Tag: science

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: Archery Physics
Archery is one of the oldest Olympic sports, but the physics involved are remarkably complex. Even looking only at the flight of the arrow, the problem is hardly simple. The heavy point of the arrow makes it front-heavy, and the fletches on the back of the arrow provide additional surface area on which air can act. This means that the center of mass of the arrow–where gravity acts–is further forward than the center of pressure–where aerodynamic forces act. This results in the aerodynamic forces helping to stabilize the flight of the arrow. To see why this is important, try throwing a dart fletching first!
When an arrow is fired from a bow, as in the high speed video above, the sudden impetus of force from the bowstring causes the arrow to flex and vibrate as it is fired. The aerodynamic forces generated by the fletches straighten the arrow’s flight, helping it reach the intended target accurately. Some fletching is designed to make the arrow spin; this can further improve accuracy but comes at the cost of speed since some of the arrow’s initial kinetic energy must be converted to rotation. For more, check out Archery Report, which features some great articles on the physics of archery and even has CFD comparing arrow tips. Mark Leach also has some great information on tuning a bow, which, if done properly, allows one to accurately shoot unfletched arrows.
FYFD is celebrating the Olympics by looking at the fluid dynamics of sports. Check out our previous posts on how the Olympic torch works and what makes a pool fast.
July 31, 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
The Olympic Torch
[original media no longer available]
Today marks the beginning of the 2012 Olympic Games in London. In the opening ceremony, the Olympic flame will complete its journey from Olympia to London, having been carried by some 8,000 torch bearers. Modern Olympic torches are expected to withstand wind, rain, snow, and human error to keep the flame alive and are specially designed and tested for these conditions. Each individual torch is fueled by a mixture of propane and butane stored as a pressurized liquid. The liquid fuel travels through a series of evaporation coils around the burner before combustion. Each torch carries sufficient fuel to burn about fourteen minutes. In addition to computer simulation, the 2012 Olympic torch design was tested in BMW’s Environmental Wind Tunnel to ensure a visible, stable flame for orientations within 45 degrees of vertical in conditions ranging from -5 degrees to 40 degrees Celsius, rain, snow, 35 mph winds, and 50 mph wind gusts. For more on the current torch and previous designs, see How Stuff Works, E&T, and the BBC.
FYFD is celebrating the Olympics by featuring the role of fluid dynamics in sports starting Monday. If you have any burning questions, feel free to ask or email!
July 27, 2012
Oil in Alcohol
In this video two droplets of oil fall through a bath of isopropyl alcohol. The oil is denser than alcohol, and the two fluids are miscible. The velocity and density gradients where the two fluids meet generate hydrodynamic instabilities that create the distinctive patterns seen in the falling drops. (Video credit: BYU Splash Lab)
July 26, 2012
Mussels
In this video, schlieren imaging is used to make visible the flow field around a mussel. Mussels are filter-feeders, drawing nearby water in to obtain their food and expelling the unneeded fluid once they’ve gathered the plankton they eat. Normally this process is invisible to the naked eye, but schlieren imaging reveals changes in density (and thus refractive index) that make it possible to visualize the outflow from the mussel. The technique is also commonly used in supersonic flows to reveal shock waves. (Video credit: Stephen Allen)
July 25, 2012
Sally Ride
Today FYFD takes a brief aside from fluid dynamics to mark the passing of Sally Ride, the first U.S. woman to travel to space. A physicist by training, Ride served as a mission specialist on STS-7 and STS-41G, shuttle missions that included deploying satellites as well as conducting scientific experiments. After her career with NASA, Ride returned to physics as a faculty member at the University of California, San Diego and dedicated herself to motivating children and young adults–most especially women–to pursue careers in science, math, and engineering. She was an inspiration and role model to more than a generation; her courage and her passion for science touched many lives, including my own. Godspeed, Dr. Ride.
July 24, 2012
How Maple Seeds Fly
Maple tree seeds flutter and spin as they descend. The above video, which shows flow visualization of a freely falling seed, demonstrates that the so-called helicopter seed’s autorotation creates a vortex along the leading edge. Watch as the seed’s “wing” sweeps through and you will notice the vortex along the upper surface. This leading edge vortex generates high lift on the maple seed, allowing it to stay in the air more effectively than other seeds, thereby increasing the maple’s reproductive range. (Video credit: D. Lentink et al.; see also Supplemental Materials)
July 23, 2012
Vibrating Mercury
A drop of mercury on a vibrating teflon surface assumes various mode shapes as the amplitude and frequency of oscillation are changed. Note the geometry and symmetry of the mode shapes. Near the end of the movie, the mercury oscillates chaotically and all symmetry and pattern is broken. (Because mercury is toxic, do not try this experiment at home.)
July 19, 2012