Aerodynamics of flapping-wing Micro-Air-Vehicle

Abstract

The interest in Micro Air Vehicles (MAVs) has stimulated continuous research activities, in view of their potential in civilian and military applications. An autonomous MAV with dedicated onboard sensors would be capable of executing mission in closed environments, such as surveillance, in door inspection in support of rescue operations and information gathering. Flapping wing MAV (also referred to as ’ornithopter’), which is the most intriguing type in the MAV family is often inspired from biological examples has triggered attention due to their outstanding manoeuvrability and flight capability at the low Reynolds number regime. Like natural flyers, flapping wing MAVs are usually equipped with a single or multiple pairs of flexible wings to generate both thrust and lift at the same time. Compared to the other locomotion, the flappingmotion benefits from a number of specific unsteady lift enhancement mechanisms. This thesis performs an integrated experimental and numerical study in order to explore the aerodynamic behaviour associated with flapping wing MAVs. The study is hereby divided into twofold: (1) Experimentally investigate the aerodynamic characteristics of flappingwingMAV, using state-of-the-artmeasurement techniques; (2) Develop a novel numerical methodology with particular focus in simulating flapping wing aerodynamics. For the experimental study, theDelFlyMicro was employed as the testbed in this thesis. The DelFly Micro uses an X-wing configuration which performs the clap-and-fling mechanisms three times during each flapping period and thus will result in complex flow behaviour. The aerodynamic characteristics were evaluated by means of extensive force production and power consumption measurements. The aero-structural effect of the DelFly Micro is fist investigated by means of a vacuum test, where the aerodynamically associated thrust component and different deformation patterns in air and in vacuum were addressed. Subsequently, the effect of wing properties, including flapping frequency, flexibility and wing aspect ratio are also examined in terms of thrust production, power consumption and propulsive efficiency. To better understand the aerodynamic force generation with respect to the flow structures, flow topology was visualized by Particle Image Velocimetry (PIV) techniques for the DelFlyMicro in both hover and forward flight configurations. The PIV measurement in the vicinity of the DelFly Micro revealed the formation, evolution of the vortical structures on the wing surface during flapping motion. A jet flow behaviour was revealed at all-time instants is induced by the clap and fling event. Three-dimensional flow structures in the wake of DelFly Micro were visualized by two reconstruction strategies: (1) a spatial reconstruction using Kriging regression with local error (can be regarded as the measurement uncertainties) estimates; (2) a temporal-spatial reconstruction based on the Taylor’s hypothesis of frozen flow. Two dominate structures generated during the instroke and outstroke alternatively shed from the wings which are quite isolated from each other for both hovering and forward flight configurations. In view of the facts that numerical simulating for flapping wing application is difficult due to the inherently large displacement during the flight. Particularly for the clap and fling event which is the most extreme situation would challenge the conventional dynamic mesh deformation techniques. A deformable mesh strategy was developed in this thesis with a specific purpose to simulate the flow with multiple bodies undergoing arbitrary large motion. The dynamic mesh strategy uses a hierarchical overset grid where the mesh quality and resolution can be independently controlled. Additionally, the deformation of the body is achieved by means of a RBF-based mapping mesh deformation technique which is proved to be efficient and robust in terms of CPU time and grid quality. The developed dynamic mesh method was embedded in an in-house programmed URANS solver and further used to simulate the DelFly II flapping wing MAV. The vortical structures demonstrate very complex flow behaviours associated with the DelFly II flappingwingMAV. Comparison between the numerical and the experimental results indicates that the numerical solver can provide a quantitative prediction of the unsteady aerodynamics of the flapping-wing MAV in terms of aerodynamic force production and flow structures.