Spectral nonequilibrium in a turbulent mixing layer

The stochastically steady buoyancy-driven turbulent mix of unstably thermal ly stratified hot and cold water in a channel is examined by means of therm ocouples placed at various stations downstream from an entrance splitter pl ate. Two-time correlations of temperature fluctuations are converted to the approximate two-point correlations of density fluctuations that would occu r in a time-evolving mixing layer. The spectral structure exhibits strong n onequilibrium near the splitter plate. Further downstream the spectral stru cture evolves to self-similarity. Evolution of the structural features is c ompared with the predictions of a turbulence spectral transport theory for the nonequilibrium behavior of a time-evolving mixing layer between two flu ids of different density. Results of the comparison give tentative validati on to most aspects of the theoretical description and suggest closure alter ations for further enhancement of the transport-equation validity. (C) 1999 American Institute of Physics. [S1070-6631(99)01308-2].