Whole-wavelength description of a wave boundary layer over permeable wall

Abstract

This paper describes the whole-wavelength, highly resolved velocity field statistics in an oscillatory boundary layer driven by gravity waves, evolving on both impermeable and permeable beds. Velocity data from PTV measurements are acquired in a small window and then extended to the whole wavelength by means of a new phase-locked slotting technique. This technique increases the robustness of statistics and resolution by improving the number of the data samples available for each spatial cell in the Eulerian representation of the oscillatory boundary layer. The into- and out-of-bed volume flux is evaluated and its effects on the velocity field are reported, together with its influence on vorticity evolution. The presence of coherent structures embedded in a shear-dominated scenario is emphasized. The Lagrangian flow features extracted by PTV are presented in the context of the generalized Lagrangian-mean theory. The Stokes drift vertical profile is estimated by comparing the Eulerian and Lagrangian velocities. A critical height-separating region where the volume flux due to ventilation has different effects on both orbital amplitudes and Stokes drift is found. This distance corresponds to the maximum height of the still-attached vorticity layer.