Velocity fields in breaking-limited waves of finite depth

Abstract

The kinematics below the strongest possible periodic water waves on intermediate depth, for wave periods Tg/h=8.75 and 11.7 (g acceleration of gravity, h water depth), is measured by PTV. The largest possible uniform waves far away from the wave maker have a height of H/h≃0.49 and a fluid velocity up to 0.5gh for these periods. Moderately breaking waves measured close to the wave maker have a turbulent surface region riding on top of a smooth flow with horizontal fluid velocity of 0.62gh at maximum, and wave height up to H/h=0.63. Strongly breaking waves have a thicker turbulent surface region, smaller maximum height (H/h=0.56) and a horizontal fluid velocity of 0.72gh at maximum. Measurement of the flow below 72 breaking wave crests illustrate the range and variation of the elevation and kinematics. Experiments are compared to fully nonlinear and second-order theories, where the former is valid for regular nonbreaking waves, and the latter gives conservative predictions for the very strong waves. Secondary streaming in the bottom boundary layer below the waves is measured.