Our experimental study focuses on the density and velocity field in two layers of fluid separated by a sharp density interface. Turbulence is generated by a non-invasive stirrer, a Taylor–Couette tank, and the interface is stabilized with a source of saline fluid and a source of fresh water at the bottom and top of the tank, respectively. The same volume fluxes are withdrawn by two sinks to maintain a constant volume of fluid in the tank. Our results confirm past experiments and show that a strong vertical exchange of fluid occurs close to the inner cylinder and across the interface, where the vertical turbulent length scales appear to be suppressed. For low values of kinetic energy supplied to the system, the interface may act as a rigid boundary for the turbulent eddies, with a reduction of the vertical length scales although it seems not to affect the horizontal length scales. The vertical buoyancy flux extracted at the top of the tank is fairly well reproduced by the measured correlation $\overline{\unicode[STIX]{x1D70C}^{\prime }w^{\prime }}$ between density and vertical velocity fluctuations across the interface. Quadrant analysis of the correlation terms reveals that the greatest contribution to salt flux is given by eddies that carry the lighter fluid from top to bottom across the interface. The mixing process is accompanied by a single wake-like disturbance, with a radial front advancing in the azimuthal direction across the interface, acting as a blade, and with a period that decreases with rotation rate. The wake favours the smoothing of the density step and, in a simplified model, we assume that the turbulent diffusion is active during a fraction of the cycle in the wake-mixing region, with diffusivity proportional to the transverse length scale and the speed of the wake. The mixing region is the domain between the nose of the wave-like perturbation and the section where the interface becomes ‘darker’ again after being mixed by the vortexes. The results of this model are in a fair agreement with the experiments. The potential energy of the interfacial perturbations is only a small part of the missing turbulent kinetic energy, defined as the difference in the turbulent kinetic energy between a well-mixed fluid and a two-layer fluid. Further analysis is needed to explain the mechanism of generating these perturbations and the factors that control their periodicity.

中文翻译：

---

稳态两层系统中的浮力传递。在 Taylor-Couette 细胞中的实验

我们的实验研究集中在由尖锐密度界面分隔的两层流体中的密度和速度场。湍流由非侵入式搅拌器、Taylor-Couette 罐产生，界面分别通过罐底部和顶部的盐水源和淡水源稳定。相同体积的通量由两个水槽提取，以保持罐中流体的恒定体积。我们的结果证实了过去的实验，并表明在靠近内圆柱体和穿过界面的地方发生了强烈的垂直流体交换，其中垂直湍流长度尺度似乎被抑制了。对于提供给系统的低动能值，界面可能充当湍流涡流的刚性边界，减少了垂直长度尺度，尽管它似乎不影响水平长度尺度。通过测量的相关性 $\overline{\unicode[STIX]{x1D70C}^{\prime }w^{\prime }}$ 之间的密度和垂直速度波动，可以很好地再现在水箱顶部提取的垂直浮力通量跨界面。相关项的象限分析表明，对盐通量的最大贡献是由将较轻的流体从顶部到底部穿过界面的涡流给出的。混合过程伴随着单一的尾流状扰动，径向前缘沿方位角方向穿过界面，充当叶片，周期随转速而减小。尾流有利于密度步长的平滑，在一个简化的模型中，我们假设湍流扩散在尾流混合区域的一小部分周期内是活跃的，扩散率与横向长度尺度和尾流速度成正比。混合区域是波状扰动的鼻部与被涡旋混合后界面再次变得“更暗”的部分之间的域。该模型的结果与实验相当一致。界面扰动的势能只是缺失的湍动能的一小部分，定义为充分混合的流体和两层流体之间湍动能的差异。需要进一步分析来解释产生这些扰动的机制以及控制它们周期性的因素。