Tag: drag

How Cycling Position Affects Aerodynamics
New FYFD video! How much does a rider’s position on the bike affect the drag they experience? To find out I teamed up with folks from the University of Colorado at Boulder and at SimScale to explore this topic using high-speed video, flow visualization, and computational fluid dynamics.
Check out the full video below, and if you need some more cycling science before the Tour de France gets rolling, you can find some of my previous cycling-related posts here. (Image and video credit: N. Sharp; CFD simulation – A. Arafat)
ETA: Please note that the video contained in this post was sponsored by SimScale.
June 30, 2017
Rio 2016: Badminton
Badminton is unusual among racquet sports because it does not use a sphere as its projectile. Instead players hit a shuttlecock, traditionally made from a cork ball and a skirt of goose feathers. Despite its unusual shape, the shuttlecock reaches some of the fastest speeds in sports – over 330 kph (200 mph)! The shuttlecock’s high-drag form quickly slows shots down but also gives the game very different trajectories compared to other racquet sports.
It’s likely that, if you’ve played badminton yourself, you’ve played with a shuttlecock that has a plastic skirt rather than a feathered one. These synthetic shuttlecocks are cheaper and more durable, but they also have different drag characteristics than their feathered cousins. At low speeds, synthetic shuttlecocks have more drag than feathered ones, but at high speeds, the opposite is true. This is because the plastic skirt deforms more easily than the feathers, causing a synthetic shuttlecock’s skirt to collapse into a shape with less drag. (Video credit: Science Friday; research credit: F. Alam et al.)
Join us throughout the Rio Olympics for more fluid dynamics in sports. If you love FYFD, please help support the site!
August 18, 2016
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: 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
Perching Physics
Compared to birds, manmade aircraft tend to be quite limited and inelegant. Fixed-wing aircraft, for example, require long, flat areas for take-off and landing, whereas birds of all sizes are adept at maneuvers like perching. This video examines the perching behaviors of large birds and extends the physics to a small unmanned aerial vehicle (UAV). As a bird approaches a perching location, it pitches its body and wings upward. This places the bird in what’s known as deep stall, where air flowing over the upper surface of the wing separates just after the leading edge. This move dramatically increases drag on the bird, slowing it for landing. At the same time, the speed of the pitch maneuver generates a vortex on the wing that helps the bird maintain lift despite the drop in speed. With the help of both forces, the bird can make a graceful, controlled landing in only a short distance. (Video credit: J. Mitchell et. al.)
January 6, 2016
Reader Question: Winglets
Reader tvargo writes:
First off… love your blog! I know very little about physics, but love reading about it. Could you potentially explain what the little upturned ends of wings do? looking on wikipedia is see this: “There are several types of wingtip devices, and although they function in different manners, the intended effect is always to reduce the aircraft’s drag by partial recovery of the tip vortex energy.” huh?
Thanks! That’s a great question. Winglets are very common, especially on commercial airliners. To understand what they do, it’s helpful to first think about a winglet-less airplane wing. Each section of the wing produces lift. For a uniform, infinite wing, the lift produced at each spanwise location would be the same. In reality, though, wings are finite and wingtip vortices at their ends distort the flow. The vortices’ upward flow around the ends of the wing reduces the lift produced at the wing’s outermost sections, making the finite wing less efficient (though obviously more practical) than an infinite wing.
Adding a winglet modifies the end conditions, both by redirecting the wingtip vortices away from the underside of the wing and by reducing the strength of the vortex. Both actions cause the winglet-equipped wing to produce more lift near the outboard ends than a wing without winglets.
But why, you might ask, does the Wikipedia explanation talk about reducing drag? Since a finite wing produces less lift than an infinite one, finite wings must be flown at a higher angle of attack to produce equivalent lift. Increasing the angle of attack also increases drag on the wing. (If you’ve ever stuck a tilted hand out a car window at speed, then you’re familiar with this effect.) Because the winglet recovers some of the lift that would otherwise be lost, it allows the wing to be flown at a lower angle of attack, thereby reducing the drag. Thus, overall, adding winglets improves a wing’s efficiency. (Photo credit: C. Castro)
July 30, 2014
Brazuca
Since 2006, Adidas has unveiled a new football design for each FIFA World Cup. This year’s ball, the Brazuca, is the first 6-panel ball and features glued panels instead of stitched ones. It also has a grippy surface covered in tiny nubs. Wind tunnel tests indicate the Brazuca experiences less drag than other recent low-panel-number footballs as well as less drag than a conventional 32-panel ball. Its stability and trajectory in flight are also more similar to a conventional ball than other recent World Cup balls, particularly the infamous Jabulani of the 2010 World Cup. The Brazuca’s similar flight performance relative to a conventional ball is likely due to its rough surface. Like the many stitched seams of a conventional football, the nubs on the Brazuca help trip flow around the ball to turbulence, much like dimples on a golf ball. Because the roughness is uniformly distributed, this transition is likely to happen simultaneously on all sides of the ball. Contrast this with a smooth, 8-panel football like the Jabulani; with fewer seams to trip flow on the ball, transition is uneven, causing a pressure imbalance across the ball that makes it change its trajectory. For more, be sure to check out the Brazuca articles at National Geographic and Popular Mechanics, as well as the original research article. (Photo credit: D. Karmann; research credit: S. Hong and T. Asai)
June 12, 2014
Hawk in Flight
For a little more than century, mankind has taken flight in fixed-wing aircraft. But other species have flown for much longer using flapping techniques, the details of which humans are still unraveling. To really appreciate flapping flight, it helps to have high-speed video, like this beautiful footage of a goshawk attacking a water balloon. The motion of the hawk’s wings is far more complex than the simple up and down flapping we imitate as children. On the downstroke, the wings and tail stretch to their fullest, providing as large an area as possible for lift. During steady flight, the bird flaps while almost horizontal for minimal drag, but as it approaches its target, it rears back, allowing the downstroke to both lift and slow the bird. In the upstroke, the bird needs to avoid generating negative lift by pushing air upward. To do this, it pulls its wings in and simultaneously rotates them back and up. Its tail feathers are also pulled in but to a lesser extent. Leaving them partially spread probably maintains some positive lift and provides stability. At the end of the upstroke, the hawk’s wings are ready to stretch again, and so the cycle continues. (Video credit: Earth Unplugged/BBC; h/t to io9)
March 10, 2014
Shooting a Bullet Through a Water Balloon
This high-speed video of a bullet fired into a water balloon shows how dramatically drag forces can affect an object. In general, drag is proportional to fluid density times an object’s velocity squared. This means that changes in velocity cause even larger changes in drag force. In this case, though, it’s not the bullet’s velocity that is its undoing. When the bullet penetrates the balloon, it transitions from moving through air to moving through water, which is 1000 times more dense. In an instant, the bullet’s drag increases by three orders of magnitude. The response is immediate: the bullet slows down so quickly that it lacks the energy to pierce the far side of the balloon. This is not the only neat fluid dynamics in the video, though. When the bullet enters the balloon, it drags air in its wake, creating an air-filled cavity in the balloon. The cavity seals near the entry point and quickly breaks up into smaller bubbles. Meanwhile, a unstable jet of water streams out of the balloon through the bullet hole, driven by hydrodynamic pressure and the constriction of the balloon. (Video credit: Keyence)
January 15, 2014