Mixing in arrays of villi-like actuators

Abstract

This paper is concerned with computational modeling of fluid mixing by arrays of villi-like actuators. There are numerous applications of such actuators motivated by the motility and mixing induced by natural villi in the small intestine, such as microbial fuel cells and swimming robots—understanding how mixing occurs from viscous-dominated to inertia-dominated flows is paramount. Here, we analyze mixing in two-dimensional arrays of actuators, where neighboring actuators perform in-phase or anti-phase oscillations. We show that in both these cases, the temporal behavior becomes progressively more complex as inertia, or the Reynolds number, is increased. This behavior is classified into three regimes or stages with distinct behaviors and flow structures. We show that mixing can be substantially enhanced in the direction parallel to the wall the actuators are mounted on. We show this mixing is effectively constrained to a peripheral region or layer above the actuator tips. This layer is thicker in the anti-phase case than the in-phase case; however, in both cases this layer thickness saturates at high Reynolds number. Particle tracking results are used to define a mixing number, which shows the anti-phase pattern to be the most effective at mixing both along and across this peripheral layer, and this is linked to the flow structures generated in each stage. Our results provide a map for a range of behaviors that can be achieved through coordinated active motions of villi-like structures that we hope will be useful for the design of future robotics and fluidic-control systems.