek demonstrates how to see gas motions that are normally invisible using a schlieren photography set-up. Schlieren techniques have been important in fluid dynamics for well over a century, and Derek’s set-up is one of the two most common ways to set up the technique. (The other method uses two collimating mirrors instead of a single spherical or parabolic one.) As explained in the video, the schlieren optical set-up is sensitive to small changes in the refractive index, making density changes or differences in a gas visible. This makes it possible to distinguish gases of different temperatures or compositions and even lets you see shock waves in supersonic flows. (Video and image credit: Veritasium; submitted by Paul)

  • The Surge in the Hourglass

    The Surge in the Hourglass

    When we watch sands running through an hourglass, we think their flow rate is constant. In other words, the same number of grains falls through the neck at the beginning and the end. In many practical granular flows, like those through industrial hoppers (left), this is not the case. Instead, emptying those containers involves a surge near the end where the discharge rate is higher.

    The surge is related to the interstitial fluid – the air, water, or other fluid in the space between the grains. On the right, you see an experiment in which brown grains submerged in green-dyed water are emptied. The dark layer is dyed water initially at the top of the grains. As the container drains, that dyed layer moves down more rapidly than the grains; this indicates that the interstitial fluid is actually being pumped by the draining of the grains. Researchers think this is an important factor affecting the final surge. (Image credits: hopper – T. Cizauskas; discharge graph – J. Koivisto and D. Durian, source; research credit: J. Koivisto and D. Durian; submitted by Marc A)