Adding polymer to a liquid was thought to reduce drag only up to a point, but new experiments have found exceptions to the usual limit.
Frictional 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  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 fluid-dynamical properties associated with MDR.
The drag-reducing effects of polymer in a fluid were discovered by chance. In 1946, B. A. Toms  was studying the mechanical degradation of long-chained polymer molecules in water flowing through a pipe. He found that dissolving a minute amount of polymer in the fluid reduced drag up to 70%, even though it had practically no effect on the fluid’s shear viscosity. To this day, the exact mechanism for this drag reduction is obscure. But the leading explanation is that the polymers interact with the flow by stretching and removing energy from the turbulent velocity fluctuations. The polymers’ response reduces momentum transport towards the wall and, in turn, drag .
Read more: Polymers Reduce Drag More than Expected
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