St. Thoroddsen et Cw. Vanatta, THE EFFECTS OF A VERTICAL CONTRACTION ON TURBULENCE DYNAMICS IN A STABLY STRATIFIED FLUID, Journal of Fluid Mechanics, 285, 1995, pp. 371-406
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.