Polymers Reduce Drag More than Expected

Adding polymer to a fluid is known to reduce drag, but the effectiveness was thought to taper off beyond a certain concentration called the maximum drag reduction asymptote (dotted line.) Hof and colleagues carefully measured the friction factor (f) in a liquid for different polymer concentrations (C) and Reynolds numbers (Re). They uncovered a window of C and Re values in which they could reduce drag below the usual limit. The friction-versus-concentration measurement shown here corresponds to Re =3150 and the polymer polyacrylamide. The insets show different types of fluid behavior: Newtonian turbulence (left); laminar flow (center); and elastoinertial turbulence (right). Red and blue regions have higher than average flow velocities.

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 [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 fluid-dynamical properties associated with MDR.

The drag-reducing effects of polymer in a fluid were discovered by chance. In 1946, B. A. Toms [2] 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 [3].

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