THE EFFECTS OF A VERTICAL CONTRACTION ON TURBULENCE DYNAMICS IN A STABLY STRATIFIED FLUID

We have experimentally studied the effects of mean strain on the evolu tion of stably stratified turbulence. Grid-generated turbulence (Re-la mbda less than or equal to 25) in a stable linear mean background dens ity gradient was passed through a two-dimensional contraction, contrac ting the stream only in the vertical direction. This induces an increa se in stratification strength, which reduces the largest vertical over turning scales allowed by buoyancy forces. The mean strain through the contraction causes, on the other hand, stretching of streamwise vorti ces tending to increase the fluctuation levels of the transverse veloc ity components. This competition between buoyancy and vortex stretchin g dominates the turbulence dynamics inside and downstream of the contr action. Comparison between non-stratified and stratified experiments s hows that the stratification significantly reduces the vertical veloci ty fluctuations. The vertical heat flux is initially enhanced through the contraction. Then, farther downstream the flux quickly reverses, l eading to very strong restratification coinciding with an increase in the vertical velocity fluctuations. The vertical heat flux collapses m uch more rapidly than in the stratified case without an upstream contr action and the restratification intensity is also much stronger, showi ng 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 subse quently transferred to smaller scales. The distance from the turbulenc e-generating grid to the entrance of the contraction is an important p arameter 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 th e various turbulence length scales is described. Two-point measurement s of velocity and temperature transverse integral scales were also per formed inside the contraction. The emergence of 'zombie' turbulence, f or large buoyancy times, is in good quantitative agreement with the nu merical simulations of Gerz & Yamazaki (1993) for stratification numbe r larger than 1.