Technical Notes Effects of Corrugated Aerofoil Surface Features on Flow-Separation Control

W ITH increased awareness of the potential engineering benefits in emulating certain aspects of insect wings or insect flight mechanics, it is not surprising that there is recent surge of interest in their investigations for lift generation or stall mitigation. Other than exploring how the exact flapping/heaving mechanisms employed by insects contribute toward their agility during flight [1–8], understanding how unique surface geometries and features of insect wings enable these insects to maneuver the way they do is also one of the major research motivations for some recent studies. Of interest to the present study are investigations conducted by Hu and Tamai [9], Murphy and Hu [10], and Levy and Seifert [11] recently, where they looked at the flow dynamics of aerofoils based on dragonfly wing cross sections. Hu and Tamai [9] and Murphy and Hu [10] studied corrugated aerofoils with cross sections resembling typical dragonfly wing cross sections and observed favorable aerodynamic behavior. They noted that flow-separation vortices trapped within the corrugationvalleys draw fluid toward the aerofoil wall region and reduce the overall extent of the flow-separation region. These unique flow features mean that flow separations can be delayed until a higher angle of attack with accompanying increases in lift-to-drag ratios for these corrugated aerofoils up to a chord Reynolds number of Re 125; 000. On the other hand, the corrugated aerofoil studied by Levy and Seifert [11] had far fewer corrugations. Instead, their aerofoil had only two corrugations close to the leading edge, followed by a “saddle” and convex trailing-edge “hump”. Because of this geometric difference, the mechanisms with which this aerofoil is able to delay flow separation are different. In this case, flow separations arising from the upstream corrugations reattach back to the trailingedge hump regularly, which translates into fewer flow-separation events propagating beyond the trailing edge. In particular, a recirculating vortex is observed to form at the saddle, which is believed to play an important role in controlling flow separation. It should be mentioned that Hu and Tamai [9] performed their experiments at Re 34; 000, and Murphy and Hu [10] conducted theirs at Re 58; 000 to 125,000, whereas Levy and Seifert [11] performed their investigations atRe < 8000. In addition, the ranges of angle of attack investigated between these studies were also different. It is clear from the earlier studies that the vortex formation and behavior along the upper surfaces of corrugated aerofoils drive the favorable flow effects seen so far. Although some insights into their behavior have been provided by the earlier studies, direct comparisons between them were difficult due to the different test conditions used. To do that, they have to be studied under similar flow conditions, and this provided the primary motivation for the present study. To accomplish that, an experimental flow visualization and particle image velocimetry (PIV) investigation was performed in this study to compare the differences in the near-field vortical behavior and the extent to which flow separation is mitigated between these two corrugated aerofoils at a fixed chord Reynolds number of Re 14; 000. The use of a relatively lowReynolds number herewill not only provide additional insights into the basic aerodynamic characteristics of dragonfly wings but also shed light on the use of different corrugated aerofoils in micro aerial vehicles as well.

Field of study: Physics