Category: Phenomena

Rio 2016: Track Cycling
Track cycling is a sport where speed is everything. As much as 90% of the resistance a rider has to overcome is aerodynamic drag. To minimize drag, riders wear form-fitting one-piece skinsuits and wear special, streamlined helmets. They have aerodynamic bikes (unique left-side-drive ones, if you’re the Team USA women) and ride with their arms close together and thrust in front of them to minimize the frontal area they expose.
Even the tactics of racing rely heavily on aerodynamics. In events like team pursuit cyclists stay extremely close to the wheel of the rider ahead of them in order to remain in that rider’s slipstream and experience less drag. When switching off the position of lead rider, cyclists use the curvature of the track to help them move to the back of the paceline quickly to minimize the time spent outside of their teammate’s draft. (Image credits: GettyImages via the Guardian and the IOC, source)
Previously: Cycling’s aero equipment helps them beat the clock; the effect of cyclists being followed by a motorbike or car
Join us throughout the Rio Olympics for more fluid dynamics in sports. If you love FYFD, please help support the site!
August 12, 2016
Rio 2016: Table Tennis
Many sports use spherical balls, but the small size and weight of a table tennis ball makes it the one where aerodynamics have the strongest effect. Spin also plays a big role in the game by creating asymmetry in the flow around the ball.
Consider a table tennis ball with topspin, meaning that its upper surface is rolling in the direction of travel. That means that air flowing over the top of the ball is moving in the opposite direction as the ball’s surface. This will tend to make the flow separate from the ball at its widest point.
On the other side, the ball’s surface is spinning in the same direction as the air flow. This helps hold the air to the surface so that it follows the curve of the ball longer and doesn’t detach until well after the ball’s widest point. As a result of both these effects, air flowing around the ball experiences a net upward force, which in turn pushes the table tennis ball downward. This is known as the Magnus effect, and it plays a significant role in many sports. (Image credits: GettyImages; AFP)
Previously: The Magnus effect and the reverse Magnus effect in soccer; curveballs and knuckleballs in baseball
Join us throughout the Rio Olympics for more fluid dynamics in sports. If you love FYFD, please help support the site!
August 11, 2016
Rio 2016: Whitewater Sports
The whitewater rapids of canoe slalom have their origins in mountain streams. Today the sport’s Olympic venues are artificial rivers, specially designed to provide world-class rapids whatever the geography of the host city. Rio’s course, like London’s, is reconfigurable; its features are controlled by the placement of Lego-like plastic blocks.
A key part of the course’s design process was building a small-scale physical model of the course. To maintain the dynamics of the rapids at a smaller physical scale, engineers used a concept called similitude. Surface waves like rapids are a function of the flow’s inertia and the effects of gravity, a ratio that’s captured in the dimensionless Froude number. To match the small-scale model to the real flow, engineers scaled the features of the real course down such that the Froude number stayed the same between the model and the full-scale course. As seen in the animations above, this meant that the model had the same general flow features as the final course, letting engineers and designers test and fine-tune features before construction. Learn more about the model and its construction in these two videos. (Image credits: kayaker – Getty Images; model comparisons – J. Pollert, source)
Previously: Physics of rowing; why that octopus kite looks so real
Join us throughout the Rio Olympics for more fluid dynamics in sports. If you love FYFD, please help support the site!
August 10, 2016
Rio 2016: Rugby

The sport of rugby returns to the Olympics in Rio this year. Rugby’s ball is somewhat similar in size and shape to an American football, but it is a little wider and more rounded. Aerodynamically, this means that the rugby ball has more drag, but it is also more stable in flight, allowing players to pass and kick accurately, with or without a spiral.
As seen in the flow visualizations above, air travels up and around the ball before separating on the far side. The more the ball is tilted, the larger this separated region is and the greater the drag. At the same time, though, that tilt provides lift on the ball. The ideal orientation is the one with the largest ratio of lift force and drag force. For a rugby ball, this occurs at about 40 degrees.(Image credits: Planet Rugby; A. Vance et al.)
Previously: The aerodynamics of the American football
Join us throughout the Rio Olympics for more fluid dynamics in sports. If you love FYFD, please help support the site!
August 9, 2016
Rio 2016: Swimming
Strange as it seems, elite swimmers are faster when swimming underwater than they are at the surface. So much so, in fact, that they’re restricted to being underwater only 15 m after a dive or turn. To see just how stark a difference this makes, check out this crazy video. (I know, right?!)
To understand how this is possible, it helps to look at the three types of drag a swimmer experiences: pressure drag, skin friction, and wave drag. Pressure drag is probably the most familiar; it’s the drag that comes from the swimmer’s shape and how the fluid moves around it. Skin friction is the drag that comes from viscous friction between the swimmer and the water. The final type, wave drag, comes from the energy expended to create waves at the surface of the water. As you might expect, energy that goes into splashing is energy that isn’t going into propulsion.
