In this paper, the longitudinal aerodynamic characteristics of an insect-like flapping wing is studied to reveal the underlying causes of the lift augmentation mechanisms, and to build an appropriate aerodynamic model that allows to accurately estimate the time-varying aerodynamic forces and moments. To this end, dynamically scale-up robotic wing models and servo-driven towing tank are developed. These are synchronized with each other for high fidelity time-varying measurements. A micro-sized six-axis sensor is used to measure aerodynamic forces/moments on the wing model. The phase-locked and/or time-resolved digital particle image velocimetry (DPIV) technique is employed to observe the vortical flows around the wing.
In order to understand the effect of the wing kinematics on the aerodynamic characteristics in hover, several motion profiles composed of the piecewise sinusoidal functions are applied into the model. In most cases, the time-varying aerodynamic forces in the stroke phase show good agreements with the translational velocity profiles. This implies that although the wings produce highly vortex-dominated unsteady flow, the aerodynamic characteristics in the stroke phase can be assumed as a steady-state configuration. The vorticity distributions in all cases indicate the strong vortex wake and unsteadiness, but the first trailing-edge vortex (TEV), which is generated by the start of the wing pitch, is rarely associated with the peak of the wing-wake interaction. The relationship between the force peaks and the flow structures implies that the characteristics of the second TEV has significant roles in the wing-wake interaction. Based on above findings, a quasi-steady aerodynamic model including the time-varying pitching moment is developed. The blade element approach with Polhamus leading-edge suction analogy is employed to describe the vortex lift in this model. Unlike previous models, the movements of the centers of pressure (CP) are also considered for higher accuracy of the aerodynamic pitching moment. The present aerodynamic model provides better predictions showing much enhanced agreements with the actual force and moment on the wing model. This clearly implies that the previous assumption of the CP at the quarter chord, which was based on the classical concept for airfoils, is no longer valid for the pitching moment prediction on the wing. The effect of advance ratio J on the lift augmentations is also investigated. Nine J cases covered from the hovering flight (J=0) to the fastest forward speed of living insects (J=1.0). The aerodynamic forces are slightly increased with increasing J in low J cases (J<0.25), but they show drastic drops approximately at J>0.25. The chordwise cross-sectional DPIV results show that the strong TEV encroaches into the LEV region from the outboard trailing edges as the J increases. The coherent substructures and substantial turbulent kinetic energy with upwash well support the attenuated LEV, lower vortex lift, and consequent poor aerodynamic performance at higher J. The aerodynamic forces reconstructed by the instantaneous stroke velocity show the negligible variations, which indicates both the small unsteadiness and the validness of the quasi-steady aerodynamic model. Rarely changed CP locations also imply the negligible unsteadiness during the stroke. Given these findings, there is the final effort to compensate the J effects in the quasi-steady aerodynamic model. Alternative form of Polhamus leading-edge suction analogy and some suppositions are reemployed to establish the aerodynamic model. A total of three sets of Polhamus coefficients KP and KV are extracted from the data at each fixed J, and the KP and KV are reconstructed as functions of J. Exponential functions of K？P and KV, which converge to the results in the case of J=∞, indirectly show the reduction of the vortex lift with increasing J. With these corrections, each time-varying estimation well follows the measurement results in the forward flight.
곤충형 플래핑 날개의 세로방향 공력특성을 분석하기 위한 실험적 연구가 수행되었다. 2축과 3축의 회전 자유도를 갖는 고해상도 로봇 모델을 설계/제작하였으며, 로봇모델과 동기화되는 랙-피니언 시스템 기반의 토우잉-탱크와 미세힘 측정용 로드셀, 위상동기 및 시분할 PIV 등을 활용해 공력특성의 비정상 거동과 유동구조를 분석하였다. 제자리비행을 기준으로 다양한 날갯짓 함수를 입력해 얻은 공력측정결과, 비정상 공력특성인 날개-후류 상호작용은 시간적, 공간적으로 매우 국부적인 효과임을 확인하였고, PIV결과 이는 주로 뒷전와류에 의해 지배되는 것을 알 수 있었다. 이를 바탕으로 반실험적 준정상 공력모델링을 진행하였으며 압력중심의 거동을 파악해 향상된 공력모멘트를 예측할 수 있도록 설계하였다. 날개-후류 상호작용이 지배적인 스트로크 초기를 제외하고, 공력모델링은 대부분의 구간에서 공력/모멘트 변화를 준수하게 예측할 수 있음을 확인하였다. 특히 제안된 공력모델은 스트로크 반전에서 나타나는 모멘트 변동을 상당수준 보완할 수 있음을 확인하였다. 더불어 전진비에 따라 달라지는 양력/항력/피칭모멘트 계수를 보상하기 위해 다양한 전진비가 앞전와류에 미치는 영향을 파악하였다. 이를 통해 특정 전진비 이상에서 앞전와류가 유지되지 못하고 와류양력을 상실하는 것을 관찰하였으며, 다양한 전진비의 측정결과를 바탕으로 이를 보상하는 공력모델을 제시하였다. 보상된 공력모델은 복잡한 날갯짓에서도 기존의 모델에 비해 향상된 예측이 가능하였으며 특히 피칭모멘트의 보상능력이 뛰어났다. 상기 결과는 곤충 모방형 초소형비행체를 개발하기 위한 다양한 방면에 쓰일 수 있을 것이다.
