Optical Distortion Induced by Cavitating Flow over Submerged Hydrofoils: A Coupled Computational Fluid Dynamics and Ray Tracing Study

Abstract

Autonomous underwater vehicles (AUVs) increasingly rely on optical sensors for navigation, reconnaissance, and environmental monitoring; however, high-speed operation near the air-water interface generates free-surface waves and cavitation that create strongly non-uniform density and refractive-index fields, causing image distortion beyond the scope of conventional ray-tracing models. In this study, a coupled hydro-optic framework is developed by integrating multiphase computational fluid dynamics (CFD) and optical ray tracing to quantify distortion in cavitating free-surface flow over submerged hydrofoils. The flow field is solved using the Volume of Fluid (VOF) method with the Schnerr–Sauer cavitation model and a standard k–ε turbulence closure, and the resulting density field is mapped to refractive index through the Gladstone– Dale relation. Light propagation is then computed by numerically integrating the Eikonal equation using a fourth-order Runge–Kutta scheme. The CFD model is validated against published hydrofoil pressure-coefficient data, and the ray-tracing solver is validated against an analytical Eikonal solution. A parametric study of hydrofoil thickness, camber, angle of attack, immersion depth, and cavitation number shows that cavitation and wave deformation jointly alter ray trajectories and compress the effective imaging field. For example, in the immersion-depth study (h/c= 0.5,0.7,1), the free-surface wave amplitude decreases from 0.09cto 0.06c, while the camera field of view narrows from to . In addition, increasing hydrofoil thickness (NACA 0009 to NACA 0015) reduces the maximum viewing angle from approximately to under the reported operating conditions. These results demonstrate the coupled role of cavitation-bubble growth and wave-shape evolution in hydro-optic distortion and provide a predictive basis for designing more robust AUV imaging systems.