The difference in turbulent diffusion between active and passive scalers in stable thermal stratification

The difference in turbulent diffusion between active scalar (heat) and pass ive scalar (mass) in a stable thermally stratified flow is investigated bot h experimentally and numerically. The experiments are conducted in an unshe ared thermally stratified water flow downstream of a turbulence-generating grid. Passive mass is released into the stable thermally stratified flow fr om a point source located 60 mm downstream from the grid. Instantaneous str eamwise and vertical velocities, the temperature of the active scalar and t he concentration of the passive scalar are simultaneously measured using a combined technique with a two-component laser-Doppler velocimeter (LDV), a resistance thermometer and a laser-induced fluorescence (LIF) method. From the measurements, turbulent heat and mass fluxes and eddy diffusivities for both active heat and passive mass are estimated. To investigate the Prandt l or Schmidt number effects on the difference in turbulent diffusion betwee n active heat and passive mass, a three-dimensional direct numerical simula tion (DNS) based on a finite difference method is applied to stable thermal ly stratified flows of both water and air behind the turbulence grid. The S chmidt number of passive mass in the DNS is set to the same value as the Pr andtl number of active heat. The results show that stable stratificaion causes a large difference in edd y diffusivities between active heat and passive mass. The numerical predict ions by the DNS are in qualitative agreement with the measurements despite the assumption of the same molecular diffusivity for active heat and passiv e mass. The difference suggests that the assumption of identical eddy diffu sivity for active heat and passive mass, used in conventional turbulence mo dels, gives significant errors in estimating heat and mass transfer in a pl ume under stably stratified conditions.