FYFD

Tag: drag reduction

  • Tour de France Physics: Time Trials

    Tour de France Physics: Time Trials

    Unlike road stages in which cyclists can draft off one another to reduce drag, in the time trial a cyclist is on a solo race against the clock with nowhere to hide. As a result, the event features lots of technologies designed to reduce both pressure drag and skin friction on the cyclist. For time trials, cyclists wear skinsuits and shoe covers to eliminate any sources of flapping fabrics and to reduce skin friction. They ride bicycles designed to be as light and aerodynamic as possible. Instead of rounded tubing in the frames, these bikes consist of elongated airfoil profiles that direct air past and prevent separation that may increase pressure drag. The rims of their tires are wider and the back wheel is replaced with a disc wheel that allows no airflow aross the wheel. Like the airfoil tubing, these changes help prevent separation. Similarly, riders wear elongated helmets designed to be as aerodynamic as possible while the rider is in the “aero” position, with arms directed out over the wheels, head level, elbows tucked, and back flat. In wind tunnel tests, the rider best able to hold this position will experience the least drag. Even the addition or subtraction of a water bottle is not left to chance, with many time trial bikes designed to be more aerodynamic with a water bottle onboard (though you probably won’t catch the cyclists breaking their aero position to get a drink)! (Photos by Veeral Patel)

    FYFD is celebrating the Tour de France with a weeklong exploration of the fluid dynamics of cycling. See previous posts on drafting in the peloton, and pacelining and echelons, and the art of the lead-out train.

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    Drafting Behind a Big Rig

    For those who like the effects of drafting in cycling backed up by Mythbusters, here’s a comparison between riding a mountain bike at 20 mph solo and on the tail end of a semi. #

  • Tour de France Physics: Lead-Out Trains

    [original media no longer available]

    One of the most impressive cycling techniques for drag reduction on a rider is the lead-out train that delivers a sprinter to the finish line. No current team is better at this than HTC-Highroad. Watch for them in the white and yellow from about ~4:00 in the above video.

    The lead-out train begins 5 km or so before the line, with the entire team in a line at the front of the peloton with the sprinter in the final position. The rider at the front will ride for as long and hard as he can, ensuring that the pace is such that no riders from the main field are able to pull ahead. This accelerates the sprinter to higher speeds while sheltering him in the wake of the rest of the team.

    One by one, the riders of the team will do their time at the front, expending their energy while protecting the sprinter. The final lead-out rider will be sprinting a few hundred meters from the finishing line; at this point the sprinter in the back may be riding 70 kph while enjoying protection from the wind. Finally, with the finish line in sight, he will swing out around his lead-out man and go all out for the line. Sprinters can hit speeds of nearly 80 kph in these short bursts.

    FYFD is celebrating the Tour de France with a weeklong exploration of the fluid dynamics of cycling. See previous posts on drafting in the peloton, and pacelining and echelons.

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    Tour de France Physics: Breakaways

    In cycling, a small group of riders often leave the protection of the peloton in a breakaway. These riders will often spend 80% or more of a stage or race outside of the peloton, trying to reach the finish line before they’re caught. Because the pressure drag is so draining on a lone cyclist, it’s vital that breakaway riders work together. When the wind comes predominantly from the front or back, riders will form one or two lines, riding with their wheels within a foot of one another (see ~0:23). This paceline rotates so that every rider takes a turn at the front, bearing the brunt of the effort while other cyclists recover in their wake, where they experience less drag.

    If the wind blows predominantly across the riders, they will form a diagonal line with the frontmost rider rotating behind for shelter from the wind after a pull. This drag reduction technique is called an echelon (see ~1:40). As seen above, for experienced riders the echelon can protect individuals even in bike-stealingly high winds.

    FYFD is celebrating the Tour de France with a weeklong exploration of the fluid dynamics of cycling. See part one on drafting in the peloton.

