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-medium vehicle 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， experiments are conducted 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 the existing research on the near-ground wake structure of multi-rotor 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 blade tip clearance exhibits the best aerodynamic performance. As the blade tip 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