MECHANICAL CHARACTERISTICS OF THE DEFORMATION OF BIRD FEATHERS IN AIRFLOW
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摘要: 鸟类羽毛在飞行中的物理性质是仿生力学关心的重要问题之一. 基于CFD/CSD数值模拟方法研究了羽毛微结构在气流作用下的变形和力学特征, 揭示了鸟类静止时羽毛蓬松、而在飞行状态下紧贴皮肤表面保持表面光滑的物理机制. 首先, 通过对鸟类羽毛在显微镜下的观察, 将羽毛分解成典型简单微结构以模仿羽枝单元, 从而对羽毛外形和结构进行建模, 之后, 采用CFD/CSD方法分析比较了两种典型羽枝模型结构(片状和枝状羽枝单元)的变形和力学特征, 最后, 基于上述片状羽枝模型进一步研究了来流方向对羽枝变形的影响机理及多根排列羽枝的变形和力学特征. 结果表明: 在一定风向的范围内, 羽毛在气流下都具有保持紧贴皮肤表面的变形趋势, 这种紧贴壁面的趋势只有在气流与羽轴几乎垂直时才会改变; 在来流侧滑角为
$ 45^\circ $ 时, 羽枝沿皮肤表面法向下压的变形最为显著, 尖端位移达原始高度的约97%; 多根排列的羽枝在顺流方向气动载荷逐渐下降, 与迎风首根羽枝最大差距约11%. 此研究工作对于理解鸟类飞行时羽毛的力学特性有明确的学术价值.Abstract: The physical properties of bird feathers during flight constitute one of the significant concerns in the field of biomimetic mechanics. In this paper, the CFD/CSD numerical simulation method is adopted to calculate and analyze the deformation as well as mechanical characteristics of feather microstructure in the airflow, revealing the physical mechanism by which feathers exhibit thick and fluffy at rest, but keep close to the bird’s skin surface to maintain the surface smoothness during flight. Initially, by observing the bird feathers with optical microscope, the feather structure is decomposed into typical simple microstructures to imitate the barb element, thus modeling the shape and structure of the feather. Then, the deformation and mechanical characteristics of two typical modeling structures (flaky barb element and branch barb element) are analyzed and compared by CFD/CSD method. Finally, based on the aforementioned flaky barb element model, further research is conducted on the influence mechanism of the incoming flow direction on the deformation feature of barb element, as well as the deformation and mechanical characteristics of multiple arranged barbs. The results show that within a certain range of inflow direction, the feather maintains the deformation tendency of keeping close to the bird’s skin surface in the airflow. This tendency will change only when the airflow is almost perpendicular to the feather rachis. When the sideslip angle of inflow is$ 45^\circ $ , the downward deformation of barb element along the skin surface normal is the most significant, with a tip displacement of approximately 97% of its initial height. The multi-row arranged barb elements experienced a gradual decrease in aerodynamic load in the flow direction, with a maximum difference of about 11% compared to the leading barb element against the wind. This research has a clear academic value for understanding the mechanical properties of bird feathers when flying.-
Key words:
- bionic mechanics /
- bird feather /
- feather microstructure /
- fluid-structure coupling /
- CFD/CSD
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图 6 羽毛结构及简化的羽枝模型: (a) 片状羽枝模型和(b) 枝状羽枝模型
Figure 6. Feather structure and simplified barb model: (a) flaky barb model and (b) branch barb model
图 14 羽枝位移随来流侧滑角
$\beta $ 的变化Figure 14. Variation of barb displacement with angle of sideslip
$\beta $ 
图 15 刚性/柔性羽枝气动力及力矩随来流侧滑角
$\beta $ 的变化Figure 15. Variation of aerodynamic force and moment of rigid/flexible barb with angle of sideslip
$\beta $ 
图 17 多根羽枝气动载荷随顺流方向位置的变化
Figure 17. Variation of aerodynamic loads on multiple barbs with positions in the downstream direction
表 1 羽枝单元材料参数
Table 1. Material properties of barb elements
Density/(kg·m−3) Young's modulus/GPa Poisson's ratio barb 2.50 3.04 0.30 barbules 2.50 0.77 0.30 
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表 2 羽枝及羽小枝面网格分布
Table 2. Grid distribution of barb and barbules
Coarse Medium Fine ${N_r} \times {N_1}$ $38 \times 81$ $48 \times 102$ $60 \times 128$ ${N_2} \times {N_5}$ $22 \times 20$ $28 \times 25$ $35 \times 32$
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表 3 羽枝位移及气动力载荷
Table 3. Displacement and aerodynamic load of barb
Flaky barb Branch barb $\Delta {\theta _\eta }/(^\circ )$ −4.81 −2.58 $\Delta {\theta _\zeta }/(^\circ )$ 6.77 10.08 $\Delta y/{y_{{\rm{tip}},{\rm{rig}}} }/\text{%}$ −21.76 −3.59 $\Delta {y_{{\rm{tip}}} }/{y_{{\rm{tip}},{\rm{rig}}} }/\text{%}$ −77.81 −23.46 $ {C_{m\eta }} $ −70.51 −19.49 $ {C_{m\zeta }} $ 108.39 79.76 ${C_{fy}}$ −60.33 −10.02
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