  • Tour de France Physics: Pelotons

    Tour de France Physics: Pelotons

    July is well underway and for cycling fans around the world that means it’s time for the Tour de France. This week at FYFD we’re going to do something a little different: in honor of cycling’s biggest race, every post this week will focus on some of the fluid dynamics involved in the sport.

    On a bicycle, except when climbing, the majority of a rider’s energy goes toward overcoming aerodynamic drag. Riders wear close-fitting clothes to reduce skin friction and loss to flapping fabric, but most of their drag is pressure-based. A blunt object disturbs the airflow around it, usually resulting in separated flow in its wake. A high pressure region forms in front of the rider and a low pressure region forms in the separated flow behind them. This pressure difference literally pulls the rider backwards. Since drag goes roughly as speed squared, adding a headwind makes matters even worse for a cyclist.

    In races, especially on flat stages, the majority of the riders will stay in a large group called a peloton in order to counteract these aerodynamics. By riding in the wakes of those in the front, riders in the peloton experience a much smaller front-to-back pressure difference and thus much less drag. For a rider in the midst of the peloton, the drag reduction can be as great as 40% (#). This allows riders to conserve energy for solo efforts near the end of the race or stage, like breaking away from the peloton in the final kilometers or winning a sprint for the finish line. (Photo credit: Wade Wallace)

  • Drafting Flags

    Drafting Flags

    Wired Science has published a gallery of fluid dynamics photos and videos, several of which have been featured here previously. There’s some neat stuff there, well worth checking out. #

    This image shows two flags oriented in line with a film flowing top to bottom. The second flag interrupts the wake of the first one, which reduces the drag experienced by the first flag and increases that on the second. This is called inverted drafting and occurs because the flags are passive objects that bend to every change in the flow. #

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    Hot Spheres Sink Faster

    New research shows that the Leidenfrost effect–which causes water droplets to skitter across a hot pan–can drastically reduce the drag on objects moving through a liquid. When raised to a high enough temperature, a sphere falling water will be coated in a protective layer of vapor (see video above) that acts like a lubricant as the sphere moves through the water. If the temperature of the object drops too low, the vapor layer will dissolve into a mess of bubbles (~35 secs into video). One way that this mechanism reduces drag is by keeping flow attached to the sphere for longer as shown in this video. Preventing this flow separation increases the pressure recovered after the point of lowest pressure (the shoulders of the sphere), which reduces overall drag.

    See also:

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    Aerodynamics with Bill Nye and Samuel L. Jackson

    Bill Nye, Samuel Jackson, golf balls, Reynolds number, dimples, and boundary layers. It doesn’t get much better than this. – Khristopher O (submitter)

    It definitely beats Jackson’s other foray into aerodynamics! The dimples on a golf ball cause turbulent boundary layers, which actually decrease drag on the ball and make it fly farther. Why bluff bodies experience a reduction in drag as speed (and thus Reynolds number) increases was a matter of great confusion for fluid mechanicians early in the twentieth century, but it’s not too hard to see why it happens with some flow visualization.

    On the top sphere, the laminar boundary layer separates from the sphere just past its shoulder. This results in a pressure loss on the backside of the sphere and, thus, an increase in drag. On the bottom sphere, a trip-wire placed just before the shoulder causes a turbulent boundary layer, which separates from the sphere farther along the backside. This late separation results in a thinner wake and a smaller pressure loss behind the sphere, thereby reducing the overall drag when compared to the laminar case. (Photo credit: An Album of Fluid Motion)

  • Bristling Scales Give Sharks Speed

    Bristling Scales Give Sharks Speed

    The shortfin mako shark is one of the ocean’s fastest and most agile hunters, thanks in part to flexible scales along its body. As water flows around the shark’s body, the scales bristle to angles in excess of 60 degrees. This causes turbulence in the boundary layer along the shark’s body and prevents boundary layer separation which would otherwise increase the shark’s drag. In this respect, the scales serve much the same purpose as dimples on a golf ball. (Abstract, National Geographic article) #