Influence of Internal Gravity Waves on Counter-Gradient Heat Flux in a Thermaly Stratified Flow.

Abstract

Effects of internal gravity waves on heat and momentum transfer were experimentally investigated in a strongly stably-stratified mixing layer with a temperature gradient of more than 1000 K/m. A high-resolution thermo-anemometer system gave simultaneous values of velocity and temperature fluctuations from which higher order moments were calculated. In the initial linear stage of streamwise development of the internal gravity waves, usual down-gradient mechanism dominated the heat and the momentum transfer across the mixing layer. As the internal gravity waves developed downstream, the time-averaged heat flux decreased apparently and the phase difference between ω and θ approached -π/2. Waveform analysis on the vertical heat flux clearly showed that the constituent frequency of the internal gravity waves and their harmonics contributed to the occurrence of the counter-gradient heat flux when the internal gravity waves began to collapse downstream through their nonlinear interaction. Where the counter-gradient heat flux occurred, the production terms of the temperature and the velocity fluctuations became negative and the amplitude of the temperature fluctuation apparently decreased contrary to the increase in intensity of the vertical velocity fluctuation.