Large electric transformers such as those used in high voltage substations radiate an annoying low frequency hum into nearby communities. Attempts have been made to actively control the noise by placing a large number of loudspeakers as control sources around noisy transformers to cancel the hum. These cancellation systems require a large number of loudspeakers to be successful due to the imposing size of the transformer structures. Thus such systems are very expensive if global noise reduction is to be achieved. The aim of this thesis is to investigate theoretically and experimentally the use of thin perforated panels closely placed to a heavy structure to reduce the radiation of unwanted harmonic noise. These panels can themselves be vibrated to form a control source radiating over a large surface surrounding the primary source. The problem of the equipment overheating inside the enclosure is alleviated because the holes in the panels still allow natural cooling. An initial study is carried out to determine the resonance frequencies of perforated panels. The use of previously determined effective elastic properties of the panels and Finite Element Analysis to theoretically calculate their resonance frequencies is examined. Secondly the attenuation provided by active noise control using perforated panels as control sources is explored by use of a coupled analysis, where the primary source is assumed to influence the radiation of the perforated control panel. This analysis was found to predict poorly the amount of attenuation that could be achieved, so an uncoupled analysis is undertaken, where both the primary and control sources are assumed to radiate independently of each other. Not only does this greatly simplify the theoretical analysis but it also enables prediction of attenuation levels which are comparable to those determined experimentally. The theoretical model is reformulated to enable comparison of the sound power attenuation provided by perforated panel control sources with that of traditional acoustic and structural control sources. Finally, the use of modal filtering of traditional acoustic error sensor signals to give transformed mode (or power mode) sensors is examined. The independently radiating acoustic transformed modes of the panel are determined by an eigenanalysis and a theoretical analysis is presented for a farfield acoustic power sensor system to provide a direct measurement of the total radiated acoustic power. The frequency dependence of the sensor system, and the amount of global sound power attenuation that can be achieved is examined. Experimental measurements are made to verify the theoretical model and show that a sound power sensor implemented with acoustic sensors can be used in a practical active noise control system to increase the amount of attenuation that can be achieved. Alternatively the sound power sensor can be used to reduce the number of error channels required by a control system to obtain a given level of attenuation when compared to traditional error criteria. The power mode sensor analysis is then applied to the perforated panel control system, with similar results. / Thesis (Ph.D.)--Engineering (Department of Mechanical Engineering), 1996.

Thèse de doctorat : Sciences de gestion : Orléans : 2009. / Titre provenant de l'écran-titre.

Herein is contained details of a comprehensive finite element survey and experimental investigation into the buckling and postbuckling characteristics of thin laminated square Carbon-Epoxy panels with various cutout geometries, subjected to uniaxial compression. The plate edges are considered to be fully fixed with constant edge displacement loading. The panels were quasi isotropic in nature with a stacking sequence of (0/90/±45)2,. Square, circular and elliptical centrally located cutouts were considered with cutout dimension/panel widths ranging from 0.1 to 0.7 in increments of 0.1. Eccentrically located circular and square cutouts were considered for cutout dimension/panel width ratios ranging from 0.1 to 0.4 with vertical and horizontal eccentricity varying from 0 to 20% of the panels width. Multiple circular cutouts with cutout dimension/panel width ranging from 0 to 0.3 with separation distance/panel width ratios ranging from 0.2 to 0.65. A finite element eigenvalue analysis was adopted to determine the critical buckling loads and buckle mode shapes for the panels. The postbuckling response of the panels were investigated by adopting a non-linear finite element analysis approach using an Incremental Newton-Raphson Iterative solution scheme. A limited experimental test programme was undertaken to act as verification to the finite element solutions. A purpose built buckling rig was designed and manufactured for the purposes of the tests. It has been confirmed that the critical buckling loads for centrally located circular and square cutouts initially reduces as the cutout size increases. After reaching a minimum value it thereafter increases with large cutout sizes, the exact changeover point being dependant upon the shape of the cutout. The orientation of ellipse major axis significantly affects the critical buckling load of a panel. A horizontally aligned ellipse exhibits similar behaviour as that to a circular or square cutout. However when the major axis is rotated relative to the horizontal axis its buckling capacity reduces monotonically till it has a buckling load less than that for an unperforated panel when vertical aligned. It has been shown when a circular cutout is eccentrically placed in a panel, for small cutout sizes the buckling load reduces with horizontal eccentricity while a small increase is experienced for vertical eccentricity. Multiple circular cutouts significantly reduce the buckling capacity of the panel for all cutout sizes and separation distances. Initial geometric imperfection in the panel does not affect the critical buckling load significantly. The postbuckled response of such panels are also insensitive to the magnitude of imperfection. Panels with circular, square and elliptical cutouts exhibit substantial postbuckled strength. The post buckling response of such panels are insensitive to cutout geometry shape.

No description available.

