Experimental study on longitudinal aerodynamic characteristics of an insect-based flapping wing

Abstract

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that the previous assumption of the CP at the quarter chord, which was based on the classical concept for airfoils, is no longer valid for the pitching moment prediction on the wing.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.