Volumetric velocity measurements of turbulent coherent structures induced by plunging regular waves

Abstract

The instantaneous turbulent velocity fields induced by the breaking of plunging regular waves on a 1 in 40 plane slope were measured using a volumetric three-component velocimetry (V3V) system. The measurement volume was located in the outer surf zone to capture the plunger vortex generated at incipient breaking and the splash-up vortex generated at the second plunge point. The measurement volume extended from a few millimeters above the bottom to just below the wave trough level. The aerated water was seeded with fluorescent particles and long-pass filters were used to block scattered light from entrained air bubbles. The volumetric velocity measurements revealed the three-dimensional (3D) structure of breaking-wave-generated vortices previously captured in two-dimensional (2D) planes using a particle image velocimetry (PIV) system. The evolution of the vortices was also captured. The most common turbulent coherent structure observed was a vortex loop with counter-rotating vorticity; two longitudinal vortices oriented obliquely upward in the direction of wave propagation are connected at the base by a transverse vortex. The vortex loop was carried in a downburst of turbulent fluid, which impinged on the bottom around the instant of maximum positive (onshore) wave-induced velocity. Downbursts carried large amount of turbulent kinetic energy and induced large apparent shear stress over the bottom. The distributions of vorticity, turbulent kinetic energy, turbulent kinetic energy flux and turbulent shear stress in the transverse segment of a vortex loop were examined. In a separate experiment, water particle velocities associated with non-breaking waves were measured at different heights above a plane slope using an acoustic Doppler velocimeter (ADV) to verify the V3V measurements.