Vortex wakes of a flapping foil

Abstract

We present an experimental study of a symmetric foil performing pitching oscillations in a vertically flowing soap film. By varying the frequency and amplitude of the oscillation we visualize a variety of wakes with up to 16 vortices per oscillation period, including von Kármán vortex street, inverted von Kármán vortex street, 2P wake, 2P+2S wake and novel wakes ranging from 4P to 8P. We map out the wake types in a phase diagram spanned by the width-based Strouhal number and the dimensionless amplitude. We follow the time evolution of the vortex formation near the round leading edge and the shedding process at the sharp trailing edge in detail. This allows us to identify the origins of the vortices in the 2P wake, to understand that two distinct 2P regions are present in the phase diagram due to the timing of the vortex shedding at the leading edge and the trailing edge and to propose a simple model for the vorticity generation. We use the model to describe the transition from 2P wake to 2S wake with increasing oscillation frequency and the transition from the von Kármán wake, typically associated with drag, to the inverted von Kármán wake, typically associated with thrust generation.

Spatio-temporal stability of the Kármán vortex street and the effect of confinement

“…Schnipper, Andersen & Bohr (2009) obtained a rich variety of these vortex wakes by varying the amplitude and frequency of the oscillation, with most of them showing strong downstream persistence. We therefore expect regular pattern of vortices to be convectively unstable in a wide range of configurations.…”

Hummingbird feather sounds are produced by aeroelastic flutter, not vortex-induced vibration

“…These predictions do not prescribe some sort of '1 motion-1 vortex' model. For fluttering filaments and actuated airfoils, it has been clearly shown that there need not be a 1:1 relationship between shed vortices and the structure's oscillatory motion (Lentink et al, 2010;Schnipper et al, 2009;Zhang et al, 2000;and references therein). We suggest that aerodynamic excitation of a feather causes it to flutter when aerodynamic energy received exceeds structural damping, and that periodicity is set in part by the structural resonance of the feather.…”

Unsteady Adjoint Approach for Design Optimization of Flapping Airfoils

“…15. For a clear definition of wake trend, we shall use "S" to denote a single vortex and "P" to denote a pair of vortices of opposite signs [25]. Accordingly, in …”