This paper provides an experimental investigation into the use of leading edge serrations as a means of reducing the broadband noise generated due to the interaction between the aerofoil's leading edge (LE) and impinging turbulence. Experiments are performed on a flat plate in an open jet wind tunnel. Grids are used to generate isotropic homogeneous turbulence. The leading edge serrations are in the form of sinusoidal profiles of wavelengths, λ, and amplitudes, 2h. The frequency and amplitude characteristics are studied in detail in order to understand the effect of LE serrations on noise reduction characteristics and are compared with straight edge baseline flat plates. Noise reductions are found to be insignificant at low frequencies but significant in the mid frequency range (500 Hz to 8 kHz) for all the cases studied. The flat plate results are also compared to the noise reductions obtained on a serrated NACA-65 aerofoil with the same serration profile. Noise reductions are found to be significantly higher for the flat plates with a maximum noise reduction of around 9 dB compared with about 7 dB for the aerofoil. In general, it is observed that the sound power reduction level (∆PWL) is sensitive to the amplitude, 2h of the LE serrations but much less sensitive to the serration wavelength, λ. Thus, this paper sufficiently demonstrates that the LE amplitude act as a key parameter for enhancing the noise reduction levels in flat plates and aerofoils.

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This work, realized in the framework of the European project TurboNoiseBB, presents an advanced aeroacoustic design methodology of Outlet Guide Vanes (OGVs) with leading edge serrations, including details on their broadband noise and aerodynamic performance. The serrated OGV corresponds to a modified stator from an aero-engine fan stage tested at the AneCom Aerotest's facility (Germany). Sinusoidal leading edge patterns with varying amplitude and wavelength along the span are designed in collaboration with Safran Aircraft Engines. Serrations are adjusted to account for the turbulence characteristics provided by Reynolds-Averaged Navier-Stokes (RANS) calculations. Optimal parameters are found using simple design rules discussed in the paper. Down selection of serrated OGV designs (patent pending) are conducted through a RANS control of aerodynamic performances and in accordance with industrial specifications, ensuring acceptable penalties on the loss coefficient and isentropic efficiency. Broadband noise simulations are performed using a computational aeroacoustic (CAA) code that solves the linearized Euler equations with a synthetic turbulence model. Additionally, the acoustic response of the serrated leading edge airfoils is also estimated using the most relevant analytical formulation in the literature, based on the Wiener-Hopf technique. Numerical predictions at approach conditions are compared to available experimental measurements (on the untreated baseline case) and analytical Amiet-based and Wiener-Hopf solutions, showing a satisfactory agreement in the sound power spectrum in the bypass duct. Finally, the acoustic performances estimated at the design stage are numerically assessed by both CAA and Wiener-Hopf methods, providing around 2.5 dB and 3.5 dB reduction on the overall power level, respectively.

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Turbulent flow interactions with the outlet guide vanes are known to mainly contribute to broadband noise emission of aeroengines at approach conditions. This paper presents a 3D CAA hybrid method aiming at simulating the aeroacoustic response of an annular cascade impacted by a prescribed homogeneous isotropic turbulent flow. It is based on a time-domain Euler solver coupled to a synthetic turbulence model implemented in the code by means of a suited inflow boundary condition. The fluctuating pressure over the airfoil surface provided by CAA is used as an input to a Ffowcs-Williams and Hawkings integral method to calculate the radiated sound field. Euler computations are first validated against an academic CAA benchmark in the case of an harmonic gust interacting with an annular flat plate cascade. Then, simulations are applied to turbulence-cascade interactions for annular configurations, in uniform and swirling mean flows, and numerical results in terms of sound power spectra in the outlet duct are compared to semi-analytical and numerical solutions, and to an available experiment.

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A benchmark dedicated to RANS-informed analytical methods for the prediction of turbofan rotor–stator interaction broadband noise was organised within the framework of the European project TurboNoiseBB. The second part of this benchmark focuses on the impact of the acoustic models. Twelve different approaches implemented in seven different acoustic solvers are compared. Some of the methods resort to the acoustic analogy, while some use a direct approach bypassing the calculation of a source term. Due to differing application objectives, the studied methods vary in terms of complexity to represent the turbulence, to calculate the acoustic response of the stator and to model the boundary and flow conditions for the generation and propagation of the acoustic waves. This diversity of approaches constitutes the unique quality of this work. The overall agreement of the predicted sound power spectra is satisfactory. While the comparison between the models show significant deviations at low frequency, the power levels vary within an interval of ±3 dB at mid and high frequencies. The trends predicted by increasing the rotor speed are similar for almost all models. However, most predicted levels are some decibels lower than the experimental results. This comparison is not completely fair—particularly at low frequency—because of the presence of noise sources in the experimental results, which were not considered in the simulations.

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