Air Entrainment Measurements of a Ship Hull Turbulent Boundary Layer*

Air entrainment occurs in turbulent free-surface boundary layers that grow along the hulls of ships. The boundary layer characteristics and air entrainment depend on the ship speed and position along the hull. A ship boundary layer simulation device consisting of a surface-piercing stainless-steel belt driven from rest by two vertically oriented rollers generates a temporally evolving boundary layer that emulates the spatially evolving boundary layer of a ship hull. The radius, depth, and streamwise position of bubbles entrained into the belt's boundary layer are extracted from images captured by a stationary high-speed camera recording at 1000 fps for 10 s in runs with final belt speeds ranging from 3.5 m/s to 5 m/s. Five runs at each belt velocity were performed to reach statistical convergence of the bubble measurements. Bubbles with radii down to 0.25 mm are resolved within the effective camera field of view (3.52 cm x 5.67 cm) and depth of field (6.5 cm). The bubbles in each image are tracked across the measurement area and the vertical and horizontal components of the bubble velocities are determined from the trajectories. Bubble characteristics are explored as functions of bubble radius and depth.