  We study experimentally temperature fluctuations in stably stratified forced
turbulence in air flow. In the experiments with an imposed vertical temperature
gradient, the turbulence is produced by two oscillating grids located nearby
the side walls of the chamber. Particle Image Velocimetry is used to determine
the turbulent and mean velocity fields, and a specially designed temperature
probe with sensitive thermocouples is employed to measure the temperature
field. We found that the ratio $\big[(\ell_x \nabla_x \bar{T})^2 + (\ell_y
\nabla_y \bar{T})^2 + (\ell_z \nabla_z \bar{T})^2\big] / <\theta^2 >$ is nearly
constant, is independent of the frequency of the grid oscillations and has the
same magnitude for both, stably and unstably stratified turbulent flows, where
$\ell_i$ are the integral scales of turbulence along $x, y, z$ directions,
$\bar{T}$ and $\theta$ are the mean and turbulent fluctuations components of
the fluid temperature. We demonstrated that for large frequencies of the grid
oscillations, the temperature field can be considered as a passive scalar,
while for smaller frequencies the temperature field behaves as an active field.
The theoretical predictions based on the budget equations for turbulent kinetic
energy, turbulent potential energy ($\propto <\theta^2 >)$ and turbulent heat
flux, are in a good agreement with the experimental results. Detailed
comparison with experimental results obtained previously in unstably stratified
forced turbulence is performed.

Fluid Dynamics