IST Austria, 3400 Klosterneuburg, Austria.
Phys Rev Lett. 2018 Mar 23;120(12):124501. doi: 10.1103/PhysRevLett.120.124501.
The drag of turbulent flows can be drastically decreased by adding small amounts of high molecular weight polymers. While drag reduction initially increases with polymer concentration, it eventually saturates to what is known as the maximum drag reduction (MDR) asymptote; this asymptote is generally attributed to the dynamics being reduced to a marginal yet persistent state of subdued turbulent motion. Contrary to this accepted view, we show that, for an appropriate choice of parameters, polymers can reduce the drag beyond the suggested asymptotic limit, eliminating turbulence and giving way to laminar flow. At higher polymer concentrations, however, the laminar state becomes unstable, resulting in a fluctuating flow with the characteristic drag of the MDR asymptote. Our findings indicate that the asymptotic state is hence dynamically disconnected from ordinary turbulence.
在湍流中加入少量高分子量聚合物可以显著降低 drag。虽然 drag 减少最初随聚合物浓度的增加而增加，但最终会达到所谓的最大 drag 减少 (MDR) 渐近线；这个渐近线通常归因于动力学降低到一个边缘但持久的抑制湍流运动状态。与这种公认的观点相反，我们表明，对于适当的参数选择，聚合物可以将 drag 降低到建议的渐近极限之外，消除湍流并导致层流。然而，在更高的聚合物浓度下，层流状态变得不稳定，导致具有 MDR 渐近线特征 drag 的脉动流。我们的研究结果表明，渐近状态与普通湍流在动力学上是分离的。
