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    London 2012: Running Aerodynamics

    Running is not an event typically associated with aerodynamics, though any runner will tell you that a headwind can slow them down.  For comparison, a swimmer on world record pace sees 40 to 50 times the drag force of a runner over the same distance. But despite the relatively small influence of drag on a runner, there are measurable effects due to wind and altitude when races are judged by hundredths of a second. Given this, it comes as no surprise that researchers (and presumably manufacturers) are starting to considering how to optimize aerodynamics in running. The video above describes results of a study on running shoes that suggests modest savings may be derived from shoes with dimpled surfaces, much like a golf ball. Socks, on the other hand, don’t show any aerodynamic savings from special surfaces. Of course, the bulk of a runner’s drag comes from their hair and clothing; this is, in part, why runners wear form fitting clothes. While there may be some aerodynamic savings to be had, I don’t think we’ll see world records falling like crazy in Rio because of the latest new shoes.

    FYFD is celebrating the Olympics by featuring the fluid dynamics of sport. Check out our previous posts on how the Olympic torch works, what makes a pool fast, the aerodynamics of archery, the science of badminton, how cyclists get “aero”, and how divers reduce splash.

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    London 2012: Archery Physics

    Archery is one of the oldest Olympic sports, but the physics involved are remarkably complex. Even looking only at the flight of the arrow, the problem is hardly simple. The heavy point of the arrow makes it front-heavy, and the fletches on the back of the arrow provide additional surface area on which air can act. This means that the center of mass of the arrow–where gravity acts–is further forward than the center of pressure–where aerodynamic forces act.  This results in the aerodynamic forces helping to stabilize the flight of the arrow.  To see why this is important, try throwing a dart fletching first!

    When an arrow is fired from a bow, as in the high speed video above, the sudden impetus of force from the bowstring causes the arrow to flex and vibrate as it is fired. The aerodynamic forces generated by the fletches straighten the arrow’s flight, helping it reach the intended target accurately.  Some fletching is designed to make the arrow spin; this can further improve accuracy but comes at the cost of speed since some of the arrow’s initial kinetic energy must be converted to rotation.  For more, check out Archery Report, which features some great articles on the physics of archery and even has CFD comparing arrow tips. Mark Leach also has some great information on tuning a bow, which, if done properly, allows one to accurately shoot unfletched arrows.

    FYFD is celebrating the Olympics by looking at the fluid dynamics of sports. Check out our previous posts on how the Olympic torch works and what makes a pool fast.

  • The Olympic Torch

    [original media no longer available]

    Today marks the beginning of the 2012 Olympic Games in London. In the opening ceremony, the Olympic flame will complete its journey from Olympia to London, having been carried by some 8,000 torch bearers. Modern Olympic torches are expected to withstand wind, rain, snow, and human error to keep the flame alive and are specially designed and tested for these conditions. Each individual torch is fueled by a mixture of propane and butane stored as a pressurized liquid. The liquid fuel travels through a series of evaporation coils around the burner before combustion. Each torch carries sufficient fuel to burn about fourteen minutes. In addition to computer simulation, the 2012 Olympic torch design was tested in BMW’s Environmental Wind Tunnel to ensure a visible, stable flame for orientations within 45 degrees of vertical in conditions ranging from -5 degrees to 40 degrees Celsius, rain, snow, 35 mph winds, and 50 mph wind gusts. For more on the current torch and previous designs, see How Stuff Works, E&T, and the BBC.

    FYFD is celebrating the Olympics by featuring the role of fluid dynamics in sports starting Monday. If you have any burning questions, feel free to ask or email!

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    Oil in Alcohol

    In this video two droplets of oil fall through a bath of isopropyl alcohol. The oil is denser than alcohol, and the two fluids are miscible. The velocity and density gradients where the two fluids meet generate hydrodynamic instabilities that create the distinctive patterns seen in the falling drops. (Video credit: BYU Splash Lab)

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    Mussels

    In this video, schlieren imaging is used to make visible the flow field around a mussel.  Mussels are filter-feeders, drawing nearby water in to obtain their food and expelling the unneeded fluid once they’ve gathered the plankton they eat. Normally this process is invisible to the naked eye, but schlieren imaging reveals changes in density (and thus refractive index) that make it possible to visualize the outflow from the mussel. The technique is also commonly used in supersonic flows to reveal shock waves. (Video credit: Stephen Allen)

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    Sally Ride

    Today FYFD takes a brief aside from fluid dynamics to mark the passing of Sally Ride, the first U.S. woman to travel to space. A physicist by training, Ride served as a mission specialist on STS-7 and STS-41G, shuttle missions that included deploying satellites as well as conducting scientific experiments.  After her career with NASA, Ride returned to physics as a faculty member at the University of California, San Diego and dedicated herself to motivating children and young adults–most especially women–to pursue careers in science, math, and engineering.  She was an inspiration and role model to more than a generation; her courage and her passion for science touched many lives, including my own.  Godspeed, Dr. Ride.

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    How Maple Seeds Fly

    Maple tree seeds flutter and spin as they descend. The above video, which shows flow visualization of a freely falling seed, demonstrates that the so-called helicopter seed’s autorotation creates a vortex along the leading edge.  Watch as the seed’s “wing” sweeps through and you will notice the vortex along the upper surface. This leading edge vortex generates high lift on the maple seed, allowing it to stay in the air more effectively than other seeds, thereby increasing the maple’s reproductive range. (Video credit: D. Lentink et al.; see also Supplemental Materials)

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    Turing Patterns

    Turing patterns form as a result of a particular kind of chemical reaction: a reaction-diffusion system. It consists of an activator chemical capable of making more of itself, and an inhibitor chemical which slows the production of the activator as well as a mechanism for diffusing the chemicals. Although Turing’s original work was theoretical in nature, scientists have since proven that Turing patterns do occur in nature, both in petri dishes and in the markings of animals. Here artist Jonathan McCabe explores multi-scale Turing patterns in a fluid-like environment. (Video credit: Jonathan McCabe and Jason Forrest; submitted by Stuart R)

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    Vibrating Mercury

    A drop of mercury on a vibrating teflon surface assumes various mode shapes as the amplitude and frequency of oscillation are changed. Note the geometry and symmetry of the mode shapes. Near the end of the movie, the mercury oscillates chaotically and all symmetry and pattern is broken. (Because mercury is toxic, do not try this experiment at home.)

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    Liquid Pearls

    Researchers create liquid pearls–a liquid droplet surrounded by a gel-like exterior–by dropping the fluid through a special bath. The initial droplet contains a mixture of the liquid core and an alginate solution. When the drop falls through a bath containing calcium ions, the alginate turns into a hydrogel shell around the liquid core. In order to prevent mixing during the droplet impact, researchers use a surfactant that helps the thin alginate layer persist while gelling takes place. The resulting liquid pearl is permeable to chemicals; researchers hope this may allow them to be used to contain microorganisms or cells in a three-dimensional environment during testing. (Video credit: New Scientist, N. Bremond et al.; see also Gallery of Fluid Motion)