The effect of microbubbles on a flat plate turbulent boundary layer.

The purpose of this paper is to investigate the drag reduction on a flat plate turbulent boundary layer in presence of microbubbles at low and moderate Reynolds number Re? =2085 and Re? =3060. The mean size of the bubbles was about 0.1mm, corresponding to 3 wall lengths at the higher Re and to about 2 wall lengths at the lower Re. Different bubbles concentrations were adopted in order to quantify the effect of the bubbles on the drag. Measurements were carried out by two-component PIV system and data were analyzed using an innovative technique, which allows for the evaluation of the velocity around the bubbles. Second order statistics of the velocity have been evaluated in all cases in order to describe the modification of the turbulent structure in the boundary layer in presence of micro-bubbles.