Institute of Geophysics, Polish Academy of Sciences, Ks. Janusza 64, 01-452, Warsaw, Poland.
Sci Rep. 2018 Jan 11;8(1):412. doi: 10.1038/s41598-017-18654-7.
Settling due to gravity force is a basic transport mechanism of solid particles in fluids in the Earth. A large portion of particles occurring in nature and used in technical applications are non-spherical. Settling of particles is usually studied in homogeneous ambient conditions, however, stratification is inherent of natural fluids. It has been acknowledged that stratification modifies the velocity of settling spheres and amorphous aggregates. However, the effect of particle shape on the dynamics of settling through density-stratified ambient fluid has not been recognized well enough. Here I show experimental evidence that continuous density transition markedly modifies the settling dynamics of a disk in terms of settling velocity and orientation of a particle. Settling dynamics of a disk are more complex than dynamics of spheres and aggregates studied previously. I found that in a two-layer ambient with density transition, a disk settling in a low Reynolds number regime undergoes five phases of settling with the orientation varying from horizontal to vertical, and it may achieve two local minimum settling velocities in the density transition layer. Moreover, I found that the settling dynamics depends on a density difference between upper and lower homogeneous layers, stratification strength and thickness of density transition.
由于重力作用而沉降是地球流体中固体颗粒的基本传输机制。自然界中存在的和在技术应用中使用的大部分颗粒是非球形的。颗粒的沉降通常在均匀环境条件下进行研究，然而，分层是自然流体固有的。已经认识到，分层会改变沉降球体和无定形聚集体的沉降速度。然而，颗粒形状对通过密度分层环境流体的沉降动力学的影响还没有得到很好的认识。在这里，我通过实验证明，连续密度过渡明显改变了盘在沉降速度和颗粒取向方面的沉降动力学。盘的沉降动力学比以前研究的球体和聚集体的动力学更为复杂。我发现，在具有密度过渡的两层环境中，在低雷诺数区域沉降的圆盘经历了五个沉降阶段，其取向从水平变为垂直，并且可能在密度过渡层中达到两个局部最小沉降速度。此外，我发现沉降动力学取决于上下均匀层之间的密度差、分层强度和密度过渡层的厚度。
