Research On Force Generation Mechanism Of Multiple-vortex System For Drag Reduction Of Large Civil Aircraft Afterbody

The drag reduction research of large civil aircrafts is related to its economy and environmental protection,which is an important topic of current development and future sustainable development.Therefore,it is supported by the National Key Basic Research Development Plan(973 Plan),"Research on drag reduction mechanism and method of large civil aircrafts".The vortical structure of large passenger aircrafts is complex,which is closely related to its resistance under cruising and high-lift layouts.However,the structural characteristics of the rear fuselage vortex system,the mechanism of drag generation,the method of calculation and the means of drag reduction are still unclear and imperfect.This paper,hence,intends to reveal the coupling mechanism between the topological structure and the drag change caused by the formation,evolution and interaction of the vortices through in-depth study of the afterbody vortex system,so as to provide an effective theoretical basis for drag-reduction method of large passenger aircrafts.To meet the above requirements,the interaction mechanism of the afterbody vortex system is studied in this paper.On this basis,the mechanism of the force exerted by the vortices is explored,and the drag reduction mechanism and design method of the afterbody are explored.The specific works are as follows:Firstly,the detailed flow visualization and qualitative vortex phenomena of the single and multiple vortex-pair flows are studied by using IDDES numerical simulation method.On this basis,the interaction kinematics model of the co-rotating vortex pairs is established by integrating the wind tunnel 2D-PIV test and the two-dimensional numerical simulation of vorticity-stream function.By changing the Reynolds number,the ratio of circulation and the ratio of radial separation distance,this paper investigates the variation of the kinematic quantities of vortices.The vortex interaction is mainly dominated by two key dimensionless parameters,circulation ratio and separation ratio,varying from 0.65 to 3.25 and 0.12 to 0.35 respectively.Reynolds numbers based on total circulation of the coherent pairs range from 1.42×10~4 to 9.47×10~4.Secondly,in order to reveal the force generation mechanism of the coherent vortex pairs,this paper establishes the vorticity-loop model to characterize the interaction,and develop and apply the Vorticity Moment Theorem to the analysis model.The contribution force of vortices is strictly defined and quantitatively decomposed,and the generation mechanism of vortices is also explained.The results show that the lift and drag of the vortex force are proportional to the time-varying rate of the circulation and the enclosed area of the vorticity loop.The enclosed area is affected by the vertical and lateral vortex-wash velocities produced by self-and mutual-induction.Specifically,the linear total lift of a system(multiple co-rotating vortex pairs)is independent of the instantaneous vortex interaction and remains constant,while the nonlinear total vortex drag of a system is naturally time-dependent.In addition,the lift and vortex drag produced by each counter-rotating vortex are also unsteady.Betz and Maskell wake integral models are used for comparative analysis,and the traditional induced drag can be regarded as the steady case of the vortex drag without the interaction of vortices.Thirdly,based on the above mechanism research,this paper uses the RANS numerical simulation method to study the drag reduction mechanism and the design method of the vortex generator(matrix)on the wing-fueselage-tail model.By changing the installation position and angle of attack of the vortex generator,this study adjusts the ratio of vortex intensity and the ratio of separation distance,and explore the coupling relationship between different afterbody vortices and the vortex drag of the afterbody.The drag reduction is realized by using the interaction mechanism of the vortices.The above research provides a theoretical basis for the rational design of the vortex generator for flow control and drag reduction.Based on the above research,this paper has made some innovative progress in the following two points:Firstly,the interaction mechanism of the afterbody vortex system of large civil aircrafts is preliminarily revealed,the vorticity-loop model characterizing the vortex interaction is established,and the generation mechanism of the vortex drag is revealed based on the Vorticity Moment Theorem.Secondly,the above research theory has been preliminarily applied to the design of the afterbody vortex generator of large passenger aircrafts,and a certain effect of vortex drag reduction has been achieved.