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How can small amounts of polymer reduce drag in turbulent water flow?

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differential manometer, Torricellian barometer, and siphon — fluid mechanics Figure 1: Schematic representations of (A) a differential manometer, (B) a Torricellian barometer, and (C) a siphon.

In the study of fluid mechanics, dissolving small amounts of suitable polymers into water can effectively reduce drag forces and increase flow rates in turbulent conditions.1

This phenomenon is notable because these additives actually increase the viscosity of the fluid, which would typically decrease flow rates in laminar regimes. In turbulent flow, however, the fluid experiences small-scale, parasitic perturbations that cascade into increasingly finer motions until the energy is dissipated as heat.1

Polymer molecules mitigate this by imposing a degree of coherence on the fluid motion over the relatively large distances they span. By inhibiting the breakdown of these perturbations into smaller, energy-dissipating scales, the polymers reduce the intensity of turbulence and the associated drag.1