Impulsive Plunging Wave Breaking Downstream of a Bump in a Shallow Water Flume

Abstract

The plunging wave-breaking process for impulsive flow over a bump in a shallow water flume is described using complementary experiments and simulations, which is relevant to ship hydrodynamics since it includes effect of wave-body interactions and wave breaking direction is opposite to the mean flow. Phase averaged measurements (relative to the time at which the maximum wave height is reached just before the first plunge) are conducted, including the overall flume flow and 2D PIV center-plane velocities and turbulence inside the plunging breaking wave and bottom pressures under the breaking wave. A total number of 226 individual plunging wave-breaking tests were conducted, which all followed a similar time line consisting of startup, steep wave formation, plunging wave, and chaotic wave breaking swept downstream time phases. The plunging wave breaking process consists of four repeated plunging events each with three [jet impact (plunge), oblique splash and vertical jet] sub-events, which were identified first using complementary CFD. Video images with red dye display the plunging wave breaking events and sub-events. The first and second plunges take longer than the last two plunges. Oblique splashes and vertical jets account for more time than plunging. The wave profile at maximum height, first plunge, bump and wave breaking vortex and entrapped air bubble trajectories, entrapped air bubble diameters, kinetic, potential, and total energy, and bottom pressures are analyzed. The simulations on four different grids qualitatively predict all four time phases, all four plunging events and their sub-events, and bottom pressure but with reduced velocity magnitudes and larger post-breaking water elevations. The medium grid results are presented and the fine grid simulations are in progress. Similarities and differences are discussed with the previous deep water or sloping beaches experimental and computational studies.