In rowing, as in any water sport, drag comes in three varieties: skin friction, form (or pressure) drag, and wave drag. Skin friction comes from the friction between the hull and water causing the boat to drag water with it as it moves. This can be mitigated with the right materials and surface finish but will never be completely negligible. In fact, the racing shells used in rowing are unusual for boats because skin friction is their major source of resistance. This is because form drag, caused by the shape of the boat cutting through the water, and wave drag, the energy lost due to the waves that form along the hull, are small in racing shells due to their long, narrow, and streamlined shape. Because skin friction dominates among the three types of drag, the force a rower overcomes to move the boat is proportional to the hull’s velocity squared, and the power required to do so is proportional to the hull’s velocity cubed. This means that it is more efficient for rowers to keep a constant hull speed throughout a race than it is to start slow and speed up or start fast and slow down because the work (power x time) needed to keep a constant speed is smaller. For more on the physics of rowing, check out Anu Dudhia’s excellent website or this video from Physics of Life. (Photo credits: Ecouterre, AP)
FYFD is celebrating the Olympics by featuring the fluid dynamics of sport. Check out some of our previous posts, including what makes a pool fast, how divers reduce splash, and the aerodynamics of badminton.
Fans of swimming will recall the controversies of the now-banned sharkskin-style swimsuits that helped break so many records in the past few years. The suits decrease drag on a swimmer both by making them more hydrodynamic in form and by drastically reducing skin friction where the water meets the swimmer’s body. In addition to decreasing the two major sources of drag on a swimmer, the compression provided by the material can help increase blood flow to muscles. These improvements came at a high material cost, though, and, since the technology was not viable for all athletes, it has since been banned.
Here laminar and turbulent flows, basic concepts in fluid mechanics, are demonstrated in the kitchen sink! While laminar flow is often desirable for decreasing drag due to friction, most practical flows are turbulent. The hissing the video author associates with the onset of turbulence is not a coincidence either. The chaotic motion of turbulent flows can produce aerodynamic noise like the roar produced by airplane propellers or the hum of electrical lines in the wind.
Unlike road stages in which cyclists can draft off one another to reduce drag, in the time trial a cyclist is on a solo race against the clock with nowhere to hide. As a result, the event features lots of technologies designed to reduce both pressure drag and skin friction on the cyclist. For time trials, cyclists wear skinsuits and shoe covers to eliminate any sources of flapping fabrics and to reduce skin friction. They ride bicycles designed to be as light and aerodynamic as possible. Instead of rounded tubing in the frames, these bikes consist of elongated airfoil profiles that direct air past and prevent separation that may increase pressure drag. The rims of their tires are wider and the back wheel is replaced with a disc wheel that allows no airflow aross the wheel. Like the airfoil tubing, these changes help prevent separation. Similarly, riders wear elongated helmets designed to be as aerodynamic as possible while the rider is in the “aero” position, with arms directed out over the wheels, head level, elbows tucked, and back flat. In wind tunnel tests, the rider best able to hold this position will experience the least drag. Even the addition or subtraction of a water bottle is not left to chance, with many time trial bikes designed to be more aerodynamic with a water bottle onboard (though you probably won’t catch the cyclists breaking their aero position to get a drink)! (Photos by Veeral Patel)
FYFD is celebrating the Tour de France with a weeklong exploration of the fluid dynamics of cycling. See previous posts on drafting in the peloton, and pacelining and echelons, and the art of the lead-out train.
