On the Effect of Stably Stratified Layer to the Development of Free Convection

气液界面下的湍流结构和输运过程在海气相互作用和许多工业操作中都具有重要意义。由于在层状稳定的情况下（有逆温层），平行于垂直方向的湍流被抑制，预计湍流结构将受到很大的影响。为了阐明这种效应，我们对水面下自由对流层的发展和统计特性进行了实验和数值研究。用激光多普勒测速仪和精密热电偶同时测量速度和温度波动。用运动热通量和对流层厚度归一化了波动的方差和相关性，得到了这些量的垂直剖面。将其剖面与中性环境（无逆温层）进行比较。水平速度方差在逆温层附近的减小速度较中性环境慢。在逆温层附近，温度与垂直速度的相关系数为正，而在中性环境下，温度与垂直速度的相关系数始终为非正。采用DNS方法对该情况进行了数值模拟。计算得到的方差曲线与实验结果吻合较好。通过对不同表面边界条件的模拟，发现在近表面出现的条纹结构应该是由表面剪切应力产生的。边界条件的不同对方差无显著影响。

Turbulence structure and transport process beneath the gas-liquid interface is important in air-sea interaction and in many industrial operations. As, in stably stratified circumstances (with inversion layer), turbulence parallel to vertical direction is suppressed, turbulence structure is expected to be affected greatly. To clarify the effect, we experimentally and numerically studied the development and statistical properties of a free convective layer under a water surface. Velocity and temperature fluctuations were simultaneously measured by a laser-Doppler velocimeter and a fine thermocouple. Variances and correlations for the fluctuations were normalized by the kinematic heat flux and the thickness of the convective layer, and vertical profiles of those quantities were obtained.Their profiles were compared with those for neutral circumstances (without inversion layer). The variance of the horizontal velocity decrease more slowly near the inversion layer, compared to neutral circumstances. The value of the correlation between temperature and vertical velocity became positive near the inversion layer, whereas the value was always non-positive in neutral circumstances.The situation was numerically simulated by DNS method. The calculated profiles of the variances were consistent with the experimental ones. By the simulation with various boundary conditions at the surface, it was found that stripe structure, which appeared in near the surface, was supposed to be generated by shear stress at the surface. There was no significant difference for the variances by the difference of the boundary conditions.