Investigations of broadband fan interaction noise using synthetic turbulence

Dieste, M. and Gabard, Gwenael (2010) Investigations of broadband fan interaction noise using synthetic turbulence. 17th International Congress on Sound and Vibration (ICSV), Cairo, Cairo, Egypt. 17 - 21 Jul 2010. 8 pp .

Abstract

The aim of this investigation is to develop computational aeroacoustics methodology to study broadband fan interaction noise. Broadband fan noise has become a major noise source on modern aircraft and it is crucial, for instance, in more fuel-efficient alternatives to turbofan engines. Solving the fluid mechanics of this process either via Direct Numerical Simulation or Large Eddy Simulation remains prohibitive within an industrial context. In the present approach, the sound source is calculated from stochastically generated synthetic turbulence instead of solving the Navier Stokes equations and combined with an aeroacoustic propagation model. This approach is computationally cheaper that DNS or LES and can provide accurate results for engineering purposes.

Synthetic two-dimensional isotropic turbulent flows are generated by filtering white noise. The filter is expressed in terms of either the correlation function or the energy spectrum. In contrast with most filter-based models, non-Gaussian filters such as those derived from Liepmann and von Karman spectra are used. While Gaussian filters perform very well from a computational point of view, Liepmann and von Karman filters are more representative of the physics of the problem.

The aeroacoustic sound propagation is modelled by the linearised Euler equations implemented in a parallel, multiblock, finite-difference code in the time domain. Particles representing vortices are launched upstream of the airfoil and convected with the mean flow following a purely Lagrangian approach. The turbulent velocity field is computed at each point along the aerofoil and implemented as source in the linearized Euler equations.

Simulation results have been compared with the analytical solution proposed by Amiet. This numerical method is being used to assess the influence of various parameters on interaction noise for the cases of frozen and evolving turbulence. The case of inhomogeneous turbulence will also be considered in the final paper.

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