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.