Professional cyclists can reduce their drag by following close behind the rider in front. Counterintuitively, downstream flags feel more drag than those flapping in front of them.
Leif Ristroph of Cornell University in Ithaca, New York, and Jun Zhang of New York University in New York City simulated onedimensional flags by tethering tiny threads in a flow of soap film (pictured). The upstream thread's flapping was constrained by the proximity of the fixed 'flagpole' — the leading portion — of its downstream partner. Meanwhile, the downstream flag was caught in the leader's oscillating wake.