Physical Characterization of the Effects of Downstream Porosity on Leading-Edge Noise Generation and Reduction

Abstract

A numerical investigation has been conducted to characterize the reduction of noise generated by the interaction of incoming turbulence with a flat plate featuring a porous region downstream of the leading edge (referred to as downstream porosity). This work builds on previous experimental and analytical studies where promising noise reduction was achieved and several physical mechanisms potentially contributing to it were identified. These include phase inversions of pressure jumps potentially linked to secondary vorticity phenomena, destructive interference between noise sources, and the alteration of coherent structures within the boundary layer. The present work aims to investigate these mechanisms in detail, corroborating and extending novel experimental findings, to quantify their relative contributions to noise reduction and correlate them with the flow behavior within the perforation holes. The analysis of the unsteady surface pressure over the porous region reveals a marked, periodic phase opposition with respect to the primary noise source at the flat-plate leading edge, persisting across the entire porous section. This behavior indicates that destructive interference underlies the noise-reduction peaks observed experimentally, providing the first evidence for this mechanism. The analysis of the vorticity field, together with the velocity and surface-pressure spectra, supports the presence of a coherent mechanism over the porous region that is associated with this phase opposition. However, its origin and nature remain to be clearly established.