Trailing-edge noise reduction through finlet-induced turbulence

Biologically inspired finlet treatments have been shown to effectively reduce the trailing-edge noise of a flat plate and hence are a viable noise-suppression technology for engineering applications. The present work performs a thorough experimental investigation on the near-field dynamics of finlet surface treatments applied to a flat plate. To examine the underlying noise-reduction mechanism, the manipulated flow field is analysed using data from detailed static, unsteady wall-pressure as well as velocity measurements and their correlations. Specifically, the densely populated dynamic transducers allow for the tracking of the turbulent boundary-layer development from upstream to the wake of the finlet-treated area (see supplementary movies), which elucidates the formation of 'finlet-induced turbulence' through flow-finlet interaction. Associated turbulence structures are found to further develop within the treated area and structures shed from the top of the finlets are observed to mix and merge with the turbulence being channelled through the space between the finlets in the finlet wake. While the mixing process increases the spanwise turbulence length scale, it significantly attenuates the unsteady wall-pressure fluctuation at the trailing edge and thus leads to broadband reduction of the trailing-edge noise. Moreover, it corroborates the findings of earlier studies suggesting that there exists an optimal distance between finlets and trailing-edge where the mixing effects are most beneficial.