Rotational relaxation time as unifying time scale for polymer and fiber drag reduction

TL;DR

This study proposes that the rotational orientation time is a universal time scale governing drag reduction in turbulent flows for both polymers and fibers, based on numerical simulations comparing their stress tensors.

Contribution

It introduces the rotational orientation time as a unifying time scale for drag reduction, supported by simulation evidence linking polymer and fiber stress behaviors.

Findings

Polymer and fiber stress tensors are similar in turbulent flow.

Viscous effects dominate the drag reduction mechanism.

Rotational orientation time sets a new criterion for drag reduction.

Abstract

Using hybrid Direct Numerical Simulation with Langevin dynamics, a comparison is performed between polymer and fiber stress tensors in turbulent flow. The stress tensors are found to be similar, suggesting a common drag reducing mechanism in the onset regime for both flexible polymers and rigid fibers. Since fibers do not have an elastic backbone, this must be a viscous effect. Analysis of the viscosity tensor reveals that all terms are negligible, except the off-diagonal shear viscosity associated with rotation. Based on this analysis, we identify the rotational orientation time as the unifying time scale setting a new time criterion for drag reduction by both flexible polymers and rigid fibers.