Swimming in density-stratified fluid: study on a flapping foil

Effect of layer thickness for the bounce of a particle settling through a density transition layer

We study numerically a spherical particle settling through a density transition layer at moderate Reynolds numbers Re_{u}=69∼259 for the upper fluid. We investigate how the transition layer thickness affects the particle's bouncing behavior as it crosses the interface. The previous intuitive understanding was that the bounce occurs when the relative thickness of the transition layer, L/D, which is characterized by the ratio of the layer thickness L to the particle diameter D, is small. Indeed, we report no bounce phenomenon for very thick interfaces, i.e., L/D>10 in the current parametric range. However, we argue that the bounce can also be inhibited when L/D is too small. Upon a fixed upper layer Reynolds number Re_{u}=207 with varying L/D, we examine the flow evolution of these cases. We propose that this inhibition is attributed to two mechanisms. First, as the interface thickness decreases, the detachment of the attached lighter fluid from the upper layer occurs more rapidly, resulting in a faster decrease in buoyancy. Second, in the case of a very thin interface (L/D=0.5-3.0), the residual light fluid accumulates and undergoes a secondary detachment, separating from the particle at an angle relative to the central axis. This secondary detachment reduces the drag force and effectively prevents the particle from experiencing a rebound motion.

Special issue: bioinspired fluid-structure interaction

Fluid-structure interaction (FSI) studies the interaction between fluid and solid objects. It helps understand how fluid motion affects solid objects and vice versa. FSI research is important in engineering applications such as aerodynamics, hydrodynamics, and structural analysis. It has been used to design efficient systems such as ships, aircraft, and buildings. FSI in biological systems has gained interest in recent years for understanding how organisms interact with their fluidic environment. Our special issue features papers on various biological and bio-inspired FSI problems. Papers in this special issue cover topics ranging from flow physics to optimization and diagonistics. These papers offer new insights into natural systems and inspire the development of new technologies based on natural principles.