Clarification of turbulent fine-scale structure in a stable stratified flow

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A stably-stratified and high Reynolds number turbulence field was tried to realize in a small wind tunnel by using an active turbulence grid and a thermal stratification generator. A digital thermo-anemometer, ar I-X probe and a multi-sensor probe were also developed and examined in order to investigate the characteristics of the stratified turbulence field. The results of the measurements using these instruments are as follows ;(1) A high Reynolds number turbulence field (R_λ>300, R_λ denotes turbulence Reynolds number) was generated under the stable stratification in which the gradient of its stable linear temperature profile was 30 K/m. Such field has the turbulence characteristics and the stratification similar to those of a nocturnal surface layer.(2) The wide inertial subrange existed in the temperature and velocity spectra. The peak of heat flux located in the inertial subrange of the co-spectrum.(3) The normal component of large-scale velocity fluctuation was effectively suppressed by the buoyancy effect in the strongly stable stratification, though the disturbance Froude number showed that the stratification was weakly stable.(4) The relation between the power index and the order of the velocity structure function of the neutral turbulence field supported the results of the other works, whereas that of the stably-stratified turbulence field showed that the field was more intermittent than the neutral field.(5) The fine-scale coherent vortices were detected by the multi-points and conditioned measurements. Their mean diameter was about 10η, which is same with the results of DNS.Only small ones of such coherent vortices contributed to energy dissipation, but the large-scale vortices in inertial subrange transferred heat effectively.