VIEWPOINT Polymers Reduce Drag

F rictional drag steals energy from a moving fluid, but the loss usually becomes greater as the flow goes from smooth, or “laminar,” to turbulent. The onset of turbulence therefore poses a problem for many situations involving fluid flow through a conduit, be it oil in a giant pipeline, blood in a human aorta, or liquid in a heat exchanger. One established solution is to add a small amount of polymer to the fluid, which reduces drag by suppressing turbulence. Decades of experiments, however, have indicated that this approach reduces drag only down to a certain level, known as the maximum drag reduction (MDR) asymptote. Beyond this limit, adding more polymer has no effect. A team led by Björn Hof [1] at the Institute of Science and Technology in Austria has now uncovered a window of flow conditions under which drag can be reduced beyond the usual MDR limit. Their experiments with water and common polymers also offer a new picture of the fluiddynamical properties associated with MDR.