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 an experimental technique that can exploit the deterministic turbulent boundary 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 experimental 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 $$\sim$$ ∼ 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 treatments, such as the porous structure, surface texture, or finlet, whose mechanisms are largely not understood very well at present. Graphical abstract

Mitigation of turbulent noise sources by riblets

“…To perform the ensemble averaging successfully, the rising edge of each input pulse signal can be set as the time of origin (t ′ = 0 ms) of an individual spot event. This allows the generation of an ensemble of velocity fluctuations, which is shown in Figure 4a from data obtained elsewhere [30] as an example. The ensemble is then averaged to obtain the mean velocity time signature ⟨u(x, y, t ′ )⟩, which is represented by the thick black line in the figure.…”

Recent Advances in Airfoil Self-Noise Passive Reduction

“…Other noise sources include BL passing over the surface [23], tip vortices [24], and velocity fluctuations from vibrations on lifting surfaces [25]. Brooks et al [13] categorized these noise generation mechanisms as TBL-TE, LBL Vortex-Shedding (LBL-VS), separation-stall, TE-bluntness-VS, and tip vortex formation (Figure 1).…”