Flow Structure on Unsteady Maneuvering Wings

The flow structure along a rotating wing in steady incident flow is compared to the structure on a rotating wing in quiescent fluid, in order to clarify the effect of advance ratio J (ratio of free-stream velocity to tip velocity of wing, ). Stereoscopic particle image velocimetry leads to patterns of vorticity, velocity, and Q-criterion (constant values of the second invariant of the velocity gradient tensor), as well as streamlines, which allow identification of critical points of the flow. Prior to the onset of motion, the wing is at high angle of attack, and the steady incident flow yields a fully stalled state along the wing. After the onset of wing motion, the effective angle of attack is held constant over the range of J, and the wing rotates from rest to a large angle that corresponds to attainment of the asymptotic state of the flow structure. After the onset of rotation, the stalled region quickly gives rise to a stable leading edge vortex. Throughout the rotation maneuver, the development of the flow structure in the leading edge region is relatively insensitive to the value of J. In the trailing-edge region, however, the structure of the shed vorticity layer is strongly dependent on the value of J. Further insight into the effects of J is provided by three-dimensional patterns of spanwise-oriented vorticity, spanwise velocity, and Q-criterion.In addition, the three-dimensional flow structures on a wing subjected to simultaneous pitch-up and rotational motion are characterized. The features associated with these simultaneous motions include: stabilization of the large-scale vortex generated at the leading-edge, which, for pure pitch-up motion, rapidly departs from the leading-edge region; preservation of the coherent vortex system involving both the tip vortex and the leading-edge vortex, which severely degrades for pure rotational motion; and rapid relaxation of the flow structure upon termination of the pitch-up component of motion, whereby the relaxed flow converges to a similar state irrespective of the pitch rate. Three-dimensional surfaces of iso-Q and helicity are employed in conjunction with sectional representations of spanwise vorticity, velocity, and vorticity flux to interpret the flow physics.To complement the foregoing investigation of the simultaneous rotating and pitching wing, the temporal development flow over a combined pitching and rotating wing is characterized. In addition, the effect of pitch rate is explored. Imaging is performed for a range of pitch rates, with emphasis on the three-dimensional structure during start-up and relaxation. Surfaces of transparent iso-Q and helicity are employed to interpret the flow physics. The onset and development of the components of the vortex system, i.e., the leading-edge, tip, and trailing-edge vortices, are strongly influenced by the value of pitch rate relative to the rotation rate. Comparisons at the same angle of attack indicate that the formation of vortical structures is delayed with increasing pitch rate. However, comparisons at the same rotation angle for different values of pitch rate reveal similar flow structures, thereby indicating predominance of rotation effects. Extreme values of pitch rate can lead to radically different sequences of development of the components of the three-dimensional vortex system. Nevertheless, consistently positive vorticity flux is maintained through these components, and the coherence of the vortex system is maintained