Settling of solid particles in a stratified ambient fluid is a process widely encountered in geophysical flows. A set of experiments demonstrating the settling behaviour (the pattern of trajectory, variation of particle orientation, and settling velocity with depth) of thin disks descending through a nonlinear density transition was performed. The results showed complex hydrodynamic interactions between a particle and a liquid causing settling orientation instabilities and unsteady particle descent in low to moderate Reynolds number regime. Five phases of settling were observed: two phases with stable horizontal, one with stable vertical disk position, and two reorientation phases; moreover, two local minima of settling velocity were identified. It was demonstrated that thresholds for local minima and the first reorientation depend on the settling dynamics in an upper layer, stratification conditions, and disk geometry. The comparison of settling behaviour of thin disks varying in diameter revealed that settling dynamics is sensitive to particle geometry mainly in the upper part of density transition with a non-obvious result that the first minimum velocity is smaller for a disk with a larger diameter than for a disk with a smaller diameter. The analysis of settling trajectory showed that two reorientations are accompanied with a horizontal drift, which may be important in the context of interactions between particles settling in a group.

中文翻译：

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具有密度转换的两层流体中薄盘沉降的动力学

固体颗粒在分层环境流体中的沉降是地球物理流中广泛遇到的过程。进行了一组实验，证明了通过非线性密度转换下降的薄盘的沉降行为（轨迹模式，粒子取向的变化以及沉降速度随深度的变化）。结果表明，在低至中等的雷诺数条件下，颗粒与液体之间复杂的流体动力学相互作用会导致沉降取向不稳定和不稳定的颗粒下降。观察到五个阶段的沉降：两个阶段具有稳定的水平位置，一个阶段具有稳定的垂直盘位置，以及两个重新定向阶段；第二阶段具有稳定的水平位置。此外，确定了沉降速度的两个局部最小值。结果表明，局部极小值和第一次重新定向的阈值取决于上层的沉降动力学，分层条件和磁盘几何形状。比较直径不同的薄盘的沉降行为，发现沉降动力学对粒子几何形状敏感，主要是在密度跃迁的上部，结果不明显：直径较大的圆盘的第一最小速度小于直径最小的圆盘。直径较小的磁盘。对沉降轨迹的分析表明，两次重新定向都伴随着水平漂移，这对于沉降在一组中的粒子之间的相互作用而言可能很重要。比较直径不同的薄盘的沉降行为，发现沉降动力学对粒子几何形状敏感，主要是在密度跃迁的上部，结果不明显：直径较大的圆盘的第一最小速度小于直径最小的圆盘。直径较小的磁盘。对沉降轨迹的分析表明，两次重新定向都伴随着水平漂移，这在沉降到一组粒子之间的相互作用的背景下可能很重要。比较直径不同的薄盘的沉降行为，发现沉降动力学对粒子几何形状敏感，主要是在密度跃迁的上部，结果不明显：直径较大的圆盘的第一最小速度小于直径最小的圆盘。直径较小的磁盘。对沉降轨迹的分析表明，两次重新定向都伴随着水平漂移，这在沉降到一组粒子之间的相互作用的背景下可能很重要。