A finite element code has been developed for the prediction of the radiated acoustic field in and around the aft fan duct of a turbofan engine. The acoustic field is modeled based on the assumption that the steady flow in and around the nacelle is irrotational as is the acoustic perturbation. The geometry of the nacelle is axisymmetric, and the acoustic source is harmonic and decomposed into its angular harmonics. The steady flow is computed on the acoustic mesh and provides data for the acoustic calculations. The jet is included in the steady flow potential flow model by separating the interior and exterior flow outside the aft fan duct with a thin barrier created by disconnecting the computational domain. The jet and exterior flow is allowed to mix at a defined distance downstream. In the acoustic radiation model continuity of acoustic particle velocity is implicitly satisfied across the shear layer by careful treatment of the surface integral which appears in the FEM formulation. Pressure continuity is enforced by using a penalty constraint on the shear layer. A reliable frontal solution routine which originally involved extensive I/O operations to minimize core storage has been updated to eliminate most of the inefficient direct access reading and writing with considerable impact on computational time and is now found to be competitive with banded LU solvers. Example calculations are given which show the success achieved in satisfying the complicated interface conditions on the shear layer and the characteristics of the solutions at relatively high frequencies where the refinement of the mesh is a limiting consideration for practical computations.


Mechanical and Aerospace Engineering

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Article - Conference proceedings

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Publication Date

01 Jan 1997