Studies On Aerodynamic Responses Of The Actively Morphing Model-wings And Effects Of The Passive Deformation During Bat Forward Flight

Bat is the only mammal capable of powered flight, and its wing is comprised of a mem-brane that is stretched between the forelimbs, the body walls and the hindlimbs. The active wing-deformation is large, complex, and controllable, which is attributed to its distinct ar-rangement of dozens of bones and joints. And the thin and flexible membrane gives rise to the passive deformation during flapping. The large and complex wing deformation is partic-ularly observed for a small bat flying at moderate/low speeds. Some questions arise. How to model the active wing-deformation, and how the deformation affects the aerodynamic forces acting on the bat wing during forward flight, and why? Further more, how does a bat control its wing morphing to modulate the aerodynamic forces to meet flight demands? And how about the passive deformation of the membrane? To answer these questions, a flexible plate is used to model a small bat wing (20g) flying at moderate speeds (U=3-4m/s). A three dimensional unsteady panel method is applied to predict the aerodynamic forces, and a non-linear finite element method is used to investigate the passive deformation of the membrane.The present works and results are summarized as follows:1. Based on the unsteady panel method and the finite element method, a computing platform for fluid-structure interaction is set up.The three-dimensional unsteady panel method has been improved. Based on the in-house code, the vortex shedding from the leading-edge of the plate-wing is taken into account by implementing Kutta condition. The numerical singularity due to the wake/plate panels interaction is avoided. A finite element method program considering the nonlinearity of the geometry and the orthotropy of the material is developed, and is coupled with the panel method to study the passive deformation of the membrane.2. The kinematics of the bat wing is modeled, which is comprised of flapping motion and large active deformation. There are four elementary wing-morphing models:twisting and bending in the spanwise direction, cambering in the chordwise direction and wing area-changing. The aerodynamic responses and mechanisms of the model-wing are investigated. The wing deforming regulatory mechanisms for bat steady flight with different load of body (feed or pregnancy) and for accelerating flight are studied at last. The concluding remarks are summarized as follows:1) A twisting wing can generate thrust, and the larger the twisting, the larger the thrust. For the combined morphing wing, the thrust is also affected by cambering and area-changing. When the amplitudes of cambering and area-changing are small, the smaller the deformation-s, the larger the thrust. When the amplitudes are large, the larger the deformations, the larger the thrust. 2) The cambering model has a great positive influence on the lift. Compared with a purely flapping wing, a cambering wing has a great positive influence on the lift, followed by area-changing model and then the bending model. Furthermore, the aerodynamic power can be reduced as the wing twisting deformation increases, while it can be increased by increasing wing cambering/area-changing.3) Vortex control is a main mechanism to produce high aerodynamic forces. The twist-ing in bat flight is of the same function of the supination/pronation motion in insect flight to produce thrust. By increasing or decreasing the effective angles of attack during the flapping motion, the asymmetric effects of the cambered wing is the mechanism of the high lift en-hancement. To amplify the positive effects in the downstroke (positive lift and thrust) and to reduce the negative effects in the upstroke (negative lift) can enhance the lift too for an area-changing/bending wing.4) A bat can control its wing morphing to modulate the aerodynamic forces according to its flight purpose for the optimal flight. In the steady-state flight, larger twisting deformation of the wing is suggested for the lower aerodynamic power output. When large accelerations are required for preying or evading predators, it is recommended for a bat to perform the maximum twisting and area-changing of the wing, and wing camber around0.1.3. A rectangular membrane with the flapping movement of its four boundaries is used to model a bat arm-wing. The passive deformation and its effects on aerodynamic performance of the flapping rectangular wing have been studied preliminarily. Based on our numerical method and physical model, conclusions can be drawn.When the elastic moduli of the ’arm-wing’ are set to be the values of a bat membrane, the passive deformation of the arm-wing is negligible and there is almost no influence on aerodynamic forces. If the elastic moduli and the prestresses are decreased to1%of that of a bat membrane, the passive deformation becomes large. But the averaged lift changes little and the drag decreases slightly, while the amplitudes of the instantaneous forces increase significantly, which results in the aerodynamic power increasing.