Influence of entrapped gas morphology at liquid-solid interface on underwater drag reduction effect

Abstract

Although superhydrophobic surfaces that achieve underwater drag reduction by entrapped gas have become a consensus, few studies have analyzed the influence of bubble morphology on the drag reduction effect. Therefore, this paper proposes a grooved surface to achieve underwater drag reduction and discusses the influence of a bubble state on the underwater drag reduction effect through a numerical study. The results indicate that the morphological deformation of bubbles at different flow velocities can be divided into three states. State I: at low flow velocities, the entrapped gas remains as bubbles are contained with each groove; state II: at intermediate flow velocities, the bubbles deform and bridge multiple grooves forming a gas layer; and state III: at high flow velocities, bubbles break off of the gas layer, but the layer persists with a reduced volume. When the morphology of the bubble changes, the values of pressure drag and viscous drag also change over time. In the flow velocity range of 15-22 m/s (corresponding to states II and III), the hydrophobic grooved surface can obtain a considerable drag reduction effect. The morphology of entrapped gas at the liquid-solid interface is continuously changing, and the drag reduction effect is related to the morphology of the gas-liquid interface. In addition, experiments were carried out to verify the numerical simulation results. The results provide a theoretical basis for the surface structure design of underwater drag reduction and are helpful to the further research and applications of engineering.