title: Bubbly turbulent drag reduction is a boundary layer effect.
abstract: In turbulent Taylor-Couette flow, the injection of bubbles reduces the overall drag. On the other hand, rough walls enhance the overall drag. In this work, we inject bubbles into turbulent Taylor-Couette flow with rough walls (with a Reynolds number up to 4 x 10(5), finding an enhancement of the dimensionless drag as compared to the case without bubbles. The dimensional drag is unchanged. As in the rough-wall case no smooth boundary layers can develop, the results demonstrate that bubbly drag reduction is a pure boundary layer effect.
authors.affiliation: Department of Applied Physics, IMPACT, and J.M. Burgers Center for Fluid Dynamics, Physics of Fluids Group, University of Twente, P.O. Box 217, 7500 AE Enschede, Netherlands.
authors.forename: Thomas H
authors.initials: TH
authors.lastname: van den Berg
authors.forename: Dennis P M
authors.initials: DP
authors.lastname: van Gils
authors.forename: Daniel P
authors.initials: DP
authors.lastname: Lathrop
authors.forename: Detlef
authors.initials: D
authors.lastname: Lohse