Spectral nonequilibrium in a turbulent mixing layer

The stochastically steady buoyancy-driven turbulent mix of unstably thermally stratified hot and cold water in a channel is examined by means of thermocouples placed at various stations downstream from an entrance splitter plate. Two-time correlations of temperature fluctuations are converted to the approximate two-point correlations of density fluctuations that would occur in a time-evolving mixing layer. The spectral structure exhibits strong nonequilibrium near the splitter plate. Further downstream the spectral structure evolves to self-similarity. Evolution of the structural features is compared with the predictions of a turbulence spectral transport theory for the nonequilibrium behavior of a time-evolving mixing layer between two fluids of different density. Results of the comparison give tentative validation to most aspects of the theoretical description and suggest closure alterations for further enhancement of the transport-equation validity.