Vorticity filaments beneath regular turbulent flow

Abstract

Two-dimensional vorticity flow fields created in the wake of a plunging breaker were investigated for regular turbulent flow at a Reynolds number of 30 000. Velocity flow fields obtained from an earlier study that had employed digital particle image velocimetry, were analysed to determine vorticity shedding patterns and the interactions between the vorticity filaments as flow progressed. Central difference approximations were applied to the velocity fields to determine vorticity at each point in the field. Most of the strong instantaneous vorticity observed in the flow field was in the form of filaments. A hierarchy of filaments of different lengths were observed, with the longest being as long as the height of the wave used. During the early phases of the flow, instantaneous vorticity tended to organise into thin filaments of counter-rotating pairs. Eventually, the co-rotating vorticity filaments coalesced and ultimately merged in the turbulent flow as flow progressed, while counter-rotating vorticity filaments were cancelled by viscous dissipation. The results suggested that filaments travel more slowly than the wave velocity and drifted towards the bed as they became elongated, and the number of filaments remaining in the flow were observed to decrease as flow progressed. Whereas phase-resolved instantaneous vorticity results showed pairs of counter-rotating vorticity filaments near the crest, the phase-averaged vorticity description of flow fields showed a dominant primary positive vorticity filament around the shear boundary layer.