The effects of a vertical contraction on turbulence dynamics in a stably stratified fluid

Abstract

We have experimentally studied the effects of mean strain on the evolution of stably stratified turbulence. Grid-generated turbulence ($Re_{\lambda \leqslant 25}$) in a stable linear mean background density gradient was passed through a two-dimensional contraction, contracting the stream only in the vertical direction. This induces an increase in stratification strength, which reduces the largest vertical overturning scales allowed by buoyancy forces. The mean strain through the contraction causes, on the other hand, stretching of streamwise vortices tending to increase the fluctuation levels of the transverse velocity components. This competition between buoyancy and vortex stretching dominates the turbulence dynamics inside and downstream of the contraction. Comparison between non-stratified and stratified experiments shows that the stratification significantly reduces the vertical velocity fluctuations. The vertical heat flux is initially enhanced through the contraction. Then, farther downstream the flux quickly reverses, leading to very strong restratification coinciding with an increase in the vertical velocity fluctuations. The vertical heat flux collapses much more rapidly than in the stratified case without an upstream contraction and the restratification intensity is also much stronger, showing values of normalized flux as strong as −0.55. Velocity spectra show that the revival of vertical velocity fluctuations, due to the strong restratification, starts at the very largest scales but is then subsequently transferred to smaller scales. The distance from the turbulence-generating grid to the entrance of the contraction is an important parameter which was varied in the experiments. The larger this distance, the larger the integral length scale can grow, approaching the limit set by buoyancy, before entering the contraction. The evolution of the various turbulence length scales is described. Two-point measurements of velocity and temperature transverse integral scales were also performed inside the contraction. The emergence of ‘zombie’ turbulence, for large buoyancy times, is in good quantitative agreement with the numerical simulations of Gerz & Yamazaki (1993) for stratification number larger than 1.