Experimental and Numerical Investigation of Visible-Light Characteristics of a Submerged Sphere Wake Under Wind-Wave Conditions

Surface wakes generated by submerged bodies are weak in visible-light images and obscured by wind waves. Towing experiments with a submerged sphere in a large basin investigated the effects of towing speed, submergence depth, wind speed, and camera viewing angle. A linear free surface potential flow model was coupled with a directional wind-wave spectrum and an empirical relationship between surface slope and grey level. Experimental and simulated images were compared using the relative amplitude ratio R95, the texture energy fraction at intermediate scales ηM, and the spatial separability metric ΔA3. Increasing towing speed strengthened the wake and promoted a continuous wave system with a V shape; at H/D = 1.5 and a wind setting of 1 m/s, experimental ΔA3 increased from 0.107 to 22.538 percentage points over towing speeds of 0.4 to 1.0 m/s. Greater submergence generally reduced texture energy and spatial separability. Stronger wind waves masked the wake: at H/D = 1.5 and a towing speed of 1 m/s, increasing the wind setting from 0 to 4 m/s reduced experimental R95 from 3.084 to 0.979. Viewing angle altered projected wake geometry and brightness symmetry. Simulations reproduced the principal trends, although discrepancies remained in local amplitude and high-response area.