Evolution of the leading edge vortex over an accelerating rotating wing
Seminar Room 1, Newton Institute
Flapping flight is a subject of interest for more than two decades. During this time it has been found that a stable leading edge vortex is responsible for the high lift that flapping and revolving wings can produce. However, many of these studies were limited to Reynolds numbers of few hundred, which characterize insects. Recently, the interest on designing and realizing miniature hovering vehicles requires expanding our understanding of the basic flow mechanism which govern such wing maneuvers at higher Reynolds numbers. In this study the flow field over an accelerating rotating wing model is analyzed in various Reynolds numbers ranging from 250 to 2000 using particle image velocimetry. These experimental results are compared with three-dimensional and time-accurate Navier-Stokes flow simulations. The study depicts the characteristic size and time scales of the leading-edge vortex. The results show that the topology of the leading-edge vortex is Reynolds number dependent; i n comparison to a diffused and detached leading-edge vortex at Reynolds number 250, at Reynolds number 2000 the leading-edge vortex is not stationary and can cover up to about 75 percent of the local wing chord. Furthermore, it is shown that the spanwise velocity component increases considerably at Reynolds number of 1000 and above. Moreover, in Reynolds number 250 the circulation within the leading-edge vortex during wing acceleration exceeds its asymptotic value which develops over steadily revolving wings. At Reynolds number 1000 and above, on the other hand, the circulation within the leading-edge vortex evolves much slower. These findings shed new insights about the differences between the aerodynamic characteristics of steady revolving wings and flapping ones and will be utilized to investigate the stability of the leading-edge vortex in wider range of Reynolds numbers.