关键词: 跨介质飞行器, 多旋翼, 近水面效应, 液滴, 水气交混流场, 喷泉效应

Abstract: When the cross-media rotorcrafts approach the water surface, the interaction between its downwash flow and the water surface generates mixed air-water flows. In this flow field, the rotor exhibits aerodynamic characteristics distinctly different from in-ground effect, a phenomenon known as the near-water effect. Compared to the near-water effect of single-rotor, the near-water effect of multi-rotor becomes more complex due to the aerodynamic interference between rotors and the coupling of their respective induced mixed air-water flows. Given the current lack of understanding of the near-water effect of multi-rotor, this paper conducts experiments to investigate the aerodynamic characteristics of multi-rotors near the water surface, measuring the aerodynamic forces and rotational speeds under different blade tip clearances and hovering heights. Combining existing research on the near-ground wake structure of multi-rotors and the observed morphology of the mixed air-water flows, a preliminary cognitive framework of the near-water effect of multi-rotor is established, categorized as "merging, contacting and separating." Under different mixed air-water flows conditions, the merging mode with the closest clearance exhibits the best aerodynamic performance. As the clearance increases, the contacting mode at a relatively higher distance from the water surface shows no significant thrust loss, whereas the contacting mode closer to the water surface results in a notable reduction in the rotor thrust coefficient. Additionally, in the separating mode with the largest blade tip clearance, the strong interaction between the water droplets generated by the rotor-water interaction and the rotor leads to a sharp increase in rotor torque.

Key words: cross-medium vehicle, multi-rotor, near-water effect, droplets, mixed air-water flows, fountain effect