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

1. 在热分层湍流中，温度波动（活动标量）引起了点状浮力，显著改变了湍流结构和输运过程。在极其稳定的分层条件下，有时会引起热的反梯度扩散（逆温度梯度扩散）。本课题研究了反梯度扩散领域中污染物（被动标量）等物质扩散的机理，以及主动标量和被动标量扩散的区别。结果1989年完成了实验和理论基础研究，1990年开展了以分层为主的各种气象和大气污染现象的应用研究。(1)风洞实验：在风洞中模拟了具有稳定和不稳定层状区域的湍流边界层，并研究了反梯度扩散场的湍流结构及其在下游方向的演变。1989年，在湍流能量、能量耗散率、温度方差及其破坏率四个偏微分方程和局部平衡假设的基础上，建立了湍流数学模型代数应力模型。在这一年里，我们用流动模型和欧拉空气污染输运/扩散/化学/沉积模型进行了各种数值模拟，其中配备了数学湍流模型。首先，将重力流作为一种典型的分层流进行研究。成功地模拟了开尔文-亥姆霍兹不稳定性的发生和K-H波浪、内孔和孤立波的演化，研究了它们的细节，并将它们的作用与重力流的整体动力学联系起来。此外，由氮氧化物、氧化剂、二次气溶胶和酸雨引起的各种空气污染，结果表明，该模型对发生在强分层扩散场（如热致局地风（从东京湾地区到内陆山区的长距离输送）和冬季夜间城市边界层）的雾和雪具有较好的预报能力，并研究了雾和雪的浓度特征、干湿沉积的分布以及污染物排放控制对雾和雪浓度的影响。少

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