Investigation of Turbulence Production and Counter-Gradient Heat Flux in a Strongly Stably-Stratified Flow Field

点击翻译按钮获取中文摘要

The present research aimed to examine the transition process of internal gravity waves to clarify relationship between the occurrence of counter-gradient heat flux and turbulence production in the collapse process of the internal gravity waves. Multi-point simultaneous measurement was conducted on temperature and velocity fields by a rake of 7 cold-and hot-wires and a multi-channel high-precision thermo-anemometer system. Streamwise change in the spatial structure of the internal gravity waves was obtained through correlation analysis on the velocity and the temperature fluctuations.(1)As the internal gravity waves developed downstream, the energy levels of the vertical velocity fluctuation, w, and the temperature fluctuation, θ, remarkably increased in the frequency range below the Brunt-Vaisala frequency, N_B. At same time, the coherence between w and θ increased and its phase difference changed from -π to -π/2.(2)With streamwise increase in the amplitude of the waves, correlations between the temperature fluctuations measured by the center cold-wire and the other six probes became larger and their phase differences approached zero in the frequency range below N_B.(3)In the initial stage of the collapse process of the waves, higher harmonic temperature fluctuations with large potential energy developed through the interaction between the velocity and the temperature fluctuations. Then, random components with higher frequencies became to intermingle with spiky wave forms intermittently.(4)At the break down of the wave fronts of the internal gravity waves, the counter-gradient heat flux was generated as negative turbulence production was induced against buoyancy force by converting the potential energy of the harmonic components to the kinetic energy of random components(5)The result of two point spatial correlation measurement showed that the phase velocity of the internal gravity waves increased as they developed in the streamwise direction.