Aerodynamic Mechanisms of Flapping Flight

The growing need for mobile aerial platforms capable of performing reconnaissance tasks and gathering intelligence in hazardous and physically inaccessible areas has given rise to an increasing interest in the development of micro aerial vehicles (MAVs), in view of their potential capabilities of versatile and highly maneuverable flight. Especially flapping-wing configurations, which often tend to be inspired by nature, have captivated researchers in view of their possible advantages over more conventional (fixed and rotary wing) flight modes. Contrary to these more conventional flight configurations, flapping wing systems benefit from the unsteady flow effects that are associated with the large-scale vortical structures separating from the wing leading and trailing edges, which create low pressure regions around the wings that lead to the generation of higher lift and thrust. In this respect, the present thesis pursues an experimental exploration of the aerodynamic mechanisms of flapping-wing phenomena by use of state-of-the-art flow measurement techniques, which are capable of providing time-resolved three-dimensional information. The study is divided into two parts, in which flapping-wing aerodynamics is investigated in the context of a flapping-wing MAV (Part I) and in a generic experimental setup to gain more fundamental understanding (Part II), respectively. The first part consists of studies performed with the flapping-wing MAV DelFly II. First, the aerodynamic performance of the DelFly II is assessed by means of extensive force, power consumption and wing deformation measurements. Force and power consumption measurements were performed under in-air and in-vacuum conditions in order to properly identify the contributions associated to the aerodynamic effects. Subsequently, the balance-mounted force measurements are compared to the forces estimated from free-flight visual tracking measurements in order to characterize the effects of fixed-model force measurements, which restrict the dynamic body modes that exist in actual free-flight conditions as well as introducing the effect of the vibrations which depends on the model clamping position. The comparison of the unsteady forces obtained by the two methods reveals that the X-force component (i.e., the force component along the DelFly fuselage as defined in the body axis) shows a good correspondence, whereas the Z-force component displays deviations depending of the flight condition. Reasons for the differences and main drawbacks of the two force determination techniques are discussed. Effects of the flapping frequency, wing surface thickness and wing geometry on the aerodynamic performance in hovering conditions are also investigated in terms of force and power consumption measurements. In order to assess the relation between the force generation mechanisms and wing deformation characteristics, dedicated structural deformation measurements were performed on the flapping wings of the DelFly. These measurements provide information regarding the effects of the flapping frequency and wing surface thickness on the force generation mechanisms and the power consumption characteristics in terms of both amplitude and temporal evolution. Next, flow field measurements were performed around the wing and in the wake of the DelFly in hovering and forward flight conditions by using planar and stereoscopic particle image velocimetry (stereo-PIV) techniques. The visualizations of the vortical structures around the wings reveal the formation, shedding and interaction mechanisms, particularly visualizing the clap-and-peel motion of the DelFly wings. A momentum-based approach in a control volume around the DelFly in hovering configuration was employed to estimate the flap-averaged X-force under a number of simplifying assumptions. This approach, which only uses the streamwise velocity at a downstream location, is still able to calculate the forces within an error range of 5-15 %. The flow field measurements in the wake of the flapping wings in the forward flight configuration are used to reconstruct three-dimensional wake structures. Two wake reconstruction strategies have been compared: (1) a spatio-temporal wake reconstruction based on the Taylor's hypothesis; (2) a spatial wake reconstruction by means of a modified Kriging regression technique, that takes into account the measurement uncertainties. It is shown that wing–wing interaction at the start of outstroke (peel motion) becomes a dominant feature for reduced frequencies greater than 0.62, while becoming relatively inactive at lower reduced frequency. In the second part, experimental studies concerning the fundamental research in flapping-wing flight are reported. First, the effects of wing flexibility on the unsteady flow around a flexible wing undergoing a clap-and-fling type flapping motion have been investigated by means of tomographic PIV measurements that were performed in a water tank. The two-dimensional Euler-Bernoulli beam equation was used to simulate the chordwise deformations of a plunging thin plate and to compare the deformation characteristics between in-air and in-water conditions. The comparison of in-air and in-water deformation characteristics reveals that it is not possible to achieve identical deformation characteristics, because the inertial forces are significantly reduced under in-water conditions. Second, the evolution of three-dimensional flow fields and unsteady forces are reported for surging and pitching flat plates in revolving motion starting from rest. Tomographic PIV measurements were carried out to acquire the three-dimensional flow fields with force measurements being simultaneously performed. The experiments were carried out for a number of parameters to explore the effects of acceleration period, Reynolds number, angle of attack, number of revolutions and pitch rate. Comparison of the force histories shows that the pitching wing generates considerably higher forces compared to the surging wing. Further analysis was performed to determine the reasons for the enhanced force generation of the revolving-pitching wing. It is shown that the increased force production is associated to a formation of additional circulation (bound and LEV), a higher growth rate of the LEV and more favourable vortex trajectories. It is also revealed that the force generation phenomenon is relatively insensitive to the Reynolds number. Moreover, the forces at the steady-state phase do not depend on the motion kinematics and force histories in the start-up phase.