REYNOLDS NUMBER EFFECTS ON A BIRD-FEATHER-INSPIRED POROUS-MEDIA

Bird feathers are inherently permeable, and their layered overlapping arrangement gives rise to anisotropic permeability characteristics. From a macroscopic perspective, a feathered wing surface can therefore be regarded as an anisotropic porous medium. Understanding how such porous-media characteristics influence the aerodynamic performance of bird wings constitutes the fundamental motivation of the present study. In this work, a bird-feather-inspired anisotropic porous-media airfoil is constructed to represent the dominant macroscopic anisotropy associated with real feather assemblies. A Monotone Integrated Large Eddy Simulation (MILES) approach is employed to investigate the aerodynamic behavior of the porous airfoil under different Reynolds number conditions. By comparing surface pressure distributions, velocity field structures, and flow separation characteristics at various Reynolds numbers, the mechanisms by which porous media affect airfoil aerodynamics are systematically examined. The results show that within a Reynolds number range of Re = 6 × 10⁴ ~ 4 × 10⁵, and at a fixed attack angle, the leading-edge separation vortex can partially penetrate into the porous medium. This penetration suppresses the downstream growth of the separation vortex and prevents the formation of a large-scale separated region, thereby reducing the aerodynamic drag and drag fluctuations of the airfoil. In contrast, at lower Reynolds numbers (Re = 4 × 10⁴ ~ 5 × 10⁴) a pronounced change in the surface flow state is observed. Under these conditions, the low-velocity flow within the porous medium enhances the formation and development of separation vortices, leading to a deterioration of aerodynamic performance. These findings indicate that bird-feather-inspired anisotropic porous-media structures are capable of effectively modulating airfoil flow characteristics within a specific Reynolds number range. The results provide new insights into the aerodynamic role of feather structures and contribute to a deeper understanding of how feather-like porous media can influence aerodynamic performance.