Publications
Multi-model Simulation for Optimal Control of Aeroacoustics
Flow-generated noise, especially rotorcraft noise has been a serious concern for bothcommercial and military applications. A particular important noise source for rotor-craft is Blade-Vortex-Interaction (BVI)noise, a high amplitude, impulsive sound thatoften dominates other rotorcraft noise sources. Usually BVI noise is caused by theunsteady flow changes around various rotor blades due to interactions with vorticespreviously shed by the blades. A promising approach for reducing the BVI noise isto use on-blade controls, such as suction/blowing, micro-flaps/jets, and smart struc-tures. Because the design and implementation of such experiments to evaluate suchsystems are very expensive, efficient computational tools coupled with optimal con-trol systems are required to explore the relevant physics and evaluate the feasibilityof using various micro-fluidic devices before committing to hardware.In this thesis the research is to formulate and implement efficient computationaltools for the development and study of optimal control and design strategies for com-plex flow and acoustic systems with emphasis on rotorcraft applications, especiallyBVI noise control problem. The main purpose of aeroacoustic computations is todetermine the sound intensity and directivity far away from the noise source. How-ever, the computational cost of using a high-fidelity flow-physics model across thefull domain is usually prohibitive and itmight also be less accurate because of thenumerical diffusion and other problems. Taking advantage of the multi-physics andmulti-scale structure of this aeroacoustic problem, we develop a multi-model, multi-domain (near-field/far-field) method based on a discontinuous Galerkin discretiza-tion. In this approach the coupling of multi-domains and multi-models is achievedby weakly enforcing continuity of normal fluxes across a coupling surface. For ourinterested aeroacoustics control problem, the adjoint equations that determine thesensitivity of the cost functional to changes in control are also solved with same ap-proach by weakly enforcing continuity ofnormal fluxes across a coupling surface.Such formulations have been validated extensively for several aeroacoustics state andcontrol problems.A multi-model based optimal control framework has been constructed and ap-plied to our interested BVI noise control problem. This model problem consists ofthe interaction of a compressible vortex with Bell AH-1 rotor blade with wall-normal3 velocity used as control on the rotor blade surface. The computational domain isdecomposed into the near-field and far-field. The near-field is obtained by numericalsolution of the Navier-Stokes equations while far away from the noise source, wherethe effect of nonlinearities is negligible, the linearized Euler equations are used tomodel the acoustic wave propagation. The BVI wave packet is targeted by definingan objective function that measures the square amplitude of pressure fluctuations inan observation region, at a time interval encompassing the dominant leading edgecompressibility waves. Our control results show that a 12dB reduction in the ob-servation region is obtained. Interestingly, the control mechanism focuses on theobservation region and only minimize the sound level in that region at the expense ofother regions. The vortex strength and trajectory get barely changed. However, theoptimal control does alter the interaction of the vortical and potential fields, whichis the source of BVI noise. While this results in a slight increase in drag, there is asignificant reduction in the temporal gradient of lift leading to a reduction in BVIsound levels.4