On the layers produced by rapidly oscillating a vertical grid in a uniformly stratified fluid

Abstract

Experiments have been made to examine the fluid motion and density perturbations caused by oscillating a grid of vertical bars horizontally in a uniformly stratified fluid at frequency ω greatly exceeding the buoyancy frequency N. A highly turbulent region is produced near the grid. Beyond this lies a region of intrusive layers as described by Ivey & Corcos (1982), while further from the grid the motion is dominated by internal waves. The boundary between the turbulent region and the intrusive region is clearly defined, and the width of the former region appears to be proportional to , where a is the amplitude and M the mesh length of the grid. The vertical scale of the layers, which sometimes form to give a regular sequence of high and low density gradients, is also proportional to y. This scaling is shown to be consistent with that found by Hopfinger & Toly (1976) in their study of motion produced by grids oscillating in homogeneous fluids, while the abrupt change in character of the motion at the edge of the turbulent region is in accord with measurements made by Dickey & Mellor (1980) in the wake of a grid drawn steadily through a stratified fluid. The scaling is also in accord with Ivey & Corcos’ observations of the width of the turbulent region, the thickness of the layers, and the vertical flux of density.The observation of the layers is considered in the light of conjectures about the instability of turbulent motion in stratified fluids. The character and conditions of generation of the layers are consistent with their being a manifestation of the instability, but the identification is not conclusive.