Aerofoil self noise reduction by innovative trailing edge treatment

Abstract

This thesis aims to develop the next level mitigation methods for the reduction of the aerofoil turbulent boundary layer trailing-edge noise (TBL-TE) self-noise radiation focusing on the addon TE passive flow control devices in the forms of slitted and modified sawtooth geometries. The research undertaken involves experimental, numerical and analytical efforts. The introduction of the flap angle represents an additional optimisation parameter for a modified sawtooth geometry, which is exploited for the design of serration as a periodic-function in the spanwise direction. The thesis shows that a more rapid spanwise waviness of the serration can outperform the noise reduction performance between the mid and high frequency range, while remain the same level at the low frequency, when compared to the conventional serration. The associated flow-topology is also investigated to provide some explanation for the mechanisms underpinning the broadband noise reduction. For the first time, slit TE is found to facilitate acoustical interference between two sources that were physically displaced in the longitudinal direction. The findings successfully establish the frequency fine-tuning capability by slit TEs for the self-noise reduction. New analytical noise prediction model based on the acoustical interference mechanism is developed, which has been demonstrated to be compatible with the experimental results. The next level of improvement is achieved in the form of a hybrid device: Double-Rooted-Trailing-Edge-Serrations (DRooTES). The DRooTES combines the acoustical destructive interference mechanism from the slit TE, and the serration effect from the sawtooth. Both effects are found to co-exist and can be exerted constructively. The DRooTES not only demonstrates larger level of broadband noise reduction, but also establishes the frequency-tuning capability. Finally, a Large-Eddy-Simulation was performed to investigate the detailed flow mechanisms causing the acoustical interference for the slit TE. The results help to explain some of the flow mechanisms observed in the experimental results.