This paper presented a numerical simulation of drag reduction on a superhydrophobic surface with a groove structure. The computational fluid dynamics (CFD) method was used to analyze the effect of the groove microstructure when droplets impacted a superhydrophobic surface. The simulation results revealed three main characteristics: (1) The distance the droplet spread was larger along the direction parallel to the groove and smaller perpendicular to the groove; (2) Two protruding small spheres were formed at the edge of the droplet along the groove direction; (3) During retraction, the droplets presented a narrow, cross-shaped morphology. The effect of the groove structure in the two-dimensional microchannel on drag reduction near the wall was analyzed based on slip theory by the coupled level set/volume of fluid (CLSVOF) method. The air in the superhydrophobic pit formed a low-velocity vortex, which made the fluid roll on the air surface. The rolling on the surface produced a velocity slip at the gas-liquid interface. In addition, the superhydrophobic surface had an obvious drag reduction effect in the laminar flow state, but the drag reduction effect in the turbulent state was not ideal and even increased the flow resistance at the wall.

The internal flow of hydrodynamic retarder is a three-dimensional, complex, viscous, and unstable gas–liquid two-phase flow. The importance of numerical simulation of a hydrodynamic retarder has been paid attention to, but the research on the two-phase flow pattern of the retarder is rarely. Flow pattern diagnosis plays an important role in the study of two-phase flow because the premise of establishing the mathematical model of the two-phase flow is to accurately determine the flow structure of the flow system. Based on the numerical simulation and experimental research, this study found the corresponding relationship between the two-phase flow distribution inside the hydrodynamic retarder and the filling rate (expressed as “q”). The result shows that, when q = 0.1–0.2, the liquid in the working chamber is less. Under the action of centrifugal force, the liquid mainly gets distributed in the outer ring of the circulating circle, forming a laminar flow. When q = 0.3–0.4, the flow pattern of the gas–liquid two-phase flow changes from the laminar flow to slug flow. When q = 0.5–0.9, with a further increase in the filling ratio, the flow becomes a dispersed bubble flow. This paper provides an effective method for the analysis and identification of the two-phase flow pattern of a hydrodynamic retarder.