Experimental Study of the Bubbly Drag Reduction in the Recovery Region of a Separated Turbulent Boundary Layer

•The bubbly friction reduction was achieved in a reattached turbulent boundary layer flow•Two bubbly flow regimes – turbulent dispersion dominant and buoyant dominant regime are evidenced.•Gas volume fraction profiles are self-similar in the turbulent dispersion regime.•gas volume fraction waves play the major role in friction reduction in turbulent dispersion dominant regime.•In buoyant dominant regime, friction reduction is achieved by bubble wall-normal oscillation under bubble deformability. Bubbly drag reduction in turbulent flows is of significant interest in the naval industry. In this paper, we investigate experimentally the transition from bubbly drag increase to bubbly drag reduction at moderate to high Reynolds numbers and analyze the contributions of different mechanisms. The experiments were performed in a water tunnel. To achieve Reynolds numbers of the boundary layer, of the same order as the ones characteristic of a flow over a10 meters long plate, the boundary layer at the upper wall was thickened by mounting a2D obstacle. The single-phase turbulent boundary layer is strongly disturbed by the obstacle: it separates and reattaches farther in the recovery region. Air bubbles of intermediate size (0.4−1.3mm) were injected in the recovery region. Four upstream velocities were tested:2,4,6 and8m/s, corresponding to a Reynolds number ranging between 21000 and 69000 (based on the momentum thickness of the single-phase disturbed boundary layer at the measurement location of the bubbly flow). The average air volume fraction in the disturbed boundary layer 〈α〉 was varied up to0.08%. The liquid-phase velocity field in the vertical plane was investigated by particle tracking technique and the friction velocity was deduced from the logarithmic law of the wall. The gas-phase velocity field, volume fraction distribution and the bubble size were characterized by means of shadowgraphy in the vertical plane. Two regimes are highlighted: the buoyancy dominant regime at2m/s (regime I) for which bubbles are sliding along the wall and the turbulent dispersion dominant regime achieved at higher velocity (regime II) for which there is a competition between buoyancy and turbulence dispersion. It is inferred that the correlation between the gas volume fraction fluctuations and the liquid wall normal fluctuating velocity plays the major role in the friction reduction by reducing the total turbulent stress of the liquid. In regime II, gas volume fraction waves are pr