Measurement and Visualization of Turbulent Free-Surface Wake Flow Using Defocusing Particle Image Velocimetry

Turbulent free-surface wakes generated by surface-piercing bodies during transient maneuvers represent a technically complex and operationally critical domain in naval hydrodynamics. The unsteady transport of mass, momentum, and energy across the dynamic air–water interface gives rise to highly anisotropic, vortical flows characterized by inherent instabilities that significantly impact the performance and safety of naval platforms. In this study, we investigate the wake flows of representative surface-piercing objects at varying angles of attack using a combination of qualitative and quantitative techniques. Fluorescent dye visualizations reveal distinct flow instabilities localized near the unsteady free surface. Complementing these observations, time-resolved 2D PIV and advanced 3D defocusing PIV measurements offer detailed insights into the spatial and temporal evolution of anisotropic Kelvin–Helmholtz instabilities and vortex shedding phenomena. These findings contribute to a deeper understanding of free-surface turbulence and lay the foundation for high-fidelity modeling of unsteady wake dynamics.