Why Fibers Are Better Turbulent Drag Reducing Agents Than Polymers

It is typically found in literature that fibers are not as effective as drag reducing agents as polymers, however, for low concentrations, when adding polyelectrolytes (charged polymers) to distilled water, it is found that they are better drag reducing agents than polyelectrolytes in salt water [1]. Since polyelectrolytes in distilled water show rod-like behavior, and polyelectrolytes in salt water show coil-like behavior, this suggests that the common notion that polymers are better at reducing drag needs to be revised.

In this study [2], 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 are able to explain why polyelectrolytes in distilled water (i.e fibers) are better drag reducing agents than polyelectrolytes in salt water (polymer coils). Additionally, 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.

References:
[1] P.S. Virk (1975), "Drag reduction by collapsed and extended polyelectrolytes", Nature, 253, 109-110
[2] A.M.P. Boelens, M. Muthukumar (Forthcoming) "Rotational relaxation time as unifying time scale for polymer and fiber drag reduction", PRE