Search results for: “drag”

London 2012: Cycling Physics
In no discipline of cycling is more emphasis placed on fluid dynamics than in the individual time trial. This event, a solo race against the clock, leaves riders no place to hide from the aerodynamic drag that makes up 70% or more of the resistance riders overcome when pedaling. Time trial bikes are designed for low drag and light weight over maneuverability, using airfoil-like shapes in the fork and frame to direct airflow around the bike and rider without separation, which creates an area of low pressure in the wake that increases drag. Riders maintain a position stretched out over the front wheel of the bike, with their arms close together. This position reduces the frontal area exposed to the flow, which is proportional to the drag a rider experiences.
Special helmets, some with strangely streamlined curves, are used to direct airflow over the rider’s head and straight along his or her back. Both helmets and skinsuits are starting to feature areas of dimpling or raised texturing. These function in much the same way as a golf ball; the texture causes the boundary layer, the thin layer of air near a surface, to become turbulent. A turbulent boundary layer is less susceptible to separating from the surface, ultimately leading to lower drag than would be observed if the boundary layer remained laminar. Wheels, skinsuits, gloves, shoe covers, and even the location of the brakes on the bike are all tweaked to reduce drag. In an event that can be decided by hundredths of a second between riders, every gram of drag counts. (Photo credits: Stefano Rellandini, POC Sports, Reuters, Paul Starkey, Louis Garneau)
FYFD is celebrating the Olympics by featuring the fluid dynamics of sports. Check out our previous posts on how the Olympic torch works, what makes a pool fast, the aerodynamics of archery, and the science of badminton.
August 2, 2012
London 2012: Badminton Physics
Unlike most racket sports, badminton uses a projectile that is nothing like a sphere. The unusual shape of the shuttlecock not only creates substantial drag in comparison to a ball but increases the complexity of its flight path. The heavy head of the shuttlecock creates a moment that stabilizes its flight, ensuring that the head always points in the direction of travel. The skirt, traditionally made of feathers though many today are plastic, is responsible for the aerodynamic forces that make the shuttlecock’s behavior so interesting.
Measuring the drag coefficient of the shuttlecock, modeling its trajectory and behavior in the four common badminton shots, and even attempting computational fluid dynamics of the shuttlecock are all on-going research problems in sports engineering. (Photo credit: Rob Bulmahn)
FYFD is celebrating the Olympics with the fluid dynamics of sports. Check out our previous posts on how the Olympic torch works, what makes a pool fast, and the aerodynamics of archery.
August 1, 2012
London 2012: Swimming Pool Physics
The era of the LZR suit may be over in swimming, but technology is still making an impact when it comes to making swimmers faster. One thing you’ll often hear from commentators is how the London Aquatic Center boasts one of the world’s fastest pools. When swimmers compete, they have to contend with all the turbulence created in the pool by eight people trying to direct as much water behind them as possible as quickly as possible. Like ripples spreading on a pond, these waves travel, reflect, and interfere, ultimately disrupting the swimmers and causing extra drag. In a fast pool, engineers have made adjustments to reduce the impact of these waves on swimmers. Firstly, the pool is 3 meters deep, meaning that vertical disruptions are mostly damped out before they reach the bottom, so any wave reflected off the bottom of the pool will be extremely weak. Along the sides and ends of the pool, a special trough captures surface waves, preventing them from reflecting back out into the pool. The lane lines are also designed to soak up wave energy so that it does not propagate as much between lanes. When waves hit the lines, their links spin, dissipating some of the wave’s energy.
Despite these advances, the outermost lanes–those against the walls–are not used in competition. This helps to equalize the turbulence between lanes. Whether there is any fluid mechanical advantage to being in a particular lane is debatable. The outer lanes have the advantage of only one competitor’s wake to contend with, but they isolate the swimmer so he or she cannot see their competition as well. In the inner lanes, you’ll sometimes see swimmers try to swim close to the lane line if their competition is ahead of them, the idea being that they may be able to draft on their competitor’s bow wave to reduce drag. Generally speaking, the lane positions are determined by seeding going into the event, where the faster swimmers are given the innermost lanes. This is why it’s rare to see gold medals coming from the outermost lanes. For more, check out NBC’s video on designing fast pools (US only, unfortunately). (Photo credits: Associated Press, Reuters, Geoff Caddick)
July 30, 2012
Reader Question: Drafting in Cycling
jonesmartinez asks:
As a cyclist, I’m curious about drafting. How fast do I need to be going for there to be a measurable benefit? Additionally, often in a time trial a single rider is often followed by the team car and I’ve heard the rider can be pushed by the air around the team car. Any truth to this rumor? Thanks, I love the blog.
