2/27/2024 0 Comments Bird wing alula![]() Near-wall flow inhibits flow separation and up-sweep flows are attenuated. The dominant geometric factors are the height, the length and the position of the VG. ![]() VGs that are positioned at an incidence angle to the local flow act as a passive flow control device by drawing high-energy freestream fluid down to the wing’s surface. The geometry and dimensions of vortex generators (VGs) were studied for a long time. Longer alula on heavier birds forms the streamwise vortex further from the main wing, which suppresses flow separation at high angles of attack. A small streamwise vortex is formed at the tip of the alula which delays a stall and increases lift at slow speeds, and in steep descents during a glide-assisted landing. The function of alula has been known from previous studies. The Its function is similar to that of an extended leading-edge slat or a VG on an aircraft. For birds, one of the common special wing structures for flow control is alula, which is a small group of feathers attached to the leading edge of a bird’s wing ( Figure 1) and usually acts as a flow control device. ![]() Passive (vortex generators, VGs) and active (modulated pulse jet vortex generator and jet actuator ) flow control techniques are both well developed in the fixed-wing aircraft design. For the flying creatures in nature, there are various ways to control the occurrence of vortex busting, which include the special wing structures, feature morphology, and wing kinematics. If vortex bursting occurs on the outer spanwise locations of wings with a high aspect ratio (AR), it has a greater effect on lift than on thrust. Unsteady large-scale vortex structures are produced near the leading and trailing edges to produce the suction regions on the lifting surface during flapping flight. The development of micro air vehicles (MAVs) motivates the study of the flapping aerodynamics (revolving/sweeping motion and stroke reversal) of natural flyers. In this study, the use of PSP technique not only helps to understand the aerodynamic effect of the alula-like vortex generator but also shows a perspective tool for bio-inspired MAVs design. A wing model equipped with a vortex generator of an 11% height/chord length ratio exhibited the greatest performance at all angles of attack. An alula-like vortex generator equipped at the leading edge of the wing enlarged the area of the suction region (negative pressure coefficient) on the upper surface in both the streamwise and spanwise directions under near-stall and deep-stall conditions, which is related to the generation of lift and avoiding the stall of wing. The global pressure distributions on the upper surface at different angles of attack were measured to determine the strength of sectional suction forces on the wing. Herein, the pressure-sensitive paint (PSP) was used to quantitatively investigate the aerodynamic effect of an alula-like vortex generator, which is a bio-inspired passive flow control structure. Department of Aeronautics and Astronautics, National Cheng Kung University, Tainan City, Taiwan.
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