FYFD

Category: Phenomena

  • Cycling Skinsuits and Vortex Generators

    Cycling Skinsuits and Vortex Generators

    It didn’t take long for an aerodynamic controversy to crop up in this year’s Tour de France. At the 14km individual time trial, riders from Team Sky wore custom Castelli skinsuits with integrated dot-like patterns on their upper arms (shown above). By the next day, a sports scientist with a competing team cried foul play, claiming that these fabrics could have given Team Sky as much as 25 seconds’ advantage over other riders. The Sky team finished with 4 out of the top 10 places on the time trial, and their leader, three-time Tour winner Chris Froome, finished some 35 seconds ahead of his expected competitors for the yellow jersey.

    Vortex generators explained

    So how could a few dots make a measurable difference? These protrusions are vortex generators meant to modify flow around a cyclist. Humans are not aerodynamic and what typically happens when air flows over a cyclist’s arms is shown in the flow visualization above: the air follows the curve of the arm part way, then it separates from the body, leaving a region of recirculation that increases drag. Vortex generators can help prevent or delay that drag-inducing flow separation by adding extra energy and turbulence to the air near the arm’s surface. Because turbulent boundary layers can follow a curve longer before separating, this helps reduce the drag by reducing the recirculation zone.

    About that time savings

    Aerodynamically speaking, those vortex generators can make a difference, but the question is, how much? In his complaint, Grappe cites a 2016 paper by L. Brownlie et al. that wind-tunnel tested different vortex generator patterns for use in running apparel. The speeds tested included those relevant to cycling. The specific numbers Grappe quotes aren’t directly relevant, however:

    As noted above, race garments that contain VG provide reductions in Fd of between 3.7 and 6.8% compared to equivalent
    advanced race apparel developed for the 2012 London Olympics which in turn provided substantially lower drag than
    conventional race apparel.

    the effectiveness of 5, 10 and 15 cm wide strips of VG applied to each flank of a sleeveless singlet revealed that the 5 cm wide
    strips provided between 3.1 and 7.1% less Fd than the 10 cm wide strips and between 1.9 and 4.3% less Fd than the 15 cm wide
    strips.  

    Here Brownlie et al. are specifically describing the savings for running apparel, which uses vortex generators in very different places than you would on a cyclist. Note the second quote even refers to a sleeveless singlet, so the vortex generators measured are definitely not in the same place as these skinsuits!

    The bottom line

    I fully expect that vortex generators give a marginal aerodynamic edge, which is why Sky and other teams have already been using them in competition. But I hesitate to declare that the savings is as high as 5-7%, and I have no way to verify Grappe’s subsequent claims that this translates to 18-25 seconds in the time trial. Those are numbers he gives without citing what model is being used to translate drag gains into time.

    In the end, what is needed is clarification of the rules. As they stand, one rule seems to allow the skinsuits because the vortex generators are integrated into the fabric, whereas another states clothing is forbidden “to influence the performances of a rider such as reducing air resistance”. Those two stances seems contradictory, and, for now, the race officials’ verdict to allow the suits stands.

    If you want to learn more about aerodynamics and cycling, be sure to check out my latest FYFD video. (Image credits: B. Tessier/Reuters; Getty Images; L. Brownlie et al. 2009; h/t to W. Küper)

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    When Walls Chirp

    If you’ve ever clapped near a wall with a corrugated surface, you may have noticed some strange echoes. Surfaces like these can cause a chirping sound to observers. The reason, as Nick Moore explains in the video above, is that the original sound reflects off the corrugations at different times and travels back to the observer such that the first reflections to arrive are closely spaced (and thus higher pitched) while the later reflections are spread further out. This creates a chirp that starts at a high pitch and then falls to lower ones. Have you ever come across structures that do this? (Video credit: N. Moore)

  • 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)

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    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)

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    Perijove

    The Juno spacecraft continues to send back incredible photos of Jupiter’s atmosphere. This video animates images from the sixth close pass of Jupiter to give you a sense of what Juno sees as it swoops by our system’s largest planet. The trajectory passes from the north pole to the south, showing Jupiter’s whitish zones, dark belts, and massive storms. Up close Jupiter looks like an Impressionist painting, all vortices and shear instabilities. The large white spots you see are enormous counterclockwise rotating vortices known as anticyclones – many of them larger than our entire planet. (Video credit: NASA / SwRI / MSSS / G. Eichstädt / S. Doran)

  • Capillary Action in Microgravity

    Capillary Action in Microgravity

    On Earth, gravity dominates over many fluid effects, but in microgravity a different picture emerges. This animation shows a two-channel apparatus partially filled with silicone oil being dropped. While in free-fall, the liquid experiences microgravity conditions and the height of the fluid in the two connected channels changes. The oil meniscus climbs up the walls of the tubes thanks to capillary action. This is the result of intermolecular forces between the liquid and solid walls. Capillary action is most effective in narrow tubes where surface tension and the adhesion between the liquid and solid can actually propel liquid up the walls, as seen here. On Earth we mostly ignore capillary action, except in very small spaces, but for space systems, it is a major force to reckon with in designing flows. (Image credit: NASA Glenn Research Center, source)

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    Building Labs on a Chip

    In their second video on microfluidics, the Lutetium Project takes viewers inside the process of creating microfluidic circuits, also known as labs-on-a-chip. When you want to build pipes only a few microns across, you need to use special techniques. As the video shows, manufacturing starts with photolithography, a process used to selectively mask parts of the substrate which are then etched away chemically. This creates a mold that’s later covered in a polymer solution. Once hardened, the polymer is removed from the mold, treated and attached to a glass slide. The result is a tiny fluid circuit that’s only a few square centimeters in total size. To really appreciate the process, check out the video, which helpfully takes you inside the clean room to see the chip manufacturing process firsthand. (Video and image credit: The Lutetium Project)