Modal response characterizations and mechanism separation modeling of amphibious vehicle body under different water entry depths

Amphibious vehicles play a vital role in flood control, emergency response, and cross-terrain operations. However, most existing studies have focused on their hydrodynamic performance, while the structural vibration characteristics of amphibious vehicles under varying submersion conditions remain insufficiently addressed-particularly in terms of dynamic frequency evolution and modal transition behavior. This study focuses on analyzing the variation of wet natural frequencies in an amphibious vehicle structure subjected to different water entry depths. To distinguish the influence of different fluid-induced effects on modal response, the interaction is decoupled into fluid-added mass and hydrostatic prestress. A coupled ANSYS-Fluent framework is used to simulate a simplified amphibious hull model and evaluate the corresponding dry, wet, and prestress-induced modal behaviors. Results indicate that fluid-added mass reduces natural frequencies by 42%-53%, while hydrostatic prestress contributes an additional 23%-38% reduction due to geometric stiffness degradation. Notably, a first-order modal transition is observed at a water depth of 0.8 m, attributed to geometry-induced coupling changes. Higher-order modes exhibit stable behavior beyond 1.0 m immersion, revealing a frequency saturation region. This study provides a systematic analysis of modal evolution and transition under varying submersion conditions and offers physically grounded insights for the dynamic performance assessment of amphibious structures.