An experimental study of falling spheres in a confined density-stratified fluid

Falling objects in a density stratified fluid have essential applications in the natural environment. For example, the vertical motion of zooplankton, and marine snow, the falling biological debris from the ocean's surface, is affected by ocean stratification. In addition, the stratification of the atmosphere can modify the settling dynamics of dust and aerosol, which directly impacts climate change. Studies have shown that density stratification can substantially alter the flow and wake structures around settling spherical particles, which results in a significant increase in the drag coefficient [1,2]. The dynamics of falling spheres are characterized by a natural reverse jet and internal waves. In this study, we investigate the transient behavior of falling spheres with moderate Reynolds number (~65 and up) in a confined linearly stratified fluid, using 3-D particle tracking velocimetry (PTV). The wake structure is compared to the homogenous counterpart. Moreover, the effect of confinement will be assessed by comparing the current results with those reported in the literature with no confinements. 

[1] Doostmohammadi, A., Dabiri, S., & Ardekani, A. (2014). A numerical study of the dynamics of a particle settling at moderate Reynolds numbers in a linearly stratified fluid. Journal of Fluid Mechanics, 750, 5-32. doi: 10.1017/jfm.2014.243 

[2] Torres, C., Hanazaki, H., Ochoa, J., Castillo, J., & Van Woert, M (2000). Flow Past a sphere moving vertically in a stratified diffusive fluid, Journal of Fluid Mechanics, 417, 211-236. doi: 10.1017/S0022112000001002