ALBERT

Description: 〈div data-abstract-type="normal"〉〈p〉This experimental study investigates the effect of a uniform lateral inflow on the aerodynamic characteristics of flapping wings. Seven designated sideward ratios in the hovering condition and in the presence of a contralateral wing and a body were taken into account as variables in order to secure a better understanding of wing–wing and/or wing–body interactions under the lateral inflow. Our results from the single-wing cases clarified that an inflow running from the wingroot strengthened the leading-edge vortex, thereby augmenting the aerodynamic force/moment. The inflow running in the opposite direction drastically bent the leading-edge vortex to the trailing edge, but the cycle-averaged aerodynamic force/moment was barely changed. This led to substantial imbalances in the force/moment on the two wings. The roll moment on a centre of gravity and the static margin suggested flight instability in the lateral direction, similar to previous studies. We found that the wing–wing interaction was not completely negligible overall under a lateral inflow. A massive downwash induced by the wing on the windward side nearly neutralized the aerodynamic force/moment augmentations on the other wing with lower effective angles of attack. The wing–wing interaction also gave rise to a low-lift high-drag situation during the pitching-up wing rotation, resulting in greater side force derivatives than the theory of flapping counterforce. Further calculations of the roll moment and the static margin with the centre of gravity showed that the wing–wing interaction can improve static stability in the lateral direction. This mainly stemmed from both the attenuation of the lift augmentation and the elimination of the positive roll moment of the flapping-wing system.〈/p〉〈/div〉

Description: Time-varying force/moment measurements and digital particle image velocimetry (DPIV) were conducted to reveal the influence of an advance ratio on an insect-like flapping wing. A scaled-up robotic model and a servo-driven towing tank were employed to investigate nine individual cases-(hovering), 0.0625, 0.1250, 0.1875, 0.25, 0.50, 0.75, 1.0 and (gliding motion)-at a high Reynolds number . At , the aerodynamic forces slightly increased from those in hover . The centres of pressure in these cases were concentrated in the outboard section, and the leading-edge vortices (LEVs) grew more conically than those in hover. Spanwise cross-sectional DPIV indicated that the wings generated more balanced downwashes, which effectively supported the slight lift increments in this range. At 0.25〉, a drastic force drop appeared as increased. The DPIV results in the case clearly showed a strong trailing-edge vortex on the outboard trailing edges encroaching into the upper surface, which had been occupied by the LEV for lower . The LEV vorticity was noticeably weakened, and coherent substructures with substantial turbulence accompanied this vorticity. In the case, such encroachment was extended to 50 % of the section, and the LEV outboard became significantly irregular. The near-wake structures also showed that the case had the narrowest downwash area, with unstable root and tip vortices, which reflected considerable attenuation in the lift enhancements. It was of note that all of these vortical behaviours were clearly distinguishable from aspect ratio effects. The even played a similar role to that of the in the Navier-Stokes equation. These findings clearly indicated that the could be an independent quantity governing the overall vortical system and lift enhancing mechanism on a flapping wing of a flapping-wing micro air vehicle. © © 2016 Cambridge University Press.

Description: This experimental study investigates the effect of a uniform lateral inflow on the aerodynamic characteristics of flapping wings. Seven designated sideward ratios in the hovering condition and in the presence of a contralateral wing and a body were taken into account as variables in order to secure a better understanding of wing-wing and/or wing-body interactions under the lateral inflow. Our results from the single-wing cases clarified that an inflow running from the wingroot strengthened the leading-edge vortex, thereby augmenting the aerodynamic force/moment. The inflow running in the opposite direction drastically bent the leading-edge vortex to the trailing edge, but the cycle-averaged aerodynamic force/moment was barely changed. This led to substantial imbalances in the force/moment on the two wings. The roll moment on a centre of gravity and the static margin suggested flight instability in the lateral direction, similar to previous studies. We found that the wing-wing interaction was not completely negligible overall under a lateral inflow. A massive downwash induced by the wing on the windward side nearly neutralized the aerodynamic force/moment augmentations on the other wing with lower effective angles of attack. The wing-wing interaction also gave rise to a low-lift high-drag situation during the pitching-up wing rotation, resulting in greater side force derivatives than the theory of flapping counterforce. Further calculations of the roll moment and the static margin with the centre of gravity showed that the wing-wing interaction can improve static stability in the lateral direction. This mainly stemmed from both the attenuation of the lift augmentation and the elimination of the positive roll moment of the flapping-wing system. © 2019 Cambridge University Press.