Flow-drag reduction performance of a resident electrolytic microbubble array and its mechanisms

Abstract

This work examines the problem of stable high-efficiency flow drag reduction for underwater vehicles using microbubble arrays. To this end, test pieces for micropore arrays capable of forming self-stabilizing arrays of microbubbles via electrolysis have been prepared. The quantitative relationships between the electrolysis voltage, micropore size, and flow velocity (v) and the growth behaviors and residence stability of electrolyzed microbubbles were examined by experimental and numerical methods. The flow drag reduction performance and mechanisms of microbubble arrays were investigated. It has been shown that an electrode-wall micropore array can be used to achieve self-adaptive start-stop control of microbubble electrolysis. A micropore diameter-to-depth ratio of k = 2 was found to be optimal for generating arrays of electrolytic microbubbles with good stability in flow. Two types of microbubbles are generated: the air-film type exhibits greater resident stability than the protruding type. The average measured flow-drag-reduction rate of a microbubble array with 250 μm micropores was found to be around 26%. The energy-loss region of free-fluid flow with a solid wall was 0.037 mm thicker than that with a microbubble-array wall (v = 0.5 m/s), meaning that more energy is consumed, leading to greater flow drag. Significant up-thrown flow induced by elastic deformation of microbubbles and air/water interface forces can suppress turbulent coherent structural bursts and reduce near-wall Reynolds shear stress. The mean values of turbulence-related characteristic parameters (vorticity = 890.55 s−1, kinetic energy = 0.08096 m2/s2, shear stress = 17.75 Pa, and v = 0.5 m/s) of the boundary-layer flow over the microbubble-array wall were found to be greatly reduced. This demonstrates the good turbulent friction drag reduction performance of the resident microbubble array.