Microbubble drag reduction on the axisymmetric body is experimentally investigated in the turbulent water tunnel. Microbubbles are created by injecting compressed air through the porous medium with various average pore sizes. The morphology of microbubble flow and the size distribution of microbubble are observed by the high-speed visualization system. Drag measurements are obtained by the balance which is presented as the function of void ratio. The results show that when the air injection flow rate is high, uniformly dispersed microbubble flow is coalesced into an air layer with the larger increment rate of drag reduction ratio. The diameter distributions of microbubble under various conditions are submitted to normal distribution. Microbubble drag reduction can be divided into three distinguishable regions in which the drag reduction ratio experiences increase stage, rapid increase stage and stability stage, respectively, corresponding to the various morphologies of microbubble flow. Moreover, drag reduction ratio increases with the decreasing pore sizes of porous medium at the identical void ratio in the area of low speeds, while the effect of pore sizes on drag reduction is reduced gradually until it disappears with the increasing free stream speeds, which indicates that smaller microbubbles have better efficiency in drag reduction. This research results help to improve the understanding of microbubble drag reduction and provides helpful references for practical applications.
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.
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