Optimal design of the acoustic treatments damping the noise radiated by a turbo-fan engine

International audienceThe noise radiated at the inlet of turbo-fan engines corresponds to a significant noise source at aircraft level, which is generally controlled by acoustic treatments (liners) applied on the engine and the nacelle. For cost and weight constraints, the positioning of the liners as well as their acoustic properties are variables in the acoustic design of the nacelle. This paper proposes a framework for the efficient solution of this optimization problem. A finite element scheme is resorted to for solving the convected Helmholtz equation that accurately describes the wave propagation in the nacelle duct as well as in the far field. Locally-reacting admittance boundary condition are used to model the acoustic liners. The optimization problem is stated with the objective to identify the positioning and impedance values of the acoustic liner in such a way that the acoustic efficiency (far-field pressure level) is minimal at a targeted frequency, given the constraints on the weight, the integration and the manufacturing capabilities of the acoustic liner. A model reduction strategy based on a condensation of the duct and exterior acoustic problem to the surface of the acoustic treatments is presented as a mean to drastically speed-up the optimization process. A numerical application is demonstrated on a typical 2D axisymmetric nacelle configuration. Comparisons between the optimal designs as well as the computational performance of the proposed approach are assessed