Spinning Wave Scattering From a Flow Pipe With Serrations

It is well known that trailing-edge serrations, which are also known as chevrons, are able to reduce the turbulent mixing noise from an aeroengine. The study of the associated control capability of the scattering of incident waves from the rotor–stator assembly is rare. To address this issue, a theoretical model is proposed to predict sound wave scattering from a cylindrical pipe with trailing-edge serrations in the presence of plug flows. The model incorporates Fourier series expansion into the Wiener–Hopf method and, therefore, is a natural extension of the previous aerofoil work by Huang (2017, “Theoretical Model of Acoustic Scattering From a Flat Plate With Serrations,” J. Fluid Mech., 819, pp. 228–257). The nature of the flow duct problem, however, leads to a much more complicated matrix kernel, and the associated factorization method is given in this article. The proposed model is validated by comparison with the numerical simulations at certain representative setups, which show the overall agreements to be satisfactory. The comparisons also show that the proposed model is so efficient that it can enable rapid predictions. A series of parametric studies are performed to study the two mechanisms behind the noise reduction of a serrated flow duct. One is the redistribution of acoustic energy to new higher cutoff modes. The other is the destructive interference due to multiple scattering from serrations. Overall, the proposed model should be helpful in offering deep physical insights and would be able to assist the aeroacoustic design and optimization of new low-noise aeroengines and flow duct systems after considering the tradeoff with aerodynamic impacts.