Flow field and load characteristics of a trans-media unmanned aerial vehicle in the water-exit process

When a quadrotor unmanned aerial vehicle crosses the water–air interface, it encounters strongly unsteady multiphase interactions that induce abrupt and irregular load variations. To resolve this process, a numerical framework integrating the overset grid method with a volume-of-fluid approach was developed, allowing detailed capture of the water-exit dynamics. The model was validated against classical sphere water-exit experiments, where the predicted free-surface peak differed by less than 6%, supporting its reliability. The flow evolution exhibits distinct stages, including initial surface disturbance, liquid film attachment, and subsequent fragmentation and shedding, rather than following a continuous progression. The influence of key control parameters on both aerodynamic loading and vehicle stability was systematically examined. Increasing rotational speed not only enhances thrust and exit velocity but also intensifies flow-induced fluctuations, which may lead to noticeable attitude instability. This trade-off is particularly sensitive to the timing of speed transition. A switching height of approximately 0.05 m yields a more balanced response, whereas earlier switching introduces torque asymmetry due to persistent fluid coupling. Vehicle attitude further affects the dynamics. Even slight deviations from vertical alignment produce asymmetric near-surface interactions, altering thrust distribution among rotors. As the inclination increases, this imbalance grows rapidly; for example, an initial tilt of 15° can result in pitch deviations exceeding 30°, indicating loss of control. These findings highlight the nonlinear nature of cross-medium transitions and emphasize the need for coordinated structural and control design.