Drafting plays a major role in cycling and its tactics (check out our previous series on cycling). In general, drag increases with the square of velocity and data show this holds for cyclists. The rule of thumb I’ve heard given is that aerodynamic drag doesn’t play a large role below 15 mph, but I have not seen the numbers that inform that claim. Moreover, you have to consider the resultant airspeed around the cyclist. For example, a cyclist moving 13 mph into a 15 mph headwind (28 mph effective) will be experiencing more drag than a cyclist moving 20 mph with a 10 mph tailwind (10 mph effective). With drag being reduced 25-40% by drafting a leading rider, it is almost always beneficial to get behind someone.
That said, I have seen no measurable benefit for a leading rider with a paceline behind him, even though this should, in theory, reduce the drag on the lead rider by closing out his wake. With a large object like a car behind a solo rider, there might theoretically be some benefit. However, the car would have to be driving extremely close to the rider–far closer than they do in reality.
That said, with the prevalence of power meters in the amateur market these days, I think it would be a neat project to go out and try a few of these things firsthand and see whether such tactics actually result in a measurable difference in a cyclist’s performance–though I don’t recommend riding a foot off the front or back of a car!
May 17, 2012
Egg Spinning
Spin a hard boiled egg in a puddle of milk and you get a sprinkler. But how? The science starts at the surface. When the egg spins, the fluid touching its surface is dragged along due to friction, and, because of the fluid’s viscosity, other parts of the fluid will also be spun. Dynamics tells us that the velocity at the surface of the object varies with radius; the velocity at the bottom of a spinning sphere is much smaller than that at its equator because a particle at the equator traverses a larger distance in a single rotation. Likewise, the fluid touching the bottom of the egg is spun slower than the fluid just above it. Bernoulli’s principle tells us that, for an incompressible fluid, the pressure decreases as velocity increases, meaning that a favorable pressure gradient exists along the spinning convex surface. It is this pressure gradient that draws the fluid up the sides of the object. Near the equator, the pressure gradient is weakest and centrifugal force flings the the fluid outward. Surface tension, angular velocity, and viscosity all play a role in the jets and sheets created by the sprinklers. (Video credit: NPR Science Friday with Tadd Truscott et al)
May 14, 2012
Supercavitating Penguins
[original media no longer available]
Penguins, already fluid dynamicists by nature, have developed clever methods of increasing their speed to escape from the leopard seals that prey on them. In the clip above, notice from 1:55 onward as the penguins swim for the surface and leap onto the ice – they leave a trail of bubbles in their wake. The penguins are using supercavitation to decrease their drag. When the penguins first dive in to the water, they splay their feathers out in the air and then lock them closed in the water, trapping pockets of air beneath them. When the need for a burst of speed arises, the penguin shifts its feathers to release the air, coating most of its body in a layer of bubbles. Because the drag in air is much less than the drag in water, this enables the bird to achieve much higher speeds than they normally do when swimming.
April 24, 2012
Flapping to Fly Efficiently
High-speed video shows that bats achieve some of their efficiency in flight by pulling their wings inward on the upstroke, as seen above. While this does affect drag forces on the wing slightly, the primary energy savings comes from the inertial ease of lifting the folded wing. Much the way it is easier to lift your arm when it is folded than when you stretch it outright, it takes less energy for the bat to lift a folded wing than one that is fully extended. (via Wired Science)
April 17, 2012
Brine Shrimp Swimming
For small creatures, swimming is dominated by viscosity. Here researchers use particle image velocimetry (PIV) to explore the flow field around brine shrimp. Its motion is divided into two vorticity-generating phases–the wide power stroke where the shrimp generates most of its forward motion and the recovery stroke where the shrimp returns its starting position while generating as little motion and drag as it can. (Video credit: B. Johnson, D. Garrity, L. Dasi)
April 5, 2012
Micro Air Vehicle Flow Viz
A smoke wire shows the deformation of streamlines around a swept-winged micro air vehicle (MAV). These crafts typically feature wingspans smaller than one foot and, thus, never develop the type of flow fields associated with larger fixed-wing airplanes. This complicates theoretical predictions of lift and drag for MAVs as well as making them difficult to control. MAVs have numerous commercial and military applications, including search and rescue operations. (Photo credit: Tom Omer)
March 28, 2012
Flying Squid
Ever seen a squid fly? Not many have, but the behavior may be more common than you think. Thanks to a set of photos from an amateur photographer, scientists have managed to estimate the velocity and acceleration of squid as they propel themselves out of the water by squirting a jet behind them. Researchers found that their speeds in air are roughly five times that in water, thanks to decreased drag. Previously it was thought that the flying behavior might be linked to escaping predators, but some now suggest that it enables migration over long distances by saving energy.
February 22, 2012

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