FYFD

Category: Phenomena

  • Sochi 2014: Ski Jump

    Sochi 2014: Ski Jump

    Great ski jumpers are masters of aerodynamics. There are four main parts to a jump: the in-run, take-off, flight, and landing. An athlete’s aerodynamics are most vital in the in-run and, naturally, the flight. During the in-run, the athlete is trying to gain as much speed as possible, so she tucks down and pulls her arms behind her back to streamline her body and keep her frontal area as small as possible. This limits her drag so that she can maximize her speed at take-off. Once in the air, though, the jumpers act like gliders. In flight, there are three forces acting on the the jumper: gravity, lift, and drag. Gravity pulls the jumper down, and drag tends to push her backwards up the hill, but lift, by counteracting gravity, helps keep jumpers aloft for a greater distance. To maximize lift, a jumper angles her skis outward in a V and holds her arms out from her sides. This configuration turns the jumper’s body and skis into a wing. The best jumpers will tweak their positions with training jumps and wind tunnel time to maximize their lift while minimizing their drag in flight and on the in-run. Technique is critical in ski jumping, but conditions play a significant role as well. Tomorrow’s post will discuss why and how judges account for changing conditions. (Photo credits: L. Baron/Bongarts/Getty Images; D. Lovetsky/AP; E. Bolte/USA Today)

    FYFD is celebrating the Games with a look at fluid dynamics in the Winter Olympics. Check out our previous posts on the aerodynamics of speed skating, why ice is slippery and how lugers slide so fast.

  • Sochi 2014: Speedskating Suits

    Sochi 2014: Speedskating Suits

    Long track speed skating is a race against the clock. Skaters reach speeds of roughly 50 kph, so drag has a significant impact. This is why skaters stay bent and spend straightaways–their fastest segments on the ice–with their arms pulled behind them. It’s also why their speedsuits have hoods to cover their hair. This year the U.S. speed skaters are wearing special suits designed by Under Armour and Lockheed Martin especially for their aerodynamics. The suits feature a mixture of fabrics including raised surface features on the hood and forearms. These bumps are designed to trip turbulent flow in these regions. It seems counterintuitive, but drag is actually lower for a turbulent boundary layer than a laminar one at the right Reynolds number range. This is because turbulent boundary layers are better at staying attached to non-streamlined bodies. The longer flow stays attached to the skater, the smaller the pressure difference between the air in front of the skater and the air in his wake. The suit’s seams and even its hot-rod-like flames were placed with this effect in mind. Only time will tell whether the suits really give skaters a competitive edge, but since Sochi’s low-altitude increases drag on skaters, they will appreciate some extra speed. For more, NSF has an inside look at the suit’s development. (Photo credits: Under Armour)

    FYFD is exploring the fluid dynamics of the Winter Olympics. Check out previous posts on how lugers slide fast and why ice is slippery, and be sure to stay tuned for more!

  • Sochi 2014: Luge

    Sochi 2014: Luge

    Like athletes in many of the gravity sports in the Winter Olympics, lugers want to be as aerodynamic as possible to minimize their drag. Once a luger has started sliding, only gravity can increase their speed – every other force, from friction to drag, pulls away valuable time. Luge sleds are built on sharp runners and athletes slide feet-first in a position much more streamlined than the head-first position of skeleton. Both contribute to the much higher speeds in luge – up to 140 kph (87 mph). Luge is also the only sliding sport measured down to thousandths of a second, so every gram of drag* makes a difference. Lugers keep their heads pulled back and wear full helmets to keep the air flow consistent and attached as much as possible. It is also typical for them to spend time in the wind tunnel, testing their sled’s aerodynamics, adjusting their position, and even testing their suits. (Photo credit: S. Botterill)

    * For those wondering, yes, drag is a force and a gram is a unit of mass, not force. However, it is not unusual when testing athletes in wind tunnels to compare drag between configurations in terms of grams.

    FYFD is celebrating the Games with a series on fluid dynamics in the Winter Olympics. Stay tuned for more!

  • Sochi 2014 Incoming

    Sochi 2014 Incoming

    The Winter Olympics are underway in Sochi, Russia, and here at FYFD, I am busy preparing a special series of posts on fluid dynamics in the Winter Games. Look for the first of those starting on Monday.  In the meantime, you can check out some of FYFD’s previous themed series now compiled into a special archive. (Photo credit: B. Armangue)

  • Convective Impressionism

    Convective Impressionism

    Buoyant convection, driven by temperature-dependent changes in density, is a major force here on Earth. It’s responsible for mixing in the oceans, governs the shape of flames, and drives weather patterns. The images above show flow patterns caused by buoyant convection. The colors come from liquid crystal beads immersed in the fluid; red indicates cooler fluid and blue indicates warmer fluid. You can see plumes of warmer fluid rising in some of the photos. At the same time, though, the images are beautiful simply as art and are strongly reminiscent of works by Vincent van Gogh. (Image credit: J. Zhang et al.)

