title: Bubble Drag Reduction Requires Large Bubbles.
abstract: In the maritime industry, the injection of air bubbles into the turbulent boundary layer under the ship hull is seen as one of the most promising techniques to reduce the overall fuel consumption. However, the exact mechanism behind bubble drag reduction is unknown. Here we show that bubble drag reduction in turbulent flow dramatically depends on the bubble size. By adding minute concentrations (6 ppm) of the surfactant Triton X-100 into otherwise completely unchanged strongly turbulent Taylor-Couette flow containing bubbles, we dramatically reduce the drag reduction from more than 40% to about 4%, corresponding to the trivial effect of the bubbles on the density and viscosity of the liquid. The reason for this striking behavior is that the addition of surfactants prevents bubble coalescence, leading to much smaller bubbles. Our result demonstrates that bubble deformability is crucial for bubble drag reduction in turbulent flow and opens the door for an optimization of the process.
authors.affiliation: Department of Applied Physics, MESA+ institute and J. M. Burgers Center for Fluid Dynamics, University of Twente, P.O. Box 217, 5 7500 AE Enschede, The Netherlands.
authors.forename: Ruben A
authors.initials: RA
authors.lastname: Verschoof
authors.affiliation: Department of Applied Physics, MESA+ institute and J. M. Burgers Center for Fluid Dynamics, University of Twente, P.O. Box 217, 5 7500 AE Enschede, The Netherlands.
authors.forename: Roeland C A
authors.initials: RC
authors.lastname: van der Veen
authors.affiliation: Department of Applied Physics, MESA+ institute and J. M. Burgers Center for Fluid Dynamics, University of Twente, P.O. Box 217, 5 7500 AE Enschede, The Netherlands.|Center for Combustion Energy and Department of Thermal Engineering, Key Laboratory for Thermal Science and Power Engineering of the Ministry of Education, Tsinghua University, 100084 Beijing, China.
authors.forename: Chao
authors.initials: C
authors.lastname: Sun
authors.affiliation: Department of Applied Physics, MESA+ institute and J. M. Burgers Center for Fluid Dynamics, University of Twente, P.O. Box 217, 5 7500 AE Enschede, The Netherlands.|Max Planck Institute for Dynamics and Self-Organisation, 37077 Göttingen, Germany.
authors.forename: Detlef
authors.initials: D
authors.lastname: Lohse