Nonlinear hydrodynamic coupling effects of free-surface and bottom boundaries on an AUV

Autonomous underwater vehicles (AUVs) frequently operate in shallow waters, where they are simultaneously subjected to the coupling effects of free-surface wave-making and bottom boundary restrictions. Existing studies have primarily focused on single-boundary conditions, leaving the complex, non-linear coupled hydrodynamics in dual-boundary shallow water environments insufficiently understood. In this study, a computational fluid dynamics (CFD) method based on the Reynolds Stress Model (RSM) and the Volume of Fluid (VOF) approach is employed to systematically investigate the hydrodynamic characteristics of the DARPA SUBOFF bare-hull model in shallow water. Numerical simulations were conducted across various Froude numbers ( F n = 0.205 ∼ 0.512) and submergence depths, with lateral comparisons made against deep water, free-surface-only, and bottom-only conditions. The results indicate that the dual-boundary coupling is not a simple linear superposition of single-boundary effects. Although this coupled environment typically amplifies the pressure drag (exceeding four times that of the deep water condition at the highest forward speed), strong non-linear interference generated at specific speeds (e.g., F n = 0.319) induces a partial cancellation effect, resulting in a drag actually lower than that in the free-surface-only condition. Furthermore, the vertical force (lift) exhibits highly non-linear characteristics. At greater submergence depths or high forward speeds, the combined action of bottom channel flow acceleration and the free-surface effect trigger a reversal in force polarity, generating a strong downward suction force whose amplitude surpasses even that of the bottom-only condition. Simultaneously, the dual-boundary shallow water environment amplifies the asymmetry of the longitudinal pressure distribution, inducing a significantly enhanced bow-down pitching moment. The findings contribute to improving hydrodynamic load prediction and operational safety assessment of AUVs operating in confined shallow-water environments.