Large electric transformers such as those used in high voltage substations radiate an annoying low frequency hum into nearby communities. Attempts have been made to actively control the noise by placing a large number of loudspeakers as control sources around noisy transformers to cancel the hum. These cancellation systems require a large number of loudspeakers to be successful due to the imposing size of the transformer structures. Thus such systems are very expensive if global noise reduction is to be achieved. The aim of this thesis is to investigate theoretically and experimentally the use of thin perforated panels closely placed to a heavy structure to reduce the radiation of unwanted harmonic noise. These panels can themselves be vibrated to form a control source radiating over a large surface surrounding the primary source. The problem of the equipment overheating inside the enclosure is alleviated because the holes in the panels still allow natural cooling. An initial study is carried out to determine the resonance frequencies of perforated panels. The use of previously determined effective elastic properties of the panels and Finite Element Analysis to theoretically calculate their resonance frequencies is examined. Secondly the attenuation provided by active noise control using perforated panels as control sources is explored by use of a coupled analysis, where the primary source is assumed to influence the radiation of the perforated control panel. This analysis was found to predict poorly the amount of attenuation that could be achieved, so an uncoupled analysis is undertaken, where both the primary and control sources are assumed to radiate independently of each other. Not only does this greatly simplify the theoretical analysis but it also enables prediction of attenuation levels which are comparable to those determined experimentally. The theoretical model is reformulated to enable comparison of the sound power attenuation provided by perforated panel control sources with that of traditional acoustic and structural control sources. Finally, the use of modal filtering of traditional acoustic error sensor signals to give transformed mode (or power mode) sensors is examined. The independently radiating acoustic transformed modes of the panel are determined by an eigenanalysis and a theoretical analysis is presented for a farfield acoustic power sensor system to provide a direct measurement of the total radiated acoustic power. The frequency dependence of the sensor system, and the amount of global sound power attenuation that can be achieved is examined. Experimental measurements are made to verify the theoretical model and show that a sound power sensor implemented with acoustic sensors can be used in a practical active noise control system to increase the amount of attenuation that can be achieved. Alternatively the sound power sensor can be used to reduce the number of error channels required by a control system to obtain a given level of attenuation when compared to traditional error criteria. The power mode sensor analysis is then applied to the perforated panel control system, with similar results. / Thesis (Ph.D.)--Engineering (Department of Mechanical Engineering), 1996.

Large electric transformers such as those used in high voltage substations radiate an annoying low frequency hum into nearby communities. Attempts have been made to actively control the noise by placing a large number of loudspeakers as control sources around noisy transformers to cancel the hum. These cancellation systems require a large number of loudspeakers to be successful due to the imposing size of the transformer structures. Thus such systems are very expensive if global noise reduction is to be achieved. The aim of this thesis is to investigate theoretically and experimentally the use of thin perforated panels closely placed to a heavy structure to reduce the radiation of unwanted harmonic noise. These panels can themselves be vibrated to form a control source radiating over a large surface surrounding the primary source. The problem of the equipment overheating inside the enclosure is alleviated because the holes in the panels still allow natural cooling. An initial study is carried out to determine the resonance frequencies of perforated panels. The use of previously determined effective elastic properties of the panels and Finite Element Analysis to theoretically calculate their resonance frequencies is examined. Secondly the attenuation provided by active noise control using perforated panels as control sources is explored by use of a coupled analysis, where the primary source is assumed to influence the radiation of the perforated control panel. This analysis was found to predict poorly the amount of attenuation that could be achieved, so an uncoupled analysis is undertaken, where both the primary and control sources are assumed to radiate independently of each other. Not only does this greatly simplify the theoretical analysis but it also enables prediction of attenuation levels which are comparable to those determined experimentally. The theoretical model is reformulated to enable comparison of the sound power attenuation provided by perforated panel control sources with that of traditional acoustic and structural control sources. Finally, the use of modal filtering of traditional acoustic error sensor signals to give transformed mode (or power mode) sensors is examined. The independently radiating acoustic transformed modes of the panel are determined by an eigenanalysis and a theoretical analysis is presented for a farfield acoustic power sensor system to provide a direct measurement of the total radiated acoustic power. The frequency dependence of the sensor system, and the amount of global sound power attenuation that can be achieved is examined. Experimental measurements are made to verify the theoretical model and show that a sound power sensor implemented with acoustic sensors can be used in a practical active noise control system to increase the amount of attenuation that can be achieved. Alternatively the sound power sensor can be used to reduce the number of error channels required by a control system to obtain a given level of attenuation when compared to traditional error criteria. The power mode sensor analysis is then applied to the perforated panel control system, with similar results. / Thesis (Ph.D.)--Engineering (Department of Mechanical Engineering), 1996.

Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2007. / Includes bibliographical references (leaves 78-82). Also available in electronic version.

Thesis (M.S.)--University of Wyoming, 2007. / Title from PDF title page (viewed on Nov. 7, 2008). Includes bibliographical references (p. 91-92).

Thesis (PhD) - Swinburne University of Technology, Faculty of Engineering and Industrial Sciences, 2008. / A thesis submitted for the degree of Doctor of Philosophy, Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, 2008. Typescript. Includes bibliographical references (p. 152-158).

No description available.