Eur. Phys. J. B 60, 469-476 (2007)
https://doi.org/10.1140/epjb/e2008-00018-0

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

¹ Laboratoire de Physique de l'École Normale Supérieure de Lyon, CNRS and Université de Lyon, 46 allée d'Italie, 69364 Lyon, France
² Laboratoire de Physique de la Matière Condensée et Nanostructures, CNRS and Université de Lyon, 43 boulevard du 11 Novembre 1918, 69622 Villeurbanne, France

Corresponding author: ^a pinton@ens-lyon.fr

Received: 30 August 2007
Revised: 12 November 2007
Published online: 16 January 2008

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% 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.