Flow-generated noise, especially rotorcraft noise has been a serious concern for both commercial and military applications. A particular important noise source for rotorcraft is Blade-Vortex-Interaction (BVI) noise, a high amplitude, impulsive sound that often dominates other rotorcraft noise sources.
In this thesis the research is to formulate and implement efficient computational tools for the development and study of optimal control and design strategies for complex flow/acoustic systems with emphasis on rotorcraft applications, especially BVI noise control problem. The main purpose of aeroacoustic computations is to determine the sound intensity and directivity far away from the noise source. However, the computational cost of using a high-fidelity flow-physics model across the full domain is usually prohibitive and it might also be less accurate because of the numerical diffusion and other problems. Taking advantage of the multi-physics and multi-scale structure of this aeroacoustic problem, we develop a multi-model, multi-domain (near-field/farfield) method based on a discontinuous Galerkin discretization. In this approach the coupling of multi-domains and multi-models is achieved by weakly enforcing continuity of normal fluxes across a coupling surface. For our interested aeroacoustics control problem, the adjoint equations that determine the sensitivity of the cost functional to changes in control are also solved with same approach by weakly enforcing continuity of normal fluxes across a coupling surface. Such formulations have been validated extensively for several aeroacoustics state and control problems.
A multi-model based optimal control framework has been constructed and applied to our interested BVI noise control problem. This model problem consists of the interaction of a compressible vortex with Bell AH-1 rotor blade with wall-normal velocity used as control on the rotor blade surface. The computational domain is decomposed into the near-field and far-field. The near-field is obtained by numerical solution of the Navier-Stokes equations while far away from the noise source, where the effect of nonlinearities is negligible, the linearized Euler equations are used to model the acoustic wave propagation. The BVI wave packet is targeted by defining an objective function that measures the square amplitude of pressure fluctuations in an observation region, at a time interval encompassing the dominant leading edge compressibility waves. Our control results show that a 12dB reduction in the observation region is obtained. Interestingly, the control mechanism focuses on the observation region and only minimize the sound level in that region at the expense of other regions. The vortex strength and trajectory get barely changed. However, the optimal control does alter the interaction of the vortical and potential fields, which is the source of BVI noise. While this results in a slight increase in drag, there is a significant reduction in the temporal gradient of lift leading to a reduction in BVI sound levels. (Abstract shortened by UMI.)