Abstract

Many owl species rely on specialized plumage to mitigate their aerodynamic noise and achieve functionally-silent flight while hunting. One such plumage feature, a tattered arrangement of flexible trailing-edge feathers, is idealized as a semi-infinite poroelastic plate to model the effects that edge compliance and flow seepage have on the noise production. The associated acoustic scattering problem is solved to identify how the noise scales with the flight velocity, where special attention is paid to the limiting cases of rigid-porous and elastic-impermeable plate conditions. Results from this analysis identify new parameter spaces where the porous and/or elastic properties of a trailing edge may be tailored to diminish or effectively eliminate the edge scattering effect and may contribute to the owl hush-kit. In complement to the acoustic analysis, steady and unsteady aerodynamic models including wing porosity are formulated and solved in anticipation of the potential trade-off of aerodynamic performance and acoustic stealth. The aerodynamic analysis yields porous extensions of the classical unsteady aerodynamic functions and further indicates how porosity may be used as a passive gust rejection strategy.