| Since ancient times,humans have been pursuing and yearning to fly in the sky like a bird.Since the take-off of”Flyer One”,humans have tried to continuously optimize the design of fixed-wing aircraft from a bionic perspective,and have made many progress and achievements.However,after more than 30 years of development,the lift-to-drag ratio of large civil aircraft has increased from 15.1 of Boeing 737 to 20.8 of Boeing 787,which is far from the theoretical lift-to-drag ratio of 80 for birds.This shows that,from the perspectives of thrust and lift,the morphological bionics alone cannot directly face and fundamentally solve the problem that the existing aircraft is relatively inefficient than the biological flapping animals.Since the 21st century,flapping wing aircraft has been constantly innovating,but it is still far from the high efficiency of insects’and birds’flapping wing flight.The main reason is that the recognition of control relationship between the formation and evolution of wake vortex in the flapping wing and aerodynamic generation and enhancement has not been revealed.Therefore,in order to achieve controllable flapping-wing flight,the first step is to study the generation and enhancement mechanism of unsteady aerodynamic forces in flapping-wing flight,and then explore effective aerodynamic gain control methods.In this paper,the unsteady enhancement of the most basic vortex structure,vortex ring,is used as an entry point.Based on the growth evolution and interaction of the vortex structure,the mechanisms of generation and enhancement of aerodynamic force of flapping wing flight are explored,and the effect is verified.The specific research content and main conclusions are as follows:First,through the study of interaction of multiple jet vortex rings,the physical control mechanism under the interaction of multiple vortex systems is revealed.In this paper,the multi-vortex ring interaction is divided into strong and weak interactions based on the Lagrangian coherent structures.There is a”flux window”in the strong interaction,and the window is closed in the weak interaction.In order to distinguish the two formation modes of enhancement and restraint of vortex ring in interaction,a non-dimensional parameter”structural stretching number”for evaluating the induced effect of interaction on the vortex ring formation in a multiple vortex field is proposed in this paper.Combining the structural stretching number and formation time,the inherent mechanism of the enhancement mode caused by formation and interaction of vortex structures in nature can be revealed,which provides an important basis for the design of bionic propulsion.Secondly,in order to investigate the wake vortex formation and evolution as well as aero-dynamic generation and gain mechanism of flapping wing flight,the aerodynamic generation and enhancement mechanisms are revealed and the corresponding prediction models are estab-lished.Based on Reynolds number,flapping wings can be divided into insects(Re=10～10~3)and birds(Re=10~4～10~6).In the insect flapping wing flight,three motion modes of”ad-vance”,”symmetry”and”delay”are proposed based on the phase difference between stroke and rotation motions of wing.In different modes,there are big differences in the growth of leading edge vortex and stable attachment.In the”advance”mode,the leading edge vortex can stably attach during the translation of the wings and interact with the wings during rotation.The physical formation time changes with the phase difference:3.8～5.3,indicating that insect can control the formation time of leading edge vortex by changing the phase difference of stroke and rotating motions,thereby controlling the required lift based on flight requirements.In ad-dition,the wake can be captured by the wake vortex interaction during the wing rotation,which can further enhance lift.In the flapping flight of birds,the wings speed up the wake jet during flapping up and down,and dual-wing has an advantage.The flapping wing motion can make the circulation of wingtip vortex change significantly,accelerate the dissipation,and enhance its instability.Unsteady lift reaches its peak at the downstroke end,while thrust has two peaks during the up-and downstrokes,which corresponds to the flow direction.As reduced frequency increases,the average lift increases slightly near the fixed-wing lift,and the average thrust in-creases significantly.The lift performance of dual-wing is better than that of single-wing.It can be seen that the flapping phase angle coupling of the arm and hand wings can improve the flapping wing aerodynamic force.The force model is mainly divided into quasi-steady terms and apparent mass terms determined by the coupling of flapping velocity and free-stream ve-locity.The quasi-steady terms are related to the shape of wing itself,while the apparent mass terms are closely coupled to flight speed and flapping frequency.Therefore,flapping wing in cruising flight are mainly to obtain enough thrust to overcome the drag.To sum up,it can be found that aerodynamic forces can be generated and improved through the coupling of flapping wing motion and wake vortex formation and evolution during flapping wing flight.Finally,by designing the flapping-wing model,the aerodynamic control method for flapping-wing flight is established and verified.By designing an insect-type passive rotating flapping wing model,driving Reynolds number is defined based on the wing translation.When the driving Reynolds number is between 120～240,the flight attitude can be changed in a wide range to achieve various flight actions,and the flight efficiency is high.By analyzing the stability of the bird-shaped flapping wing model,it can be found that the flapping wing flight can be better stabilized only when the flight speed is coupled with flapping wing frequency,and dual-wing flapping has better control effect on flight stability.In the formation flight of the bird-shaped flapping wing model,it is found that the optimal phase difference between the front and rear flapping wing models is between-18?～0?,the wingtip vortex attenuation is obvious,and the proportion of turbulent kinetic energy is also large,and can reach a large value within a wide range of wingtip spacing.Flapping wing formation flight is easier to introduce vortex structure interaction than fixed wing,and has the advantage of drag reduction.In summary,it can be found that based on the phase angle coupling effect of flapping wing motion,the aero-dynamic gain of flapping wing flight can be better controlled,and efficient and stable flapping wing flight can be achieved.Based on the unsteady gain of interacting vortex structures,this paper explores the un-steady aerodynamic gain mechanisms and control methods of flapping-wing flight under differ-ent Reynolds numbers,designs the flapping-wing flight model and verifies the effectiveness of aerodynamic control.The innovation lies in:(1)discovering two modes of vortex interaction and proposing a multi-vortex interaction analysis model based on dual physical parameters;(2)establishing an aerodynamic prediction model based on the coupling of wake vortex formation and evolution and flapping wing motion;(3)proposing a flapping-wing aerodynamic gain con-trol method under the coupling of motion phase angles,and design a flapping-wing flight model for verification.The above work has important guiding significance for the design of efficient flapping air vehicle and drag reduction of large civil aircraft. |
