Trailing-edge noise generation from a flat-plate aerofoil interacting with a prescribed vortex

Abstract

Aerodynamic noise generated by aerofoil-vortex interaction has widely been studied in the past where the primary noise source mechanism is the scattering of the vortex at the leading edge (LE) of the aerofoil. In this paper the secondary source mechanism – the subsequent vortical scattering at the trailing edge (TE) – is investigated in detail, which shows that this mechanism is essentially due to nonlinear effects as predicted by some analytical studies in the past. The present study is performed by employing high-resolution numerical simulations based on a prescribed non-singular vortex impinging on a flat-plate aerofoil with zero mean loading. The present work investigates both inviscid and viscous flow conditions. The inviscid flow condition is intended to support and extend from the existing theoretical works, whereas the viscous one leads to more realistic findings. The current viscous study involves laminar boundary layers and their convective instability travelling with the impinged vortex. One of the most notable observations made in this work is that the vortical scattering at the TE (in the absence of turbulent boundary layers) may become a dictating source of noise at high frequencies (surpassing the primary source at the LE) across a wide range of observer angles for both inviscid and viscous flow conditions. It is found that the high-frequency dominant source is produced by secondary near-wall vortices that are induced as a result of nonlinear interactions between the aerofoil and the impinging vortex. This new discovery makes a contrast to the existing knowledge on aerofoil-vortex interaction noise in which the secondary source is normally assumed inferior to the primary at all frequencies.