23 research outputs found
날개 유연 효과가 플래핑 날개 공력 특성에 미치는 영향에 관한 실험적 연구
No full text학위논문(박사) - 한국과학기술원 : 항공우주공학과, 2021.8,[viii, 89 p. :]In this study, the unsteady aerodynamic characteristics of a flexible flapping wing undergoing hovering flight was studied. Although a rectangular wing planform was used, the wing could exhibit similar wing deformation features as natural fliersnegative wing twist and positive camber. This wing deformation was possible by designing the wing root to have an offset angle termed the slack angle. A dynamically scaled-up robotic wing model equipped with a six-axis sensor was immersed a 3-ton water tank to measure the time-varying aerodynamic forces and moments. The digital particle image velocimetry (DPIV) technique was deployed to observe the vortical structures around the wing. The rigid wing was included throughout this study to help understand the contribution of wing deformation in unsteady aerodynamics.
First, the aerodynamic load characteristics of the flexible wing revealed that the presence of wing deformation caused the pressure forces to act in the tangential and normal directions. This effect resulted in the effective angle of attack range to be beyond the 90° for the flexible wing case. To this end, the existing aerodynamic model, which was built for the rigid wing, was revised to account for wing flexibility. The newly-extracted force coefficients were well-fitted with a cubed-sine function. The model was rigorously validated with various wing kinematics, giving a good estimation of the experimental results. The estimated error was less than 5%, 6%, and 8% for the lift, drag, and moment, respectively, considering fast to moderate wing kinematics.
Based on the revised model, the optimum angle of attack for maximum lift generation was experimentally obtained for each change in slack angle. An increase in slack angle tend to increase the optimum angle of attack for maximum lift generation. However, the practical angle of attack for application in flapping wing micro-air vehicles (FWMAVs) was found to be from 75° to 85°. Generally, the flexible wing generated less drag, consumed less power, and was more efficient than the rigid wing. This was an indication that the presence of wing flexibility requires natural fliers and FWMAVs to undertake less pitching motion in order to reduce the mechanical power and increase the efficiency of their wings. In addition, there was a conspicuous phase delay between the aerodynamic characteristics of the rigid and flexible wings. The delay was found to be very sensitive to wing kinematics.
In order unravel the effects of wing kinematics on the unsteady aerodynamic characteristics, experiments were conducted with two main wing kinematic parameters, sweep duration and timing of wing rotation. This study found that the conspicuous phase delay in the flexible wing was more sensitive to the change in sweep duration than the timing of wing rotation. The transient negative lift associated with rigid wings undergoing delayed and advanced wing rotations were observed to have totally disappeared in the flexible wing case. In general, the flexible wing with symmetric and delayed wing rotations generated the highest wing efficiency. The corresponding net force vectors were observed to be tilted in an almost vertical direction for the flexible wing in comparison to the rigid wing. The vorticity distribution at the middle of stroke revealed a slight difference in the vortical structures surrounding the rigid and flexible wings in terms of proximity to the shed trailing-edge vortices (TEVs). The linearly twisted nature of the flexible wing caused the TEVs at the outboard section of the wing to be closer to the surface of the wing. The wing twist again caused the coherent leading-edge vortex (LEV) to be stable along the wingspan, and caused the vortex lift to be sustained at the outboard section of the flexible wing regardless the aspect ratio AR. The finding in this study shows that the wing twist of natural fliers could help generate and sustain sufficient amount of lift at the outboard wing sections. Thus, the spanwise deformation of their wings could be an essential wing feature for maintaining the LEV stability across the wingspan even with high AR wing.한국과학기술원 :항공우주공학과
예인 수조를 이용한 날개 유연도가 플래핑 날개 성능에 미치는 영향 분석
No full textWing flexibility for flapping fliers is essential to reduce the overall body weight and effectively minimize the inertia force. In this study, experiments are conducted to assess the flapping performance of flexible wings based on the aspect ratio AR and advance ratio J. The wing flexibility is defined based on the novel slack-angle beta design concept. The measurements are conducted with a servo-driven towing tank system equipped with a two-degree-of-freedom robotic manipulator. The flapping motion consists of the back-and-forth movement of the wing, leading to the upstroke and downstroke phases of the motion with a fixed horizontal stroke plane. A detail examination of the AR, J and beta effect on the transient lift and drag coefficients in each stroke reveals two distinct peak forces that only appeared in the upstroke. The peak forces are found to be strongly dependent on J than the other two geometric parameters of AR and beta. In addition, the study investigated the coupling effect of AR and J on the aerodynamic lift production. It is revealed that for wing flexibility defined by beta, the maximum lift is concentrated from J=similar to 0 to similar to 0.38 and AR=similar to 2.5 to similar to 3.3 for beta=5 degrees. These AR and J ranges further emphasize the dominance of low AR wings and J in flapping wing flight for relatively low Reynolds number regime.
Wing Kinematics Effect on the Aerodynamic Performance of Flexible Flapping Wing in Hover
No full text
Wing-Wake Interaction of Insect-Like Flapping Wing in Hover: Effect of Aspect Ratio and Kinematics at Re~104
No full textIn this paper, the effect of wing aspect ratio and kinematics on wing-wake interaction at Re~104, which matched the flight regime of flapping-wing micro air vehicle (FWMAV), was investigated. The dynamically scaled-up robotic model submerged in a water tank environment revealed that the wing-wake interaction augmented lift across a decrease in both aspect ratio and wing pitching duration. At such high Re, a time-course digital particle image velocimetry (DPIV) measurement showed the entire flow was strongly dominated by trailing-edge vortices (TEV). A pair of counter-rotating TEV was found to induce a jet-like flow towards the windward side of the wing at stroke reversal. The transfer of momentum from the accelerated flow to the wing caused the enhanced lift. The size of the pair vortex decreased for an increase in both aspect ratio and wing pitching duration. The size of the TEV pair was the key feature found to generate the observed aerodynamic force characteristics
Effects of sweep-motion profile on rigid and flexible flapping-wing aerodynamics
No full textExperiments were conducted to examine the effect of sweep motion profile on the aerodynamic characteristics of flexible and rigid flapping wings in hover. The sweep motion was varied from triangular to sinusoidal profiles. The study revealed that the flexible wing was more sensitive to the change in motion profile by inducing a certain amount of phase delay. The sinusoidal and triangular profiles were found to be the most effective and efficient wing motions, respectively, for both wings. The presence of wing deformation increased the net force angle and led to a superior aerodynamic performance than the rigid wing. The flow visualization also revealed that the fully developed wake from the flexible wing motion was rather beneficial to augment the lift. This was contrary to the detrimental effect observed for the rigid wing. The wake increased the effective angle of attack, and subsequently augmented the lift in comparison with a quiescent flow. However, the overall wake was massively mitigated with stronger root vortex when the flexible wing was pitched at 45°. This implies that the insect wing deformation coupled with the surrounded wake effect during hovering flight might play a significant role in enhancing the lift
