Lorna J. Ayton

Aerodynamic noise from rigid trailing edges with finite porous extensions

A. Kisil acknowledges support from Sultan Qaboos Research Fellowship at Corpus Christi College at University of Cambridge. L. Ayton acknowledges support from a Research Fellowship at Sidney Sussex College.

Vortex Sound Models: Passive and Active Noise Control

© 2018 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. Simple models of vortex sound, considering the motion of a line vortex past an object, have been used to determine the far-field noise due to an idealised hydrodynamic disturbance, modelling, for example, instabilities within a turbulent boundary layer. Extensions of such models to investigate devices designed for control of trailing-edge noise, both passively and actively, are proposed. This simple mathematical framework potentially gives insight into the mechanism behind experimentally observed reductions in far-field noise.

Analysis of Mean Loading Effects in Fan Broadband Noise Simulations

A low-order method for predicting broadband interaction noise downstream of a fan stage in a turbofan engine is extended to include more realistic vane geometry. Vane thickness and camber are included by utilizing an asymptotic unsteady gust response method. Currently, a single airfoil model for the exit guide vanes is used to obtain preliminary insight into the effect of real geometry on the broadband noise prediction. In this work, the Source Diagnostic Test geometries are used and the results are compared to the experimental data. It is shown that results produced using the single airfoil flat-plate gust response model do not match those produced using a flat-plate cascade gust response model. However, the sound power trend across rotor speeds is still predicted. Comparison between results obtained using the flat-plate single airfoil and the real geometry airfoil response models show that including both thickness and camber only moderately affects the sound power level. Further investigation is needed to fully describe the results from a physical point of view.

Analytic solutions for reduced leading-edge noise aerofoils

This paper presents an analytic solution for the sound generated by an unsteady gust interacting with a semi-infinite at plate with a piecewise linear periodic leading edge. The Wiener-Hopf method is used in conjunction with a non-orthogonal coordinate transformation and separation of variables to allow analytical progress. A fully analytic solution is obtained in terms of a modal expansion for the far-field noise which is obtained by summing only a finite number of cuton modes, allowing very quick evaluation. The analytic solution is compared to experimental results for five test case leading-edge geometries. Good agreement is seen indicating the analytic model is capturing the key features of the interaction such as the destructive interference from the tip and root. In four of the five test cases the serrated edges show large reductions of noise compared to the straight edge at mid and high frequencies, however the square wave geometry is seen to be ineffective at noise reduction for high frequencies.

An analytic solution for the noise generated by gust–aerofoil interaction for plates with serrated leading edges

This paper presents an analytic solution for the sound generated by an unsteady gust interacting with a semi-infinite flat plate with a serrated leading edge in a background steady uniform flow. Viscous and non-linear effects are neglected. The Wiener-Hopf method is used in conjunction with a non-orthogonal coordinate transformation and separation of variables to permit analytical progress. The solution is obtained in terms of a modal expansion in the spanwise coordinate, however for low- and mid-range incident frequencies only the zeroth order mode is seen to contribute to the far-field acoustics, therefore the far-field noise can be quickly evaluated. The solution gives insight into the potential mechanisms behind the reduction of noise for plates with serrated leading edges compared to those with straight edges, and predicts a logarithmic dependence between the tip-to-root serration height and the decrease of far-field noise. The two mechanisms behind the noise reduction are proposed to be an increased destructive interference in the far field, and a redistribution of acoustic energy from low cuton modes to higher cutoff modes as the tip-to-root serration height is increased. The analytic results show good agreement in comparison with experimental measurements. The results are then compared against numerical predictions for the sound generated by a spanwise invariant line vortex interacting with a flat plate with serrated leading edge. Good agreement is also seen between the analytical and numerical results as frequency and tip-to-root ratio are varied.

Comparison of analytical, numerical, and experimental results for unsteady aerofoil interaction noise

© 2015, American Institute of Aeronautics and Astronautics Inc, AIAA. All Rights Reserved. An analytical solution to high-frequency gust-aerofoil interaction in background steady flow that is uniform far upstream is compared to numerical results for realistic aerofoils with non-zero thickness. Currently, numerical codes struggle at high frequencies, and are typically only validated against flat-plate analytical solutions. Analytical solutions presented in this paper for realistic aerofoil geometries are obtained using matched asymptotic techniques, and comparison with numerical solutions not only validates the numerical codes, but also confirms the validity of the analytical solution and its accuracy. Particular attention is given to the unsteady Kutta condition at the trailing edge, which is neglected in one numerical solution discussed in this paper; if the singularity in the surface pressure is incorporated into the analytical solution, the agreement between this numerical solution and the analytical solution is improved. Analytical solutions for the turbulent pressure spectrum close to the stagnation point of a thick aerofoil in uniform flow in both highand low-frequency regimes are also presented, and are seen to be in good agreement with experimental data.

Concluding Remarks and Further Work

This dissertation has considered various ways in which sound is generated in an aeroengine, with a particular focus towards the sound generated by blade-blade interactions within the engine. We have constructed semi-analytic solutions for certain interactions; gust-aerofoil interaction in steady, subsonic, uniform and shear flows; and sound-aerofoil interaction in steady, subsonic uniform flow. To find these solutions we had to separate the solution domain into various asymptotic regions wherein different behaviour dominated the production of sound. The regions were matched using Van Dyke’s matching rule.

Leading-Edge Stagnation-Point Noise Generated by Turbulence in Subsonic Uniform Flow

As illustrated in Chap. 3, Sect. 3.8.2, the unsteady pressure on the nose of an aerofoil during gust-aerofoil interaction in uniform flow was predicted to be singular, which violates the small perturbation assumption, and hence the asymptotic results become invalid in a small region close to the nose of the aerofoil.