Polymer and surface roughness effects on the drag crisis for falling spheres

TL;DR

This study investigates how polymer additives and surface roughness influence the drag crisis in falling spheres, revealing methods to significantly reduce or suppress drag through surface modifications and fluid properties.

Contribution

It introduces experimental insights into how surface roughness and polymer solutions affect drag crisis, highlighting new ways to control drag in fluid dynamics.

Findings

Low concentration polymers and surface grooves induce early drag crisis.

Surface roughness and high polymer concentrations suppress the drag crisis.

Drag reduction can exceed 50% with specific surface and fluid modifications.

Abstract

We make time resolved velocity measurements of steel spheres in free fall through liquid using a continuous ultrasound technique. We explore two different ways to induce large changes in drag on the spheres: 1) a small quantity of viscoelastic polymer added to water and 2) altering the surface of the sphere. Low concentration polymer solutions and/or a pattern of grooves in the sphere surface induce an early drag crisis, which may reduce drag by more than 50 percent compared to smooth spheres in pure water. On the other hand, random surface roughness and/or high concentration polymer solutions reduce drag progressively and suppress the drag crisis. We also present a qualititative argument which ties the drag reduction observed in low concentration polymer solutions to the Weissenberg number and normal stress difference.