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    The Reynolds Experiment

    One of the most famous and enduring of all fluid dynamics experiments is Osborne Reynolds’ pipe flow experiment, first published in 1883 and recreated in the video above. At the time, it was understood that flows could be laminar or turbulent, though Reynolds’ terminology of direct or sinuous is somewhat more poetic:

    Again, the internal motion of water assumes one or other of two broadly distinguishable forms-either the elements of the fluid follow one another along lines of motion which lead in the most direct manner to their destination, or they eddy about in sinuous paths the most indirect possible. #

    There had, however, been no direct evidence of these eddies in a pipe. Reynolds built an apparatus that allowed him to control the velocity of flow through a clear pipe and simultaneously introduce a line of dye into the flow. He carefully varied the velocity and temperature (and thus viscosity) in his apparatus and not only documented both laminar and turbulent flow but found that the transition from one to another could be described by a dimensionless number he derived from the Navier-Stokes equation. This number was dependent on the fluid’s velocity and kinematic viscosity as well as the diameter of the pipe. This was the birth of the Reynolds number, one of the most important parameters in all of fluid dynamics. (Video credit: S. dos Santos; research credit: O. Reynolds)

  • Wind and Waves Visualized

    Wind and Waves Visualized

    Much like the wind map we featured previously, designer Cameron Beccario’s visualizations of wind and ocean surface current data draw from near-real-time sources to create a stunning picture of fluid dynamics on a planetary scale. The number of options in terms of projections and data are really quite incredible, and you’ll want to play around to get a real sense for it. Want to see the wind and total precipitable water at 1000 hPa? Here you go. Maybe you prefer studying Pacific ocean currents. All the data are there to play with. People often wonder why weather forecasts aren’t always right, but, when you look at the scale and complexity of these flows, it’s almost a wonder that we can predict them at all. (Image credits:C. Beccario/earth; via skunkbear and io9)

  • Protostellar Jets

    Protostellar Jets

    As young stars form, they often produce narrow high-speed jets from their poles. By astronomical standards, these fountains are dense, narrowly collimated, and quickly changing. The jets have been measured at velocities greater than 200 km/s and Mach numbers as high as 20. The animation above (which you should watch in its full and glorious resolution here) is a numerical simulation of a protostellar jet. Every few decades the source star releases a new pulse, which expands, cools, and becomes unstable as it travels away from the star. Models like these, combined with observations from telescopes like Hubble, help astronomers unravel how and why these jets form. (Image credit: J. Stone and M. Norman)

    ETA: As it happens, the APOD today is also about protostellar jets, so check that out for an image of the real thing. Thanks, jshoer!

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    Hydraulic Jump in the Lab

    When fast-moving liquids encounter regions of slow-moving liquids, they decelerate rapidly, trading their kinetic energy for potential energy and creating a hydraulic jump. Flow in the video above is from left to right. The depth difference between the incoming and outgoing water can be directly related to the velocity of the incoming fluid. Hydraulic jumps in rivers and spillways are often extremely turbulent, like the one in this video, but laminar examples exist as well. In fact, with the right height and flow rate, you can create stable hydraulic jumps right in your kitchen sink. The hydraulic jumps formed from a falling jet are typically circular, but with the right conditions, all sorts of wild shapes can be observed. (Video credit: H. Chanson)

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    Vibrations from Vortices

    Vortex shedding frequently happens in the wakes of non-streamlined bodies as a result of flow around the obstacle. Newton’s third law states that forces come in equal and opposite pairs, meaning that the vortex shedding behind an obstacle is accompanied by a force on the obstacle. For a fixed cylinder, this is not always apparent, but for a pendulum, like the ones demonstrated in this video, this vortex-induced vibration causes significant motion. This same effect can make traffic lights and industrial chimneys sway. You’ve likely experienced it yourself as well, if while swimming you’ve ever spread your fingers underwater and spun in place. Try it sometime with your arm out and you’ll feel the vortices make your arm vibrate up and down as you spin.  (Video credit: Harvard Natural Sciences Lecture Demonstrations)