INVESTIGATION OF OBSTACLE EFFECTS ON THE AERODYNAMIC PERFORMANCE OF FLAPPING WINGS

The study of highly unsteady wing flapping includes the large scale vortices, complicated locomotion/dynamics and deformable wing structures. When flapping insects/birds approach or perch on some objects, such as ground, wall or obstacle, the solid boundary dissipates, absorbs and bounces the leading edge, trailing edge and wing tip vortices, which are generated and shed during the flapping flight. Such phenomenon creates a high pressure area, leads to cushion effect and influences the aerodynamics, stability and maneuverability significantly. This paper uses immersed boundary method (IBM) to numerically study the aerodynamic performance of flapping wing in proximity of obstacles, investigate the distance, flapping motion and wing flexibility effects and relevant symmetric/asymmetric flow patterns, research the influence of vortex generating and shedding to the lift/drag change, explore the key distance and reveal the mechanism how insects/birds adjust the flapping motion to achieve ideal flight. Such research could theoretically support the development of micro-bionic flapping wing vehicle. NOMENCLATURE A Flapping amplitude f Flapping frequency α Rotation amplitude c Chord length of flapping wing d Diameter of obstacle D Distance between wing and obstacle Address all correspondence to this author. INTRODUCTION Scientists have been studying the micro-aerial vehicle (MAV) for decades. Among these researches, aerodynamics of insect flight has drawn considerable attention in recent years due to its promising application in the development of biomimetic MAV [1]. Ideal biomimetic MAV with flapping wings is of small scale, high maneuverability, low energy consumption and noise, high lift and thrust generation, and high controllability. Insects adjust the flapping amplitude, frequency and mode to accommodate to various environments, which may be more efficient than fixed wings [2]. In nature, insects often approach or perch on some objects such as flowers, grass and rocks. Under such circumstances, the boundary effect influences aerodynamics and stability greatly [3, 4]. To study the characteristics of flapping flight in finite space, especially interacting with ground, cliff or obstacle, will help us understand how the aerodynamic forces, flight mode and stability are affected and provide new perspective on MAV design. Researchers have done extensive work on the infinite ground effect numerically and experimentally. However, the knowledge of finite obstacle effect is very limited. Therefore the purpose of this paper is to investigate the obstacle effect on the hovering flight by constructing simplified computational model. Because the quasi-steady theory fails to explain the unsteady, large angle of attack (AoA) and high lift flapping flight, Ellington [5] pointed out that the stall delay mechanism due to the leading-edge vortex (LEV) remarkably enhance the lift force. Wake capture and quick rotation also contribute to the lift increase and force peak at the end of flapping cycle [6,7]. Wang [8] and Shyy [9] numerically simulated the insect flight, analyzed the unsteady aerodynamics and validated the aforementioned mechanisms. Further, ground effect has been considered when studyProceedings of the ASME 2017 Fluids Engineering Division Summer Meeting FEDSM2017 July 30-August 3, 2017, Waikoloa, Hawaii, USA