  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.
