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

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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