Reconstruction of the deterministic turbulent boundary layer for the study of aerofoil self-noise mechanisms

Abstract

In the experimental aeroacoustics, it is always a challenge to study the far field radiation and near field hydrodynamics simultaneously, and be able to firmly establish the causality between them. The main objective of this paper is to present a novel experimental technique that can exploit the deterministic turbulent bound ary layer generated under a base flow of either mildly separated or laminar boundary layer to either disrupt an existing acoustic scattering mechanism, or reconstruct a new acoustic scattering scenario to enable the ensemble-averaging and wavelet analysis to study the aerofoil trailing edge noise source mechanisms in the spatial, temporal and frequency domains. One of the main attractions of this technique is that the experi mental tool does not need to be extremely high fidelity as a priori in order to fully capture the pseudo time-resolved boundary layer instability or turbulent structures. In one of the case studies presented here, roll-up vortices of the order of ∼kHz can be captured accurately by a 15-Hz PIV. A single hot-wire probe is also demonstrated to be capable of reconstructing the turbulent/coherent structures in a spatio−temporal domain. The proposed experimental technique can be extended to other self-noise scenarios when the aerofoil trailing edge is subjected to different flow control treat ments, such as the porous structure, surface texture, or finlet, whose mechanisms are largely not understood very well at present.