| The overall aim of this investigation is to characterize, the unsteady three-dimensional flow field generated by low aspect ratio rotating and flapping wings. An experimental system generates simple wing maneuvers in a water facility, and the flow structure is visualized via techniques of dye visualization, particle image velocimetry (PIV) and stereoscopic particle image velocimetry (SPIV).;The flow structure on a rotating plate of low aspect ratio is characterized well after the onset of motion, such that transient effects are not significant, and only centripetal and Coriolis accelerations are present. A stable leading-edge vortex is maintained over effective angles of attack from 30° to 75°, and at each angle of attack, its sectional structure at the midspan is relatively insensitive to Reynolds number over the range from 3,600 to 14,500. The structure of the leading-edge vortex is classified into basic regimes along the span of the plate and compared with the equivalent of the purely translating plate, which does not induce the foregoing flow structure.;In addition, the three-dimensional flow structure on a rotating wing during the early stage of rotation is addressed, and compared with the structure during the late stage. The flow structure during the early stage is associated with a highly-ordered system of stable leading-edge, root and tip vortices. Existence of a coherent tip vortex is associated with identifiable concentrations of vorticity oriented in the chordwise direction. Loss of the tip vortex at large angles is accompanied by loss of the chordwise-oriented vorticity due to eruption of the spanwise flow from the wing surface. The local scale and degree of vorticity concentration of the leading-edge vortex, as well as its proximity to the surface of the wing, are related to: regions of large spanwise velocity, concentrations of chordwise-oriented vorticity located along the span, and downwash along the wing.;Investigation of the effect of aspect ratio at a high angle of rotation, corresponding to a steady-state lift plateau, shows that increasing aspect ratio results in degradation of the organized swirl of the leading-edge vortex. At large radial distances, separated layers from the leading-and trailing-edges indicate that the effects of rotation are no longer effective. Simultaneously, there is loss of an identifiable tip vortex; it is coherent only for the smallest aspect ratio. Despite these significant changes of the patterns of the leading-edge and tip vortex vortices, the structure of the root vortex remains the same with changes of aspect ratio.;To complement the foregoing investigation of the flow structure on a rotating wing in still fluid, the flow patterns were characterized along a wing (rectangular flat plate) subjected to periodic flapping motion in presence of a uniform stream. Regions of spanwise flow exist along the wing surface; and depending on the location along the span, the flow is either toward or away from the tip of the wing. The onset and development of large-scale, streamwise-oriented vortical structures occur at locations inboard of the tip of the wing, and they can attain values of circulation of the order of one-half the circulation of the tip vortex. Time-shifted images indicate that these streamwise vortical structures persist over a major share of the wing chord. Space-time volume constructions define the form and duration of these structures, relative to the tip vortex.;The flow structure generated by a flapping wing is fundamentally altered if the leading-edge has a sinusoidal shape. It is possible to attenuate both the positive and negative spanwise flows along the plate surface, as well as the onset and development of large-scale concentrations of positive and negative streamwise vorticities at inboard locations. These alterations of the inboard flow structure do not have a significant influence on the structure of the tip vortex. |
