双扩散对流中台阶结构的实验研究

LABORATORY STUDIES ON THE STAIRCASE STRUCTURE OF DOUBLE-DIFFUSIVE CONVECTION

线性层结的盐水系统在底部加热和顶部冷却时,会产生明显的温盐台阶结构,这是双扩散对流现象.温盐台阶由界面和混合层组成.温度垂向廓线的变化可以反映台阶的生成与合并.根据盐水系统上下边界之间温差的变化,系统演化过程分为两个阶段.在温差增加阶段,主要发现在已有台阶顶部生成新台阶,而最下层台阶与其上层台阶发生合并过程.在此阶段,由于台阶的生成速率大于合并速率,系统内台阶越来越多.在温差减小阶段,系统仅存在台阶的合并过程,因此台阶数目越来越少.最后,系统内仅存在一个大尺度环流.同时,发现台阶的生成与合并过程虽然发生在局部区域,但会影响到系统的其他区域.例如,由于最下层台阶的合并,系统下边界的温度梯度发生明显变化.实验中,由于最下层台阶不断与其上层台阶合并,最下层台阶厚度比其他台阶厚度大,而其他台阶厚度变化不大.最下层台阶厚度(h)随时间(τ)的变化满足h～τ0.7的关系.

In the present study, the evolution of double-diffusive convection is explored in a linear stratified salt-water system, where the uniform heating is provided from below and the constant temperature is applied from above. By using the variances of vertical profiles of temperature and shadow graph images, it is found that the generating and merging of the staircases occur during the experiment. Based on the temperature difference across the system, the whole evolution process can be divided into two stages. In the temperature difference increase stage, the generation of new staircase happens at the top of the old ones, while the merging event typically occurs between the lowest staircase and its overlaying one. The staircase number increases because that the generating rate of new staircase is faster than the merging rate. In the temperature difference decrease stage, the exi^ting staircases disappear progressively. It is interesting that the bottom boundary of system can feel the happen of the merging events of the lowest staircase, which means that the dynamics process in the local region can have effect on the other regions of system. The thickness of the lowest staircase （h） is much thicker than those of others, and its temporal dependence is in the form of h～τ^0.7, here T is the dimensionless time.