Coupled mixed convective and stratified systems are common in natural flows. To study experimentally the associated dynamics, we use a singular property of water: its nonlinear equation of state is characterized by a maximum density close to $4°\mathrm{C}$. By heating the top of a tank at $35°\mathrm{C}$ and cooling the bottom at $0°\mathrm{C}$, a two-layer configuration spontaneously appears. The convective motion in the bottom layer consists mostly of a large-scale circulation and rising cold plumes. This turbulent flow generates propagating internal gravity waves in the upper stably stratified layer. Particle image velocimetry measurements are performed and spectral characteristics of the convection and internal gravity waves are presented. An horizontal large-scale reversing flow in the stratified layer is observed which is viscously driven by a third, intermediate layer. This buffer layer is located between the convective and stratified layers and is thermally coupled with the convective one, hence sustaining a strong horizontal shear. Three-dimensional direct numerical simulations with geometry and physical parameters close to the experimental ones corroborate our experimental results.

中文翻译：

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对流和内部重力波在4°C的最大密度附近在水中激发

自然流动中通常会使用混合的对流和分层混合系统。为了实验研究相关的动力学，我们使用水的奇异性质：其非线性状态方程的特征在于最大密度接近$4°\mathrm{C}$。通过加热罐的顶部$35°\mathrm{C}$ 然后冷却底部 $0°\mathrm{C}$，自发出现两层配置。底层的对流运动主要由大规模的环流和上升的冷羽组成。该湍流在稳定的上分层层中产生传播的内部重力波。进行了颗粒图像测速仪测量，并给出了对流和内部重力波的光谱特征。在分层层中观察到水平的大规模逆向流动，该流动由第三中间层粘性地驱动。该缓冲层位于对流层和分层层之间，并与对流层热耦合，因此保持了很强的水平剪切力。