The characteristics and mechanism of microbubble drag reduction on the axisymmetric body

To investigate the characteristics and mechanism of microbubble drag reduction on the axisymmetric body, both experiments and the numerical simulation of microbubble drag reduction have been conducted in this paper. Based on the experiments, the morphology of microbubble flow and the characteristics of microbubble drag reduction were analyzed. The size distribution of microbubbles and the influence of microbubble size on the drag reduction efficiency were quantitatively investigated. Based on the numerical simulation, the influences of microbubbles on the turbulence intensity and vorticity were analyzed to investigate the mechanism of microbubble drag reduction. The results indicate that the diameters of microbubbles basically obey the normal distribution at various conditions, and the microbubble flow presents the uniform microbubble, roll-up and cavity. As the air injection rate increases, the drag reduction respectively shows microbubble drag reduction, mixture drag reduction and air-layer drag reduction. Besides, the drag reduction ratio correspondingly presents the increasing stage, rapidly increasing stage and stable stage. At the same void ratio, the smaller microbubbles show the higher efficiency in drag reduction. The microbubbles injected into the boundary layer can reduce the turbulence intensity and the frequency of the bursting events in the flow field, which results in the drag reduction.