Hovering in Oscillatory Flows

TL;DR

This study models the unsteady aerodynamics of a 2D flyer in oscillating flows, revealing that the effort to hover is intrinsic to the flyer and independent of flow frequency and speed, with implications for UAV design and biological flight.

Contribution

It introduces a vortex sheet model to analyze the unsteady fluid-structure interaction of a flyer in oscillating flows, showing the effort needed to hover is a property of the flyer itself.

Findings

Hovering effort is independent of flow frequency and speed.

Aerodynamic lift depends on unsteady vortex shedding.

Flyer characteristics determine the intrinsic effort for hovering.

Abstract

We investigate the hovering dynamics of rigid bodies with up-down asymmetry placed in oscillating background flows. Recent experiments on inanimate pyramid-shaped objects in oscillating flows with zero mean component demonstrate that the resulting aerodynamic forces are sufficient to keep the object aloft. The mechanisms responsible for this lift production are fundamentally unsteady and depend on the shed vorticity. Here, we consider a model system of a two-dimensional flyer and compute the unsteady, two-way coupling between the flyer and the surrounding fluid in the context of the vortex sheet model. We examine in detail the flow properties (frequency and speed) required for hovering and their dependence on the flyer's characteristics (mass and geometry). We find that the aerodynamic effort required to hover is an intrinsic property of the flyer itself: a given flyer requires a constant amount of effort to hover, irrespective of the frequency and speed of the oscillating flow. This physical insight will potentially have significant implications on the design of unmanned air vehicles as well as on understanding active hovering of live organisms that can manipulate their flapping motion to favor a larger oscillation amplitude or frequency.