Three-dimensional flow structure along simultaneously pitching and rotating wings: effect of pitch rate

The flow structure along a simultaneously pitching and rotating wing is investigated using quantitative flow visualization. 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.