柔性仿羽毛结构抑制边界层转捩的初步探索

Suppression of flexible feather-like structure on boundary layer transition

  • 摘要: 通过观察,发现存在展向流动时鸟类柔性的羽毛侧缘会卷起,这对展向流动起到了额外的阻碍作用,即流向和展向的阻力特征是不同的。为了研究羽毛表面的这种各向异性阻力特征对边界层转捩流动的影响,本文从宏观角度出发建立了一种唯象力学模型来刻画表面的各向异性。然后,运用直接数值模拟的方法研究了该模型对平板边界层转捩的影响。自由来流马赫数为0.2,基于入口处边界层排移厚度的雷诺数为732。研究结果表明,在模型抑制展向流动的作用下,平板边界层转捩明显推迟,不同的参数下至少可以推迟一倍以上距离。即使在流动进入湍流状态后,壁面的摩擦阻力系数也大大降低,维持在与层流摩擦系数相当的水平。转捩位置的推迟和湍流区摩擦阻力的降低都有利于降低平板的阻力。如果考虑卷起的羽毛侧缘对流向流动带来额外阻力,这会使得转捩提前,减阻的效果降低。此研究结果一方面揭示了鸟类飞行的部分奥秘,另一方面也为边界层被动控制措施提供了一种新的思路。

     

    Abstract: It is observed that when there is spanwise flow, the soft side edges of bird feathers are blown up, and thus the spanwise flow is resisted, i.e., the drag characteristics for streamwise direction and spanwise direction on the feather surface are quite different. In order to study the influence of this anisotropic characteristics of feathers on the laminar-turbulent transition in boundary layer flow, a phenomenological mechanical model, from the macroscopic view, is established to describe the anisotropic characteristics of bird feathers. By means of direct numerical simulation with finite difference method, the model is imposed to the transitional flow over a flat plate, where the freestream Mach number is 0.2 and the Reynolds number based on the displacement thickness of boundary layer at the inlet is 732. Numerical results show that when the spanwise flow is damped by the anisotropic model, the laminar-turbulent transition is significantly delayed, and even the friction in turbulent region is reduced to be almost the same level as that in laminar region. The stronger the damping for spanwise flow by the model, the more obvious the transition delay and friction reduction. Both the delay of transition and the reduction of friction drag in turbulent region are helpful to reduce the overall drag of the flat plate. When considering the influence of blown-up feathers on streamwise direction in the form of resistance, the resistance in streamwise direction results in the earlier onset of transition. Although friction drag is further reduced in this case, while the total drag of flat plate is increased. Overall, this study reveals some of the mysteries of bird flight, and also provides a new idea for the passive control of the boundary layer flow.

     

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