

Article
Passive control of a falling sphere by ellipticshaped appendages
Authors: 
Lacis, U., Olivieri, S., Mazzino, A., Bagheri, S. 
Document Type: 
Article 
Pubstate: 
Published 
Journal: 
Physical Review Fluids 
Volume: 
2
033901 
Year: 
2017 
AbstractThe majority of investigations characterizing the motion of single or multiple particles in fluid flows consider canonical body shapes, such as spheres, cylinders, discs, etc. However, protrusions on bodies—either surface imperfections or appendages that serve a function—are ubiquitous in both nature and applications. In this work, we characterize how the dynamics of a sphere with an axissymmetric wake is modified in the presence of thin threedimensional ellipticshaped protrusions. By investigating a wide range of threedimensional appendages with different aspect ratios and lengths, we clearly show that the sphere with an appendage may robustly undergo an invertedpendulumlike (IPL) instability. This means that the position of the appendage placed behind the sphere and aligned with the freestream direction is unstable, similar to how an inverted pendulum is unstable under gravity. Due to this instability, nontrivial forces are generated on the body, leading to turn and drift, if the body is free to fall under gravity. Moreover, we identify the aspect ratio and length of the appendage that induces the largest side force on the sphere, and therefore also the largest drift for a freely falling body. Finally, we explain the physical mechanisms behind these observations in the context of the IPL instability, i.e., the balance between surface area of the appendage exposed to reversed flow in the wake and the surface area of the appendage exposed to fast freestream flow.

