  We investigated experimentally stably stratified turbulent flows in a lid
driven cavity with a non-zero vertical mean temperature gradient in order to
identify the parameters governing the mean and turbulent flows and to
understand their effects on the momentum and heat transfer. We found that the
mean velocity patterns (e.g., the form and the sizes of the large-scale
circulations) depend strongly on the degree of the temperature stratification.
In the case of strong stable stratification, the strong turbulence region is
located in the vicinity of the main large-scale circulation. We detected the
large-scale nonlinear oscillations in the case of strong stable stratification
which can be interpreted as nonlinear internal gravity waves. The ratio of the
main energy-containing frequencies of these waves in velocity and temperature
fields in the nonlinear stage is about 2. The amplitude of the waves increases
in the region of weak turbulence (near the bottom wall of the cavity), whereby
the vertical mean temperature gradient increases.
