Numerical Study On Aerodynamic Characteristics Of Micro Bionic Flapping Wings

FMAV(Flapping Micro Aerial Vehicle)is a new type of MAV mimicking the flapping mode of creatures such as insects and birds.They has a series of advantages,such as high maneuverability,high hovering capacity,high concealment and low power dissipation which bring them great potentials in both military and civilian applications.Many key technologies need to be solved in the development of FMAVs,where the aerodynamic mechanism is one of the most critical issues.Although there have been many studies on aerodynamic mechanisms of flying creatures,the effects of airfoil parameters,flapping trajectories,asymmetrical flapping parameters and wing flexibility on hovering and forward flapping flight are still not fully understood.In this thesis,the characteristics of rigid and flexible wings during the hovering and forward flapping flight are systematically studied using the computational fluid dynamics methods.The effects of airfoil parameters,flapping trajectories,and asymmetrical flapping parameters,active and passive wing deformation on aerodynamic forces,energy consumption and flight efficiency are analyzed to provide new ideas and theoretical instructions for the development of FMAVs.Firstly,the effects of the airfoil geometric parameters and wing corrugation on aerodynamic performance of forward flapping wings are studied.Based on the characteristics of insects and birds wing structure,the computational models of the rigid airfoil with different geometric parameters and corrugated strucure are establised.The effects of the wing corrugation,thickness and camber on aerodynamic forces,flight efficiency and flow structure of the forward flapping wing with different inflow velocities are systematically analyzed.The results show that the effects of wing corrugation on aerodynamic performance of flapping wings are less than those of the wing thickness and wing camber.Secondly,the effects of the wingtip trajectories and asymmetric flapping paramters on aerodynamic performance of hovering flapping wings are systematically analyzed.Based on the observations of flying creatures,seven hovering trajectory patterns including the linear pattern,the clockwise/anticlockwise oval pattern,the clockwise/ anticlockwise figure-eight pattern and the clockwise/anticlockwise double-eight pattern are designed.The aerodynamic forces and flow characteristics of the rigid wing hovering with seven trajectory patterns are analyzed by solving the computational hovering model in horizontal stroke plane.At the same time,the computational models of the time and attacked angle asymmetry stroke with inclined stroke plane are eatablished through the analysis of the time asymmetry stroke and attacked angle asymmetry stroke in dragonfly flight.The effects of the asymmetric stroke duration ratio,asymmetric angle of attack and coupled asymmetric angle of attack and stroke duration ratio on aerodynamic forces,flight efficiency and flow structure of the hovering flapping wings are systematically analyzed.Thirdly,the aerodynamic characteristics of flexible flapping wings with active flexibilty are systematically analyzed.An active flexural deformation forward flapping model and an active cambered deformation hovering flapping model are built,respectively.The numerical studies on aerodynamic characteristics of active flexure forward flapping wings and active cambered hovering flapping wings are carried out.The effects of phase angle and the amplitude of deformation on aerodynamic forces,energy consumption and flight efficiency of the flapping wings are investigated.The results reveal the behavious how active deformation influences the hovering and forward flapping wings,and provide new idea for aerodynami design of smart deformation flapping aerial vehicles.Finally,the aerodynamic characteristics of flexible flapping wings with passive flexibilty are systematically analyzed.Considering that the deformation of flying creatures are usually passive deformation under the inertial force and aerodynamic force,a lumped-torsional hovering flapping model and a beam structured forward flapping model are establised,respectively.The influences of frequency ratio and mass ratio on aerodynamic performance of hovering and forward flapping wings are investigated by using loosely fluid-structure-interation methods.The results show that appropriate wing flexibility can enhance the thrusting efficiency and lifting efficiency simultaneously,and excessive wing flexibility will make the aerodynamic performance of the flapping wings worse.The present results have provided physical insight into the understanding of mechanism of flapping flight,and useful guidance and technical reference for the development of FMAVs.