旋转圆柱尾流中柔性丝线的拍动模态研究

Study on flapping modes of a flexible filament in the wake of an upstream rotating cylinder

本文采用罚浸入边界/格子玻尔兹曼方法研究了旋转圆柱尾流中的柔性丝线拍动模态.通过改变圆柱旋转率(α)及圆柱与丝线的水平间距(G),分析了α=0.1–3.0,G/D=0.05–3.5(D为圆柱直径)的参数平面内柔性丝线的拍动模态,探讨了模态间的转变特性及不同模态的形成机理.根据拍动丝线形成的包络图,本文发现了反向、缠绕和正向拍动三类主要模态,其中反向和正向拍动模态分别包含以下典型亚模态:反向单侧拍动模态、反向双侧拍动模态、反向静止模态;正向单侧拍动模态、正向双侧拍动模态、正向静止模态;并基于参数空间的模态相图,获得了间距比G/D渐增导致的反向-正向模态的主要模态转变路径;通过分析不同模态的典型尾涡结构,探讨了柔性丝线拍动模态形成的流动机理;此外,发现在特定参数下,丝线拍动可产生明显的反馈作用:完全抑制旋转圆柱尾流脱涡或促进尾涡生成脱泻,导致旋转圆柱减阻(甚至产生推力)或明显增阻.

Flapping mode of a flexible filament in wake flow behind a rotating circular cylinder has been studied numerically by using of an immersed-boundary-based lattice Boltzmann method. In a parameter space of a=0.1-3.0 and G/D=0.05-3.5, where a is the cylinder rotation rate and G/D is the distance ratio between the filament and cylinder which has a diameter of D, flapping modes of the filament are analyzed in the phase diagram to examine the mode transition features and the mechanism of formation for different modes. Specifically, three kinds of flapping modes have been found which are labelled as reversed, coiled, and normal flapping modes; six sub-modes： reversed one-side, two-side and steady modes, and normal one-side, two-side and steady modes have been identified. Based on the phase diagram in the （a,G/D） plane, the transition pathways from the reversed to normal modes have been revealed as a consequence of increasing G/D. The critical value of G/D for transitions to the normal flapping modes demonstrates a decreasing trend for a small rotation rate, then becomes unaltered for a large a. The forming mechanism of different modes has been explored by analyzing the vortical wake behind the rotating cylinder and flexible filament. Moreover, great feedback effect of filament flapping on the cylinder wake flow has been revealed. Specifically, at specific parameters, the interaction between the flapping filament and the cylinder wake flow acts to totally suppress or trigger on the unsteady vortex shedding behind the cylinder, which correspondingly leads to great drag reduction even generation of thrust for the rotating cylinder, or significant drag enhancement.