Turbulence, waves and mixing at shear-free density interfaces. Part 2. Laboratory experiments

Abstract

A laboratory experimental study was performed to investigate turbulence, waves and mixing at a sharp density interface (with a jump in buoyancy Δb), subjected to shear-free turbulence induced by oscillating grids with typical velocity and length scales of u_H and L_H, respectively. The cases where turbulence is present on one side (single-sided stirring) or on both sides (double-sided stirring) of the interface were considered. Extensive flow visualization studies and quantitative measurements were performed on the motion field and mixing characteristics at the interface. It was found that, rather than any one mechanism controlling the mixing process, different mechanisms (namely engulfment, generation of waves and their breaking, eddy impingement and Kelvin–Helmholtz billows) play dominant roles over different ranges of the bulk Richardson number Ri(Ri =ΔbL_H/u²_H). For the Ri range where wave generation is significant, certain hypotheses and predictions of the companion paper by Fernando & Hunt (1997) were tested in detail, by flow visualization studies of the qualitative properties of interfacial motions and quantitative measurements of the r.m.s. fluctuations of interfacial velocity and displacement, the local gradient Richardson number within the stratified layer, the frequency spectra and the related fractal properties of the interface. The results are consistent with the hypothesis that, at high values of Ri(>35), the density interface consists of linear internal waves driven by turbulence at high frequencies and breaking waves with sharp horizontal gradients of density at low frequencies.