Stable vortex structures: A flat plate model of dragonfly hovering

Abstract

To understand the unsteady separated flow mechanisms associated with the hovering flight of certain insects, flat plates of varying thicknesses were sinusoidally driven back and forth using a broad range of motion amplitude and rate conditions in the absence of a mean flow. Flow visualization and hotwire anemometry techniques were used to characterize the resulting vortical structures and velocity profiles. The initiation, development, and perseverance of vortical structures were quantitatively and qualitatively assessed. Only a narrow range of oscillation parameters elicited the cohesive types of flow structures that appear to be utilized by dragonflies. These structures were selectively sensitive to variations in specific parameters. For thin plates, vortical structures were influenced most by changes in oscillation amplitude, not oscillation frequency. The reverse was observed for thick plates. Vorticity accumulation, and the initiation, development and shedding of vortex structures from an oscillating flat plate are accounted for using a simplified physical model.