Effects of microbubble injection in a turbulent boundary layer.

A quantitative description of the effects associated with small concentrations of microbubbles (void fraction Cv ? 10-3 ) released in the turbulent boundary layer over a flat plate is presented here for the case where buoyancy tends to push the bubbles towards the wall ("plate on top"). Measurements of the bubble concentration and of the velocity of the carrier (liquid) phase have been performed by means of optical techniques. Bubbles are found to preferentially accumulate in the buffer region of the boundary layer and to strongly interact with the vortical structures. The main effect of this interaction is a substantial decrease of the mean velocity in the proximity of the plate, accompanied by a marked reduction of the Reynolds stress. The analysis of two-point correlation maps suggest that the mechanism underlying the drag-reduction phenomenon concerns the disruption of the coherence of the near-wall vortical organization.