Flow-field evolution and hydrodynamic resistance during water exit of a foldable-wing vehicle

The water-exit process of foldable-wing unmanned aerial-underwater vehicles (F-UAUVs) involves strongly unsteady free-surface deformation, structural drainage, and rapidly varying hydrodynamic loading. These effects become particularly complex when realistic internal structures are retained, yet their contribution to nonlinear resistance remains insufficiently understood. This study investigates the full-scale Longbow II F-UAUV using a volume-of-fluid (VOF)-based CFD method validated against experimental resistance histories at two representative exit conditions. The calculated error in hydrodynamic resistance is less than 8.76%. Constrained water-exit simulations are conducted over 0.28 ≤ Fr ≤ 0.42 and 75°≤ θ ≤90°to examine the coupled evolution of the free surface, entrained and enclosed water, and component-wise hydrodynamic resistance. Four successive stages are identified according to the emergence of the main body, wing-body configuration, and foldable wings. The nonlinear resistance evolution is closely associated with waterplane-area variation and the redistribution and fallback of entrained and enclosed water. Internal structural features alter drainage paths and generate localized unsteady interactions, while different vehicle components dominate the resistance response at different stages. Within the investigated range, exit velocity mainly changes resistance magnitude and water redistribution, whereas pitch angle primarily affects the timing and intensity of characteristic resistance events. These findings provide a physical basis for hydrodynamic modelling and water-exit design of structurally complex F-UAUVs.