Bubble drag reduction requires large bubbles

TL;DR

This study reveals that large bubbles are essential for effective drag reduction in turbulent flows, showing that surfactants which prevent bubble coalescence significantly diminish the drag reduction effect.

Contribution

It demonstrates that bubble deformability, influenced by bubble size, is critical for drag reduction, highlighting the importance of bubble size control in turbulent flow applications.

Findings

Surfactants reduce bubble size by preventing coalescence.

Drag reduction drops from over 40% to about 4% with surfactant addition.

Bubble deformability is crucial for effective drag reduction.

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.