Study On The Drag-Reduction Mechanisms Of Moving Wingtip Based On The Unsteady Formation Of Tip Vortx

In unsteady flow,the growth and evolution of vortex structure is popular and main fluid characteristic.Through the researching of the unsteady mechanisms included in the growth and evolution of vortex structure,the control mechanisms of vortex structure can be constructed in order to give some further tutorial to the related design.In flight drag force of airplanes,the induced drag and the aeroelastic distortion caused by the obvious vortex structure at the wingtip reduces the flight performance.In contrast,birds and insects can control the growth of tip vortex by the motion and deformation of wings,which improves the flight performance.Therefore,this paper will deeply study the tip vortex and the induced drag,in order to reveal the mechanisms of growth and evolution of tip vortex as well as the mechanisms of the occurrence of induced drag.Further,this paper will explore the methods of controlling tip vortex through the motion and deformation of wings to reduce induced drag.In order to reveal the characteristic of the growth and evolution of vortex structure in motion,this paper firstly establishes the coupling relationship among the growth of vortex structure,unsteady force and motion based on study on free falling plate.Then,through the researches on the starting process of wing,the growth and evolution process is analyzed from the wingtip vortex's initial dropping to its development to quasi-steady state,and the growing mechanisms are parameterized.Through wind tunnel experiments,the physical features of steady tip vortex are studied,the induced force is measured and calculated,and the induced-drag generating mechanisms of tip vortex are analyzed.Based on the researches listed above,the motion of the wings can be controlled in order to decrease the induced drag,and the drag reducing mechanisms are revealed.The research works and related conclusions are mainly cataloged into three aspects:(1)Based on the experimental researches of free falling plate,the mechanisms of growth of the wake vortex structure and the occurrence of unsteady force are revealed,and quantified relationship is founded.It is found that by coupling among the growth of vortex structure,unsteady force and motion,there are some typical dropping models in the falling process: stable falling,fluttering,tumbling,and chaotic motion.Based on the principle of vortex formation time,the vortex structure presents different formation time in different falling models.Using quasi-steady theory and Newton second theorem and analyzing the motion characteristic of falling plate,the unsteady force caused by the growth and evolution of vortex structure is calculated.It is found that there is periodic changing regulation between force and motion,but different motion patterns may correspond to different phase difference.Therefore,by analyzing the vortex moment theorem,it is found that the quantified analytical model of vortex growth and the unsteady force reveals the growing mechanisms of the wake vortex structure and the reduction of induced drag.(2)Through PIV experiments,the entire growing process of tip vortex is studied.Further,the change of the circulation of tip vortex and the change of formation time is analyzed.A linkage machine based on continuous laser PIV system is set up.The growing process of tip vortex is recorded.It is found that when the formation time of tip vortex reaches a critical number,the circulation of tip vortex and its outline reaches a quasi-steady state.Therefore,in the wind tunnel experiments,when the dimensionless formation time of the wingtip vortex reached its maximum,the dimensionless circulation surrounded by the tip vortex also reached its maximum,and the corresponding unsteady fluid force also reaches its maximum.In addition,the induced drag can be reduced by controlling and utilizing the physical parameters,such as the non-dimensional intensity of tip vortex and the dimensionless distance between the main wing and the tip vortex.Specifically,the induced drag reduction can be achieved by inhibiting the non-dimensional intensityor controlling the movement of tip vortex,which changes the interaction between the main wing and the tip vortex.(3)The effect of tip vortex motion on the drag force is studied.Thus,the physical mechanisms based on the physical characteristic of tip vortex are revealed.In the wind tunnel experiment,wings are mounted with simplified planar winglet.It is found that drag force can be reduced by controlling the motion of winglet.By comparing the induced drag in different motion model and the static condition,the average drag caused by tip vortex can be reduced by 5% at most.Through Stereoscopic PIV,the physical characteristics of wingtip vortex under the winglet are studied.It is found that controlling the formation time and location of vortex core can guide the motion wingtip,further reducing the induced drag.Beyond that,it can also provide the theoretical support for exploring the methods of utilizing the aeroelastic effect to instruct the design of the wingtip.Based on the studies stated above,the key novel contributions achieved in this dissertation are as follows: 1)the coupling mechanism between the formation of the basic vortex,the unsteady force and the motion is established in order to obtain the growth mechanism of the tip vortex based on non-dimensional parameters;2)the relationship between the physical characteristics of tip vortex and the induced drag is built,and therefore the idea for drag-reduction design is proposed;3)the moving tip model is designed,which can reduce the drag effectively,and its drag-reduction mechanism is studied.Therefore,it can provide the theoretical support for exploring the methods of utilizing the aeroelastic effect to instruct the design of the wingtip.