Adjoint-based Aeroacoustic Design-Optimization of Flexible Rotors in Forward Flight

This paper presents the development and application of a time-dependent adjoint-based method for aeroacoustic design optimization of flexible helicopter rotors in forward flight. The rotor noise signature at a farfield observer is computed with the hybrid aeroacoustic approach in which a near-body coupled computational fluid dynamics/ computational structural dynamics solver provides flow and geometry data to an FW-H acoustic module that propagates the acoustic waves to a farfield observer. Forward and adjoint sensitivity formulations are derived that correspond to analogues of the multidisciplinary flexible aeroacoustic analysis problem. The newly implemented aeroacoustic capability is first verified to effectively perform forward flight noise prediction by comparison with the PSU-WOPWOP acoustic tool. Upon successful validation, a gradient based optimization method is used to minimize the required torque of the flexible HART-II rotor in trimmed forward flight while reducing the acoustic signature at a farfield observer by changing the shape of the blades. The adjoint formulation developed in this work is used to efficiently compute the sensitivity required by the optimization algorithm.