Development of a new class of noise environment assessment codes

Abstract

A novel linearised Navier-Stokes equations method including turbulence information is developed to simulate sound propagation especially considering the inhomogeneous medium with non-uniform flows. The emphasis is on the development of methodology and the applications to further the understanding of sound propagation in a turbulent flow. To govern the acoustic-vorticity interaction, which is important in shear flows,perturbation Reynolds stress is used to introduce the turbulent information. A linear hydrodynamic stability analysis demonstrates that the turbulence can stabilize the flow;therefore this method can overcome the instability issue associated with variants of the linearised Euler equations method. Numerical simulations are performed to validate the stability of the proposed method dealing with sound propagation in shear flows. An algebraic turbulence model is used in a benchmark case with a prescribed mean flow profile. Moreover, the turbulent viscosity solved numerically is employed to study turbofan aft noise radiation. The numerical experiments demonstrate the acoustic-vorticity interaction can be resolved by this linear acoustic propagation model to some degree. The resulting effects are evaluated in respect of the near-field and far-field results. Moreover, the attenuation of sound in fully developed turbulent pipe flows is solved and validated against experimental results. Subsequently, the acoustic damping induced by turbulence at high-order duct modes is investigated. Finally, this method is used to investigate the effects of the wall boundary layer on the external noise radiation in respect of two aspects, namely the engine noise impact on the fuselage at cruise conditions and the far-field noise distribution at take-off/landing conditions. These studies demonstrate the stability of the proposed method in the presence of shear layers and density gradients, while show the ability of the method to qualify the attenuation of sound by turbulence.

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