Polymers Tame Turbulent Flow
Adding polymers to a liquid speeds up its flow through a pipe by reducing the frictional drag at the pipe’s inner surface. This so-called Toms effect has long been used in the oil industry, for example, to improve flow rates in pipelines. Models can explain the drag reduction, but it has been unclear how the polymers affect bulk flow—that is, away from pipe walls. Now Heng-Dong Xi from Northwestern Polytechnical University in China and colleagues have found that the polymers suppress eddy formation and thus reduce the loss of flow energy to heat [1]. “Our study in bulk flow provides a clearer view of the [polymer] mechanism without the complicating presence of walls,” Xi says.
The researchers added long-chain polymers at various concentrations to a water-based solution and poured the mixture into a transparent cylindrical tank. Counterrotating baffles inside the tank generated a swirling, turbulent flow. An imaging system captured the flow velocity in the center of the tank, and from this 3D velocity map, the team identified different flow structures, such as vortex-like eddies and sheet-like streams.
In turbulent flows, energy cascades from large-scale streams down to small-scale eddies before being lost as heat. Xi and colleagues showed that polymers reduce this dissipation by suppressing the formation of eddies below a certain length scale. This idea is not new, but the researchers found that the dissipation stops decreasing above a threshold polymer concentration. They also showed that this threshold occurs once the size of the suppressed eddies reaches the maximum possible eddy size, called the integral scale. As the suppressed-eddy scale depends on certain polymer parameters, engineers might use these results to select ideal polymers for particular flow applications.
–Michael Schirber
Michael Schirber is a Corresponding Editor for Physics Magazine based in Lyon, France.
References
- Y.-B. Zhang et al., “Maximum dissipation reduction in bulk polymeric turbulence,” Phys. Rev. Lett. 135, 154001 (2025).