Three-dimensional effects of geometries and acoustic treatments on bypass-duct noise

Sugimoto, Rie, Astley, R. Jeremy, Gabard, Gwenael and Tsuchiya, Naoki (2007) Three-dimensional effects of geometries and acoustic treatments on bypass-duct noise. 13th AIAA/CEAS Aeroacoustics Conference (28th AIAA Aeroacoustics Conference), , Rome, Italy. 21 - 23 May 2007. 11 pp .

Abstract

This article focusses on aft radiated noise in the case where 3D features in the bypass duct are significant. Aft radiation presents a more challenging computational task than the corresponding intake problem. The main complicating factor is the presence of a shear layer between the bypass stream and the external flow. This mitigates against the use of frequency domain methods based on the acoustic velocity potential, which have proved effective for intake problems. Such methods can be applied directly to aft radiation only with difficulty, by assuming the presence of a non-physical vortex sheet which extends for a finite distance downstream of the bypass exhaust. The CPU time however scales poorly with problem size.
Alternatively, time-domain, high order Finite Difference schemes can be applied to the Linearised Euler Equations (LEE), as can time-domain Finite Element models based on the Discontinuous Galerkin Method (DGM). They scale better with problem size and are more readily parallelised, however, they raise new problems due to the presence of hydrodynamic instabilities in the bypass shear layer.
In all numerical approaches, the size of the discrete problem is also a major obstacle when multi-source, multi-frequency predictions are required.
In this article a numerical-analytic approach is proposed for the solution of such problems. In the proposed method, the in-duct and radiation problems are uncoupled to reduce problem size. The in-duct problem is solved by a conventional Finite Element (FE) model by using a commercial code ACTRAN/TM within a shell programme which has been developed by authors. The shell code automatically creates a 3D mesh for a parametrically defined bypass duct and projects onto it a compressible Euler mean flow solution. Modal coordinates are used to represent the solution at each end of the duct. The in-duct FE solution is then coupled to a Wiener-Hopf radiation solution in the exterior region.

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Contributors

Author: Rie Sugimoto

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