This thesis investigates the turbulence-aerofoil interaction noise, which is regarded as a significant contributor to the broadband noise in turbofan engines. In order to produce affordable means to leading edge noise predictions, synthetic turbulence method is developed in the Computational AeroAcoustics (CAA) simulations to reproduce the turbulence statistics, which will then be combined with the Linearised Euler Equations (LEEs) solved by a high-order CAA code to simulate the noise generation and propagation.

In this work, a Direct Anisotropic Filter (DAF) method is proposed to produce desired velocity spectra of three-dimensional anisotropic turbulence, based on a direct form of anisotropic filter that can be implemented directly to obtain non-Gaussian spectra. In addition...[ Read more ]

This thesis investigates the turbulence-aerofoil interaction noise, which is regarded as a significant contributor to the broadband noise in turbofan engines. In order to produce affordable means to leading edge noise predictions, synthetic turbulence method is developed in the Computational AeroAcoustics (CAA) simulations to reproduce the turbulence statistics, which will then be combined with the Linearised Euler Equations (LEEs) solved by a high-order CAA code to simulate the noise generation and propagation.

In this work, a Direct Anisotropic Filter (DAF) method is proposed to produce desired velocity spectra of three-dimensional anisotropic turbulence, based on a direct form of anisotropic filter that can be implemented directly to obtain non-Gaussian spectra. In addition, three-dimensional and pseudo three-dimensional isotropic filters of von Kármán energy spectrum and Liepmann energy spectrum are also derived based on the energy spectrum function of isotropic turbulence. The DAF method contains less model constraints than the existing methods, getting rid of the inconvenience induced by complex hypergeometric functions and Gaussian superposition with Mach number dependent model parameters. In addition, compared with the numerical results given by existing methods, this method offers improvement in accuracy and cost.

In this study, the DAF method is combined with the LEEs in the CAA simulations to study the turbulence-aerofoil interaction noise, with the results showing good agreement with benchmarking analytical solutions and experimental measurements. The numerical study also investigates the effect of anisotropy on leading edge noise for thick aerofoils, revealing that the anisotropic turbulence stretched in the axial direction will alleviate the thickness-induced noise reduction. This study also highlights the contribution of streamwise energy spectra to noise generation when considering the effects of anisotropy.

The DAF method is also used to investigate the distortion of turbulence spectrum in the vicinity of the aerofoil leading edge, which has a significant impact on the turbulence-aerofoil interaction noise. By applying different degrees of anisotropy in the incident turbulence spectrum, the effects of aerofoil thickness, camber and angle of attack on distorted turbulence spectrum are investigated in the CAA simulations. On the basis of the asymptotic results around bluff bodies obtained by the generalised rapid distortion theory, a theoretical solution of the distorted turbulence spectrum is proposed considering the influence of anisotropy. Additionally, a modified far field noise prediction model is proposed by using the distorted turbulence spectrum in the vicinity of aerofoil leading edge, in order to account for the effect of real aerofoil geometries.

Finally, the DAF method is extended to synthesise the turbulence downstream of the fan which is highly inhomogeneous and non-stationary, through the combination with the fan wake model featured by the cyclostationary variations in the turbulent kinetic energy, which represents a better fit to the physical truth of fan wake-Outlet Guide Vanes (OGVs) interaction noise. By using the extended DAF method with cyclostationary anisotropic filter, the effects of cyclostationarity and anisotropy of the fan wake on the turbulence spectra and far-field noise spectra are investigated.