Research on the water surface effect of a propeller during vertical takeoff

The UAUV (Unmanned Aerial Underwater Vehicle) is a novel aerial vehicle capable of flight, underwater navigation, and multiple active transitions between air and water. Multi-rotor and hybrid-wing UAUVs require ascending to the water surface for takeoff, at which stage the propeller is positioned extremely close to the water surface. When the propeller interacts with the free surface, its lift magnitude and flow phenomena differ from those of a propeller operating in undisturbed air. This study investigates the propeller 3090 and the water surface effect through high-fidelity computational fluid dynamics, focusing on the height and rotational speed. The key difference between this study and IGE (in-ground effect) research is the presence of a deformable water surface and the lower operational height. The thrust of the propeller increases as the height decreases, and the power requirement decreases with height. No abnormal thrust variation analogous to that in IGE is detected. It also produces lower lift compared to IGE at greater heights, and higher lift very close to the water surface. The rate of lift increase varies with height, with more pronounced nonlinearity in growth as the propeller approaches the water surface. At last, a three-parameter empirical formula with an average percentage error of lift coefficient ratio of 0.53% is derived based on the summarization and analysis of these phenomena, and its accuracy and generalization capability are validated.