Experiments on homogeneous turbulence in an unstably stratified fluid

We have studied experimentally the effects of an unstable linear density profile on the evolution of homogeneous grid-generated turbulence. The experiments were performed in a thermally stratified wind tunnel employing hot- and cold-wire anemometry. The unstably stratified air stream was passed through a biplanar grid and the down-stream evolution of the turbulence statistics were studied. Contrary to the stably stratified case, where vertical velocity fluctuations are inhibited, here the buoyancy forces feed energy into the vertical motions. This injection at the actively turbulent length scales continues up to a critical time at which large-scale overturning of the entire fluid volume occurs. This overturn is due to the Rayleigh-Benard-type instability, which is manifest in the mean temperature field by a large plume rising up from the tunnel floor at the centerline. During the viscous decay, downstream of the grid, the r.m.s. vertical velocity fluctuations are relatively enhanced becoming 1.45 times the stream-wise fluctuations, before this overturn. The normalized buoyancy flux is also enhanced by the destabilizing buoyancy forces, reaching values as large as 0.88, significantly larger than those observed in the passively stratified case. Spectral ratios identify the length scales most affected. Results for different grid mesh-sizes show that the statistics are Reynolds number independent over the range of values studied here, i.e., Re(lambda) between 21 and 66. The probability density functions of temperature and velocity remain Gaussian in the homogeneous region. As the large-scale overturn takes over the flow, the scalar and velocity fluctuations are enhanced dramatically and qualitative changes are observed in the pdf of the temperature fluctuations. (C) 1998 American Institute of Physics. [S1070-6631(98)00512-1].