Curling is a deceptively engrossing sport with some unique physics among Winter Olympic events. Athletes slide 19kg granite stones at a target 28 meters away. Along the way, teammates sweep the pebbled ice with brooms, melting it with frictional heating to help the stone slide further. The underside of the stones is concave, so they only touch the ice along a narrow ring. Researchers think roughness in the leading edge of the sliding stone cuts into the ice, leaving scratches that the trailing edge tries to follow. This is what causes the stone’s trajectory to curl. By melting the ice, sweeping also prevents curling, so competitors must know exactly when and how much to sweep. Ice conditions shift throughout a match, and the best players can read the ice to keep their stones where they want them. (Image credit: AP; W. Zhao/GettyImages)
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)
When it comes to winter sports, not all ice is created equal. Every discipline has its own standards for the ideal temperature and density of ice, which makes venue construction and maintenance a special challenge. Figure skating, for example, requires softer ice to cushion athletes’ landings, whereas short-track speed skating values dense, smooth ice for racing. The Gangneung Ice Arena hosts both and can transition between them in under 3 hours. Gangneung Oval hosts long-track speed skating and makes its ice layer by layer, spraying hot, purified water onto the rink. This builds up a particularly dense and therefore smooth ice.
The toughest sport in terms of ice conditions is curling, which requires a finely pebbled ice surface for the stones to slide on. Forming those tiny crystals on the ice sheet can only be done at precise temperature and humidity conditions. This is a particular challenge for Gangneung Curling Center due to its coastal location. To keep the temperature and humidity under control at full crowd capacity, officials even went so far as to replace all the lighting at the facility with LEDs! (Image credit: Pyeongchang 2018, 1, 2, 3)
In bobsleigh, two- and four-person teams compete across four runs down an ice track. The shortest cumulative time wins, and since typical runs are separated by hundredths of a second, teams look for any advantage that helps them shave time. The size, weight, and components of a sled are restricted by federation rules; for example, teams cannot use vortex generators to improve their aerodynamics. Instead bobsledders work with companies like BMW, McLaren, and Ferrari to engineer their sleds. Both computational fluid dynamics and wind tunnel tests with the actual team in the sled are used to make each sled as aerodynamic as possible. (Image credit: IOC, Gillette World Sports, source)
These days artificial snow-making is a standard practice for ski resorts, allowing them to jump-start the early part of the season. Snow guns continuously spray a mixture of cold water and particulates 5 or more meters in the air to generate artificial snow. The tiny droplet size helps the water freeze faster and the particles provide nucleation sites for snow crystals to form. As with natural snow, the shape and consistency of the snow depends on humidity and temperature conditions. Pyeongchang is generally cold and dry, so even the artificial snow there tends to be similar to snow in the Colorado Rockies. Recreational skiers tend to look down on artificial snow, but Olympic course designers actually prefer it. With artificial snow, they can control every aspect of an alpine course. For them, natural snowfall is a disruption that puts their design at risk. (Video credit: Reactions/American Chemical Society)
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)
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)
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)
Moguls are bump-like snow mounds featured in freestyle skiing competitions and also frequently found on recreational ski courses. Although competition runs are man-made, most mogul fields form naturally on their own. As skiiers and snowboarders carve S-shaped paths down the slope, their skis and snowboards remove snow during sharp turns and deposit it further downhill. Over a surprisingly short amount of time, these random, uncoordinated actions form bumps large enough that they force skiers and snowboarders to begin turning on the downhill side of the bump. That action continues to carve out snow on the uphill side and deposit it downhill, effectively causing the downhill bumps to migrate uphill, as seen in the timelapse animation below. As more moguls form, their motion organizes them into a checkerboard-pattern that moves in lockstep. Observations show that mogul fields can move about 10 meters uphill over the course of a season. Seemingly, the only way to prevent mogul formation on steep slopes is to regularly groom them back to a flat state! (Image credits: J. Gruber/USA Today; J. Huet; D. Bahr; research credit: D. Bahr et al.)
The Olympic Charter declares that winter sports must be practiced on snow or ice. Both are frozen forms of water, which despite its ubiquity, is one of the strangest substances around. In addition to its tendency to expand as it freezes, ice is inherently slippery, and no one’s quite certain yet why.
Most people have heard the theory that ice skating is possible due to high pressure melting the ice beneath the narrow blade. But realistically, pressure melting should only work for ice down to about -3.5 degrees Celsius. By contrast, the ideal temperatures for figure skating and ice hockey are -5.5 and -9 degrees Celsius, respectively. Melting due to friction might account for slipperiness a few more degrees below freezing, but it doesn’t explain why ice can be slippery when you’re just standing on it.
When physicist Michael Faraday suggested in 1850 that ice has a thin liquid-like layer at its surface, many discounted the theory. But modern experimental techniques and computer simulations have shown that Faraday was right. Ice has a liquid-like layer some 1 to 100 nanometers thick at its surface, and this layer persists to temperatures below -30 degrees Celsius. The process is known as surface pre-melting and what causes it is an area of active research for physical chemists. Current theories include hydrogen bonding and even quantum mechanical effects. (Image credit: AP Photo/B. Armangue; research credit: R. Rosenberg; Y. Li and G. Somorjai; F. Paesani and G. Voth)
This opens FYFD’s two-week series on the physics and fluid dynamics of the Winter Olympics. Stay tuned! – Nicole