FYFD

Tag: drag reduction

  • The Protection of the Peloton

    The Protection of the Peloton

    It’s well-known by professional cyclists that sitting in the middle of the peloton requires little effort to overcome aerodynamic drag, but now, for the first time, there’s a scientific study to back that up. Researchers built their own quarter-scale peloton of 121 riders to investigate the aerodynamic effect of cycling in such a large group versus riding solo. Through wind tunnel studies and numerical simulation, they found that riders deep in the peloton can experience as little as 5-10% of the aerodynamic drag of a solo cyclist. 

    Tactically, this means teams should aim to position their protected leader or sprinter mid-way in the pack, where they’ll receive lots of shelter without risking one of the crashes common near the back of the peloton. It also suggests that teams wanting to isolate another team’s leader should try to push them toward the outer edges of the peloton rather than letting them sit in the middle. It will be interesting to see whether pro teams shift their race strategies at all with these numbers in hand.

    Of course, this study considers only a pure headwind. But other groups are looking at the effects of side winds on cyclists. (Image credit: J. Miranda; image and research credit: B. Blocken et al.; submitted by 1307phaezr)

  • Hydrofoils and Stability

    Hydrofoils and Stability

    Today’s fastest boats use hydrofoils to lift most of a boat’s hull out of the water. This greatly reduces the drag a boat experiences, but it can also make the boat difficult to handle. One style of hydrofoil boat, called a single-track hydrofoil, uses two hydrofoils in line with one another to support and steer the boat. The pilot can steer the lead hydrofoil into the direction of a fall to correct it. Stability-wise, this is the same way that you keep a bicycle upright. On a boat, the situation is a bit tougher to manage, and, like riding a bike, it takes practice. A group of students published a full mathematical model for the dynamics of this kind of boat, which allows designers to test a prototype’s stability early in the design process and enables student teams to use computer simulators to train their pilots to drive a boat before putting them out on the water, similar to the way that airplane pilots train. (Image credit: TU Delft Solar Boat Team, source; research credit: G. van Marrewijk et al., pdf; via TU Delft News; submitted by Marc A.)

  • The Hairyflower Wild Petunia

    The Hairyflower Wild Petunia

    Dispersing seeds is a challenge when you’re stuck in one spot, but plants have evolved all sorts of mechanisms for it. Some rely on animals to carry their offspring away, others create their own vortex rings. The hairyflower wild petunia turns its fruit into a catapult. As the fruit dries out, layers inside it shrink, building up strain that bends the fruit outward. Once a raindrop strikes it, the pod bursts open, flinging out around twenty tiny, spinning, disk-shaped seeds. That spin is important for flight. The best-launched seeds may spin as quickly as 1600 times in a second, which helps stabilize them in a vertical orientation that minimizes their frontal area and reduces their drag. Researchers found that these vertically spinning seeds have almost half the drag force of a spherical seed of equal volume and density. That means the hairyflower wild petunia is able to spread its seeds much further without a larger investment in seed growth. (Image and research credit: E. Cooper et al., source; via NYTimes; submitted by Kam-Yung Soh)

  • PyeongChang 2018: Cross-Country Skiing

    PyeongChang 2018: Cross-Country Skiing

    Cross-country skiing, also known as Nordic skiing, is a part of many longstanding disciplines in the Winter Games. Unlike downhill skiing, cross-country events typically involve mass starts, which allow athletes to interact, using one another for pacing and tactics. Drafting can be a valuable method to save energy and reduce drag. A following skier sees a 25% drag reduction while drafting; the lead skier gets about a 3% reduction in drag compared to skiing solo. Competitors usually wear tight-fitting suits to minimize drag, but unlike speedskating, for example, cross-country skiers don’t get much benefit from roughened surfaces and impermeable fabrics. Typical race speeds are 4 – 9 m/s, and most of these high-tech fabrics don’t provide tangible benefits until higher speeds. (Image credit: Reuters/S. Karpukhin, US Biathlon, GettyImages/Q. Rooney)

  • PyeongChang 2018: Speedskating

    PyeongChang 2018: Speedskating

    Four years ago in Sochi, Under Armour’s suits for the U.S. speedskating team took a lot of flak after the team failed to medal. The company defended the physics and engineering of their suits, and an internal audit of the speedskating program ultimately placed blame on flaws in their training regimen, unfamiliarity with the new suits, and overconfidence.

    This time around Under Armour has taken a more hands-on approach with the team, helping with training regimens in addition to providing suits. Under Armour spent hundreds of hours testing the suits in Specialized’s wind tunnel, including testing many fabrics before settling on the slightly rough H1 fabric used in patches on the skater’s arms and legs. Like the previous suit’s dimpled design, the roughness of the fabric promotes turbulent flow near it. Because turbulent flow follows curved contours better than laminar flow does, air stays attached to the athlete for longer, thereby reducing their drag. The suit is also designed with asymmetric seams that help the athlete stay low and comfortable in the sport’s frequent left turns.

    U.S. speedskaters have been competing in a version of the suits since last winter, ensuring that athletes are familiar with the equipment this time around. Whether the new suits and training program will pay off remains to be seen. After their disastrous experience in Sochi, both the team and the company are shy about setting expectations. (Image credits: D. Maloney/Wired; US Speedskating)

  • PyeongChang 2018: Skeleton

    PyeongChang 2018: Skeleton

    Skeleton, the sliding event in which athletes race down an ice track head first, is a fast-paced and punishing sport. Skeleton racers can reach speeds of 125 kph (~80 mph) during their descents. This is a little slower than the feet-first luge, in part because the skeleton sled runs on circular bars rather than sharp runners. 

