<p> The goal of this research is to investigate the feasibility and potential effectiveness of using vacuum-sealed, double-leaf partitions for applications in noise control. Substantial work has been done previously on double-leaf partitions where the acoustics of the inner chamber and mechanical vibrations of structural supports are passively and actively controlled. The work presented here is unique in that the proposed system aims to eliminate the need for active acoustic control of transmitted acoustic energy by removing all the air between the two panels of the double partition. Therefore, the only remaining energy paths would be along the boundary and at the points where there are intermediate structural supports connecting the two panels. The eventual goal of the research is to develop a high-loss double-leaf partition that simplifies active control by removing the need for control of the air cavity and channeling all the energy into discrete structural paths. </p><p> The work presented here is a first step towards the goal of designing a high-loss, actively-controlled double-leaf partition with an air-evacuated inner chamber. One experiment is conducted to investigate the effects of various levels of vacuum on the response of a double-leaf partition whose panels are mechanically coupled only at the boundary. Another experiment is conducted which investigates the effect of changing the stiffness of an intermediate support coupling the two panels of a double-leaf partition in which a vacuum has been applied to the inner cavity. The available equipment was able to maintain a 99% vacuum between the panels. Both experiments are accompanied by analytical models used to investigate the importance of various dynamic parameters. Results show that the vacuum-sealed system shows some potential for increased transmission loss, primarily by the changing the natural frequencies of the double-leaf partition.</p>

<p> This dissertation is primarily concerned with the development of an effective methodology for reducing structure-borne sound radiation from an arbitrarily shaped vibrating structure. There are three major aspects that separate the present methodology from all the previous ones. Firstly, it is a non-contact and non-invasive approach, which is applicable to a class of vibrating structures encountered in engineering applications. Secondly, the input data consists of a combined normal surface velocity distribution on a portion of a vibrating surface and the radiated acoustic pressure at a few field points. The normal surface velocities are measured by using a laser vibrometer over a portion of the structural surface accessible to a laser beam, while the field acoustic pressures are measured by a small array of microphones. The normal surface velocities over the rest surface of the vibrating structure are reconstructed by using the Helmholtz Equation Least Squares (HELS) method. Finally, the acoustic pressures are correlated to structural vibration by decomposing the normal surface velocity into the forced-vibro-acoustic components (F-VAC). These F-VACs are mutually orthogonal basis functions that can uniquely describe the normal surface velocity. The weightings of these F-VACs represent the relative contributions of structural vibrations into the sound radiation. This makes it possible to suppress structure-borne acoustic radiation in the most cost-effective manner simply by controlling the key F-VACs of a vibrating structure. The effectiveness of the proposed methodology for reducing structure-borne acoustic radiation is examined numerically and experimentally, and compared with those via traditional experimental modal analyses. Results have demonstrated that the proposed methodology enables one to reduce much more acoustic radiation at any selected target frequencies than the traditional approach.</p><p>

Active noise control (ANC) has remained a challenging problem in the controls community, even with the rapid development of computers and state-of-the-art digital signal processors (DSP). Despite some limited success in commercial applications, there exists very little published theoretical work that can be used for design purposes. One possible obstacle is that the area of ANC comprises a combination of unrelated backgrounds. In general, a control engineer does not have sufficient knowledge in the physics of acoustics. This may explain why much of the ANC research focuses on adaptive control solutions which does not require deep insight into the acoustics. An adaptive system, however, is nonlinear and has its own open problems. For example, closed-loop stability and achievable performance of an adaptive ANC system cannot be predicted a priori and is generally difficult to analyze. This is the main reason we choose to discuss only linear, fixed-filter ANC systems. In this research we study in detail ANC systems designed to cancel noise in acoustic ducts. Due to the geometry of duct aspect ratio, we can simplify the description of the sound wave propagation along the duct to one-dimensional. Our emphasis is on analysis and design of fixed-controllers using at most two sensors and one actuator. The main thrust of this dissertation is to develop fundamental limitation theories tailored to such duct ANC problems. In particular, we apply the recently developed theories on single-input-two-output (SITO) feedback system. The alignment angle approach is found to be well suited to our ANC setup and has become the main machinery used in the development of stability and performance analysis, as well as in the selection of sensor and actuator locations. We present a new tradeoff between closed-loop stability margins and achievable performance resulting from constraints imposed by certain duct configurations. This inherent limitation can be hard to overcome with one-sensor design schemes such as feedforward controllers prevalently used in ANC literature. We demonstrate via analysis and simulations the advantages of using a two-input-single-output (TISO) controller.

<p>Turbomachinery flow fields are inherently unsteady and complex which makes the related CFD analyses computationally intensive. Physically based preliminary design tools are desirable for parametric studies early in the design stage, and to provide deep physical insight and a good starting point for the later CFD analyses. Four analytical/semi-analytical models are developed in this study: 1) a generalized flat plate cascade model for investigating the unsteady aerodynamics of a blade row with non-uniformly spaced blades; 2) a multistage interaction model for investigating rotor-stator interactions; 3) an analytical solution for quantifying the impeller wake convection and pressure wave propagating between a centrifugal compressor impeller and diffuser vane; and 4) a semi-analytical model based Lifting line theory for unified propeller and horizontal-axis turbine optimization. Each model has been thoroughly validated with existing models. </p><p> With these models, non-uniformly spaced blade rows and vane clocking are investigated in detail for their potential use as a passive control technique to reduce forced response, flutter and aeroacoustic problems in axial compressors. Parametric studies with different impeller blade numbers and back sweep angles are conducted to investigate their effect on impeller wake and pressure wave propagation. Results show that the scattered pressure waves with high circumferential wave numbers may be an important excitation source to the impeller as their amplitude grows much faster as they travel inwardly than the lower order primary pressure waves. Detailed analysis of Lifting line theory reveals the mathematical and physical equivalence of Lifting line models for propellers and horizontal-axis turbines. With a new implementation, the propeller optimization code can be used for horizontal-axis turbine optimization without any modification. The newly developed unified propeller and horizontal-axis turbine optimization code based on lifting line theory and interior point method has been shown to be a very versatile tool with the capability of hub modelling, working with non-uniform inflow and including extra user specified constraints. </p>