Efficient Design Optimization of Acoustic Liners for Engine Noise Reduction

Abstract

Engine noise reduction using acoustic liner panels is an established technology in the design of bypass turbofan engines. Using acoustic impedance modeling techniques in aeroacoustic simulations, quick assessment of different liner configurations can be made. To minimize the engine noise in the best possible way, optimal liner configurations are ideally determined by employing numerical optimization methods. In general, to exhaust the full potential of the lining technology, it is desired to use a large number of design parameters in the optimization process. In the present work, a multilevel optimization framework is presented for the design of acoustic liner panels, which can efficiently handle large design vectors. The multilevel method combines a global search strategy with a local gradient-based optimization method. The discrete adjoint solver, which is used to evaluate the liner sensitivities required in the local search, has been developed using algorithmic differentiation techniques. This approach allows consistent treatment of boundary conditions in the adjoint part, thereby increasing the robustness and accuracy of the adjoint solver. In this way, the discrete adjoint solver fully incorporates the features of the underlying aeroacoustic solver, such as the impedance modeling and nonreflecting boundary conditions. The feasibility and the efficiency of the multilevel optimization strategy are demonstrated by finding the optimal liner parameters in a turbofan engine bypass duct configuration.