Difference Between Active(Heat)-and passive Scalar Diffusions in Thermally Stratified Flow

在热力层流中，温度波动引起浮力，引起湍流结构和输运过程的剧烈变化。例如，当层结非常稳定时，它会引起热量的逆梯度扩散。然而，质量(如稀释污染物)通常沿其浓度梯度方向扩散，即使在这样的流场中也是如此。本研究的目的是从实验和理论上阐明主动标量(热)扩散和被动标量扩散的区别。1.标量扩散机制的实验研究：首先，建立了一种同时测量两种物质浓度波动的方法，它基于激光诱导发光和米氏散射，由线或面光探测器阵列组成。该技术的优越性通过以下几点得到证实

In the thermally stratifed flows temperature fluctuation induces buoyancy force and causes drastic changes in turbulence structure and transport porcesses.When the stratification is extremely stable,for example,it causes a counter-gradient diffusion of heat.However,mass(e.g.diluted pollutant) usually diffuses in the direction of its concentration gradient,even in such a flow field.The purpose of the present study is to make clear experimentally and theoretically the difference between the active-scalar(heat)and passive scalar diffusions.1.Experimental investigation on scalar diffusion mechanisms:First a simultaneous measuring technique of two-species concentration fluctuations was established,which was besed on the laser-induced luminescence and Mie-scattering and consisted of a line-or area-photodetectory array.Superiority of this technique was confirmed by