Study On The Mechanism Of The Vortex-induced Drag Of Plate Tip Vortex

Tip vortex is the primary vortical structure in the wake of both civilaircrafts and flapping wings. The vortex-induced drag and lift are critical toresearches on aircraft aerodynamics and high lift-to-drag ratio in flappingwings. Ellington and Dabiri studied the evolution of this vortical structure toanalyze the mechanism of generating aerodynamic forces, which provided anew roadmap for researches on relation between tip vortex and the draginduced. Tip vortex is generated due to finite aspect ratio and pressuredifference. Hence, studying the tip vortex of a flat plate is appropriate.Ringuette et al. investigated the startup of a flat plate with various aspect ratioexperimentally. It was reported that the vortex-induced drag stayed constantwhen growth of tip vortex stopped. However, the evolution of tip vortex andthe mechanism of the generation of vortex-induced drag were beyond theirdiscussions.In this paper, both numerical and experimental studies are conducted.Above all, in the numerical analysis, evolution of tip vortex was investigatedat first, and secondly change of vortex-induced drag was analyzed withvorticity-moment theory to study the mechanism of the induced draggeneration.Three stages were found during evolution of tip vortex: rapid growthwhen vortex core sticks to plate; vortex shedding and slow growth in thedownstream; stabilization of position of vortex and injecting circulationsteadily. From the viewpoint of momentum transfer, it was found that the generation of unsteady force attributed to both injecting circulation to increasemomentum and exchanging energy with ambient fluid to change momentum.Meanwhile, vortex-induced drag experienced corresponding stages: induceddrag increasing quickly due to rapid growth of tip vortex, decreasing becauseof the growth slowing down and vortex stabilization leading to steady induceddragFurthermore, experimental investigation was conducted to explore theevolution of three dimensional vortical structure of tip vortex, and similarstages were observed as that in two dimensions.