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

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.