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

采用主动湍流网格和热分层发生器，在小型风洞中尝试实现稳定分层的高雷诺数湍流场。为了研究分层湍流场的特性，还研制了数字热风速仪、AR I-X探头和多传感器探头。实验结果表明：（1）在稳定层结下产生了一个高雷诺数湍流场（R_λ>300，R_λ为湍流雷诺数），其稳定线性温度剖面的梯度为30 K/m。这种场具有类似于夜间近地面层的湍流特征和层结特征。(2)温度谱和速度谱存在较宽的惯性子区。热通量峰值位于共谱的惯性子区。(3)在强稳定层结中，浮力效应有效地抑制了大尺度速度脉动的法向分量，而扰动弗劳德数表明层结是弱稳定的。(4)中性湍流场的幂指数与速度结构函数的阶数之间的关系支持了其他工作的结果，而稳定分层湍流场的幂指数与速度结构函数的阶数之间的关系表明，稳定分层湍流场比中性湍流场更具有间歇性。(5)通过多点测量和条件测量，发现了细尺度相干涡。这些涡的平均直径约为10η，与DNS的结果一致，这些涡中只有小尺度的涡对能量耗散有贡献，而大尺度的涡在惯性子区能有效地传递热量。

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.