Study On The Mechanisms Of Drag Reduction Of Civil Aircraft Afterbody Based On Vortex Structures Formation And Interaction

The drag reduction of civil aircraft is an everlasting topic in aircraft design.The generation parts of drag in civil aircraft are the winglet and the afterbody section.The drag resulted from the large-upswept afterbody accounts for 30%-50% of the total drag of civil aircraft.The effective drag reduction of afterbody is significant for the drag reduction of civil aircraft.This paper,in view of the aircraft drag reduction especially the afterbody drag reduction problem,based on the interaction of afterbody detached vortex and horizon tail tip vortex,studies the afterbody flow structure and force generation mechanism,thus providing the theoretical support to civil aircraft afterbody drag reduction and also other components.This paper firstly studies the vortex rings,which are the basic vortex structure in three-dimensional flows,mainly the interacting mechanism of multiple vortex rings and its influence on unsteady force.In this paper,the capturing method of multiple vortex structures is built based on Lagrangian coherent structures.Three phenomena are found by LCSs,which are leapfrogging,merging(due to structure collapse and due to deformation itself)and detachment.These phenomena correspond to the obvious increase or decrease of unsteady force.The further study indicates that besides formation time,another timescale exists in vortex interaction,together determining the force generation mechanism of vortex rings.Thus,this paper proposes the concept of two timescales(formation time and interaction time),which determining the trend of unsteady force in vortex interaction.Under the condition of small formation time,the average force increases with the decrease of interaction time,while when the formation time is large,the transient force and average force will decrease to some degrees.This paper sets up the model of vortex interaction,and finds that vortex rings generated by jets and flow over a body share unified physical structure and characteristics.These studies on the vortex interaction guide the researches on the interaction of multiple vortex structure of afterbody.By analyzing the LCS contours,we can find that the vortex structure of a single afterbody consists starting vortex,detached vortex pair and bound vortex.Its evolution can be explained by formation time.The tip vortex of horizon tails interacts with the detached vortex pair,and this interaction influence the drag a lot.Based on the mechanism of vortex interaction,the tip vortex of horizon tail will rotate periodically around the detached vortex pair in the wake of a well-designed layout of afterbody.Thus,the formation of vortex will be restrained and the average force will decrease.Regarding to the mechanism of drag reduction of civil aircraft,this paper sets up the two-timescale analytic model of unsteady force based on the study of multiple interacting vortex rings,which is significant to the design of drag reduction.