Drag reduction by microbubbles in a turbulent boundary layer

An experimental characterization of the turbulent boundary layer over a flat plate in the presence of small amounts of microbubbles is performed. The average diameter of the injected bubbles is comparable with the local Kolmogorov lengthscale, and the bulk void fraction C- is approximately 0.1%. The velocity field of the liquid phase, as well as the bubble characteristics, is acquired by optical techniques. Even at the small void fraction typical of this investigation, the interaction between microbubbles and turbulence leads to significant modifications of the underlying flow field. In accordance with previous investigations, the main global effect consists in a reduction of the magnitude of the viscous drag. This has been checked with preliminary tests conducted in a towing tank, as well as by inferring the wall stress from the boundary layer velocity profile measured in a laboratory facility at the same conditions. Here, the local wall stress is found to drop by approximately 25%. An analysis of the turbulent statistics shows that the reduction in drag is accompanied by a substantial reduction of the momentum flux throughout most of the inner region and by a decrease of the characteristic dimension of the turbulent scales involved in the production of kinetic energy. The distribution of turbulent kinetic energy among different scales is found to be substantially altered in the two flows. In particular, an enhancement of the energy content at smaller scales is observed in the bubbly case. These effects are discussed in relation with the decrease of the coherence of near-wall flow structures induced by the bubble forcing. © 2010 American Institute of Physics.