    Body positioning is key in the sport. It’s the athlete’s primary method of steering, and it controls how much drag slows them down. But skeleton runs are brutally taxing; athletes pull 4 or 5g in the turns – more than astronauts experience during a launch! All that jostling means athletes cannot stand more than about 3 trips down the track in a day. To practice positioning without the bone-jarring descent, athletes can work in a wind tunnel. While the wind tunnel provides the aerodynamic equivalent of their usual speed, athletes focus on holding their bodies in the most streamlined position. Some wind tunnels are even able to provide screens that let the athletes see their drag values in real-time, letting them adjust to learn what works best for them. (Image credit: N. Pisarenko/AP, Bromley Sports)

  • PyeongChang 2018: Ski Jumping

    PyeongChang 2018: Ski Jumping

    No winter sport is more aerodynamically demanding than ski jumping. A jump consists of four parts: the in-run, take-off, flight, and landing. The in-run is where an athlete gains her speed, and to keep drag from slowing her down, she descends in a streamlined tuck that minimizes frontal area. The biggest aerodynamic challenge comes during flight, when the jumper wants to maximize lift while minimizing drag. The athlete spreads her skis in a V-shape and flattens her body, using her hands to adjust her flight. Flying the farthest requires careful management of forces while in the air. Wind plays a major role as well, with headwinds helping athletes fly farther. To compensate, scoring includes a wind factor calculated based on conditions for each jump. (Image credit: B. Pieper, Reuters/K. Pfaffenbach, PyeongChang 2018)

  • Hair in the Flow

    Hair in the Flow

    Humans are hairy on the inside. Not in the way that we are on the outside, but in the sense that many interior surfaces of our bodies are covered in small, flexible, hair-like protrusions like the papillae on our tongues or the cilia in our intestines. Many of these fibers are immersed in fluids, raising the question of how the flow and the hairs interact. An elastic fiber immersed in a flow will bend in the direction of the flow (bottom); this helps reduce the drag and widens the channel flow goes through compared to a stiff, upright fiber. 

    But what happens when the fibers are all mounted at an angle? In this case, researchers found an asymmetric response. If flow moves in the direction of the fibers’ bend, the hairs don’t impend the flow at all. If flow moves against that direction, however, the hairs start to stand upright, blocking the flow channel and increasing the drag. The researchers suggest this sort of mechanism could be use in micro-hydraulic devices in the same way as a diode in a circuit – allowing flow in only one direction. For another biological example of flow control, check out how a shark’s denticles can prevent flow separation. (Image credits: hairy surface – J. Alvarado et al., flow around a hair – J. Wexler et al.; research credit: J. Alvarado et al.)

  • Chains of Salps

    Chains of Salps

    Salps are small, jellyfish-like marine invertebrates that swim by ejecting a pulsatile jet. They are unusual creatures whose lives have two major stages: one in which salps swim individually and one in which they link together and swim in large chains. In the chain, salps don’t synchronize their jetting; each salp jets with its own phase and frequency. A new study suggests that, in spite of this lack of synchronicity, the salp chain’s swimming reduces the animals’ drag. There are several  factors that contribute to this result. One is that drag is generally lower on a body moving at constant speed compared to one moving in bursts. When linked together and firing randomly, all the individual jets tend to average out into one continuous swimming speed. There’s even a benefit to being out of sync: previous work showed that synchronized jets lose some of their thrust when they are too close together. Salps avoid that loss by keeping to their own beat. (Image and research credit: K. Sutherland and D. Weihs, source; via Gizmodo)

  • Reader Question: Drafting in Time Trials

    Reader Question: Drafting in Time Trials

    In a comment on this recent post regarding drafting advantages to a leader, reader fey-ruz asks:

    in cycling, team follow cars are required to maintain a minimum distance from their riders during time trials for this very reason (although i imagine the effects in that context are much smaller and dependent on the conditions, esp the wind speed, direction, and strength). FYFD, is there a simple way to understand where this upstream influence comes from? or a specific term in the navier-stokes equations that it results from?

    Cars following riders during a time trial can actually make a huge difference! One study from a couple of years ago estimated that a car following a rider in a short (13.8 km) time trial could take 6 seconds off the rider’s time. The images up top show a simulation from that study with a car following at 5 meters versus 10 meters. The colors indicate the pressure field around the car and rider. Red is high pressure, blue is low pressure. Both the car and the rider have high pressure in front of them; you can think of this as a result of them pushing the air in front of them.

    A large part of the rider’s drag comes from the difference in pressure ahead and behind them. (For a look at flow around a cyclist that focuses on velocity instead, check out my video on cycling aerodynamics.) When a car drives close behind a cyclist, it’s essentially pushing air ahead of it and into the cyclist’s wake. This actually reduces the difference in pressure between the cyclist’s front and back sides, thereby reducing his drag. Because cars are large, they have an oversized effect in this regard, but having a motorbike or another rider nearby also helps the lead cyclist aerodynamically.

    As for the Navier-Stokes equation – this effect isn’t one that you can really pin down to a single term since it’s a consequence of the flow overall. (Image credits: TU Eindhoven; K. Ramon)