Diffusion of Passive Scalar in Counter-gradient Diffusion Field of Stratified Turbulent Flows.

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1. PURPOSE In the thermally stratified turbulent flows temperature fluctuation (active scalar) induces spotty buoyancy force and changes the turbulence structure and transport processes significantly. It sometimes causes counter-gradient diffusion of heat (diffusion against temperature gradient) under extremely stable stratification conditions. In this research project mechanism of diffusion of mass such as pollutants (passive scalar) and so the difference between diffusions of active and passive scalars were investigated in the counter-gradient diffusion field.2. RESULTS In 1989 experimental and theoretical basic researches were completed and so in 1990 application researches were made on the various meteorological and air pollution phenomena of which mechanisms were predominantly controlled by the stratification.(1) WIND TUNNEL EXPERIMENT Turbulent boundary layers both with stable and unstable stratified regions were simulated in a wind tunnel and turbulence structures in the counter … More -gradient diffusion fields and their evolution in the downstream direction were investigated.(2) TURBULENCE THEORY AND PREDICTION In 1989 a mathematical turbulence model called the algebraic stress model was established, on the basis of four partial differential equations of turbulence energy, energy dissipation rate, temperature variance and its destruction rate and local equilibrim assumpion. In this year various numerical simulations were performed by a flow model and Eulerian transport /diffusion /chemistry/deposition model of air pollution in which the mathematical turbulence model was equipped. At first, as a typical stratified flow, gravity current was investigated. Simulating successfully the occurrence of Kelvin-Helmholtz instability and evolution of K-H billows, internal bore and solitary wave, their details were examined and their roles were related to the dynamics of the gravity current as a whole. Moreover, all types of air pollution due to NOx, oxidants, secondary aerosols and acid rain, fog and snow which occur in the strongly stratified diffusion fields such as thermally-induced local winds (long range transport from the Tokyo Bay area to the inland mountainous region) and nocturnal urban boundary layer in winter time) were shown to be predicted well by the sophisticated model and their concentration features and distribution of the dry and wet depositions and effects of emission controls of pollutants on their concentrations were investigated. Less