Noise reduction by perforated cascades in annular ducts

One of the latest trends in noise control relating to aeroacoustics is to mimic the silent flight capability of owls. Particularly, porosity is most often applied on cascades in ducts with axial flows, such as stator structures in an aero-engine. However, current acoustic scattering models of perforated cascades are based on two-dimensional methods without including the three-dimensional effects. In this paper, we present a fully three-dimensional acoustic scattering model for perforated cascades based on the lifting surface theory in which the dominant sound source reduces to dipoles alone under the thin airfoil assumption. Accordingly, the acoustic scattering of perforated cascades with single-mode incident wave was studied and obvious noise reduction was observed. The optimum Rayleigh conductivity and the maximum noise-reducing capability of the porosity varied substantially with different incident duct modes, whilst a larger cascade chord-length could achieve more noise reduction at optimum porosity. With a background flow, the Kutta condition can greatly influence the overall distribution of the unsteady loading on vanes. Additionally, the unsteady vortex shedding at the trailing edge offers extra sound energy dissipation mechanism. Therefore, the implementation of porosity on cascades is much different to the design of a traditional acoustic liner.