LEADER OF THE PACK. A new study shows why it’s sometimes better to stay out front.
Lance Armstrong, the cyclist who won the Tours de France seven times, often came in first because he spent so much time in second. That is, he would regularly pedal right behind a teammate whose job was to obligingly break up the stream of oncoming air, making it easier for Armstrong to save his own energy for a sprint later on. Stock cars also often maneuver to be in the draft of the car in front, thus reducing drag. A new study, however, suggests that this strategy of staying right behind a leader can backfire.
Bikes and stock cars are rigid bodies which cast a definite wind shadow. But if the object out front is a flapping body, such as a wiggling fish, a waving flag, or a bird beating its wings, then the disturbed flow set up by the flapping can increase, rather than decrease, aerodynamic drag for the follower. Not only does the follower experience more drag---forcing him to expend more energy go keep up---but the leader feels less drag.
This hypothesis is difficult to test on living animals such as birds or fish so two scientists performed an experiment with tiny waving flags. Leif Ristroph of Cornell University and Jun Zhang at New York University used two flags. Instead of a stream of air they used a flowing soap film that allowed clear images to be taken of the complex patterns set up when the fluid comes past the flags.
The result was surprising. Not only was the drag for the following flag made worse by the swirling fluid, but the measured drag felt by the leader was reduced, by as much as 50 percent, below the drag it feels when it is by itself. This is because the commotion set up by the following flag can mitigate the drag felt by the leader.
Zhang says that it’s too early to confirm that for some animals-such as migrating birds and schools of fish-being the leader of the pack is better because it reduces the energy needed to counteract drag. So far the experiment has been carried out with two flags and with six flags, and Zhang and his colleague would like to study their ideas with real animals. He believes that his results might have industrial applications, where reducing energy input is almost always a good thing. P.F. Schewe
( Physical Review Letters, 7 November 2008))