When swimming at the surface, swimmers experience a lot of wave drag. At least one experiment showed that wave drag accounted for most of a surface swimmer’s drag. In contrast, at a depth of more than 0.5 m, a swimmer’s wave drag is virtually negligible. The submersion does come at the cost of higher skin friction (since more of the swimmer is in contact with the water), but there is also more opportunity for useful propulsion since both sides of a kick can move water (and not air.) Bonus read for those interested in more: Is the fish kick the fastest stroke yet? (Image credits: AP; B. Esposito)
Previously: what makes a pool fast?
Join us throughout the Rio Olympics for more fluid dynamics in sports. If you love FYFD, please help support the site!
August 8, 2016
Rio 2016: Cycling
Today marks the official start of the 2016 Summer Olympics in Rio. Here at FYFD we’ll be celebrating by taking a look at how fluid dynamics affects Olympic sports. You can check out our previous series on the London Olympics here. Since this weekend features the men’s and women’s cycling road races, we’ll get started with cycling!
In road cycling, equipment and race strategy are all built around aerodynamic efficiency. It’s understood that following a car or motorbike gives a cyclist an unfair advantage, and officials can be quick to punish infractions. What the rules don’t account for, though, is the advantage a cyclist gets when they’re followed by a motorbike (or car). These vehicles are significantly larger than a cyclist, and when they are trailing a cyclist, they have a significant upstream effect. Essentially the higher pressure traveling ahead of the motorbike will counter the low pressure region immediately behind the cyclist. The result is that the cyclist, despite being in front, experiences less drag than they would if the motorbike weren’t there.
The difference isn’t tiny either: if a motorbike follows a rider at a distance of 0.5 m for just 1 km, the rider saves more than 2 seconds. When events can be won or lost by fractions of a second, those gains are significant. (Image credits: DCMS; B. Blocken et al., GettyImages, Reuters; research credit: B. Blocken et al.; submitted by Marc A.)
August 5, 2016
Spillway Waves
Earlier this summer, the spillway of the Banja Dam was opened for the first time, releasing a stream of excess water from the reservoir. As you can see above, waves quickly formed at the surface of the falling water. You’ve likely noticed this yourself in the run-off along the street after a storm. It turns out that shallow water running down an incline is unstable. A disturbance to the flow – from surface roughness, vibration, or a change in curvature – will grow, just like a ball sitting at the top of a hill will roll down as soon as it’s prodded. For more about this kind of instability, check out this post or my video about boundary layer stability and the Space Shuttle. (Image credit: Guillaume TYTECA, source; via Gizmodo)
August 3, 2016
Starting a Lighter
Lots of fluids are transparent, which makes it hard for us to appreciate their motion. One technique for making these invisible motions visible is schlieren photography, which makes differences in density visible. Here it’s combined with high-speed video to show what happens when you use a lighter (minus the spark!). When the fuel starts flowing, it’s unstable and turbulent, but after that initial start-up, you can see the jet settle into a smooth and laminar flow. Wisps of fuel diffuse away from the jet as the fluid disperses. As the valve shuts off, the flow becomes unstable again, and the remains of the lighter fluid diffuse away. (Video credit: The Missing Detail)
August 2, 2016
Crisscrossing Clouds
This natural-color satellite image shows crisscrossing cloud patterns off coastal Africa. These distinctive lines in the sky are gravity waves, and they form when air masses get displaced upward by terrain or other conditions. In this case, dry air cooled overnight on land before moving out over the ocean. That displaced warm, humid air above the water and forced it upward, where it eventually cooled and condensed into clouds. Gravity created the ripple-like waves; as the moist air cooled, gravity again pulled it downward – leaving behind a clear sky. Once the humid air sank, the dry air pushed it up again, creating another line of clouds and continuing the cycle. (Image credit: NASA; via NASA Earth Observatory)
July 28, 2016
Crash Course Fluids
Crash Course Physics returns to the subject of fluids with their video on fluid dynamics. They stick with ideal fluids (i.e. incompressible, inviscid, laminar flows) for simplicity and cover some of the basics by discussing conservation of mass (also called continuity) and a simple form of Bernoulli’s equation. Despite keeping things basic, the video does a nice job introducing these topics; I especially like that they explain Bernoulli’s equation as a form of conservation of energy. Sometimes it’s easy to let the terminology in fluid dynamics mask the fact that the equations we use are just alternative forms of the classical equations for conserving mass, momentum, and energy. As with their fluids at rest video, the information is densely packed, so expect to pause and rewind. (Video credit: Crash Course)
July 27, 2016