1,721,495 research outputs found
Active control of viscoelastic metamaterials
No full textMetamaterials have been the subject of significant interest over the past decade due to their ability to produce novel acoustic behaviour beyond that seen in naturally occurring media. As well as their potential in acoustic cloaks and lenses, of particular interest is the appearance of band gaps which lead to very high levels of attenuation across the material within narrow frequency ranges. Unlike traditional periodic materials which have been employed at high frequencies, the resonant elements within metamaterials allow band gaps to form within the long wavelength limit; at low frequencies where it is most difficult to design satisfactory passive isolation solutions. Hence metamaterials may provide a useful path to high performance, low frequency isolation. Passively these band gaps occur over a narrow bandwidth, however the inclusion of active elements provide a method for enhancing this behaviour and producing attenuation over a broad band. Two active metamaterials are investigated in this thesis, first a novel viscoelastic metamaterial is developed that achieves double negativity and could be employed as a high performance vibration isolator at low frequencies. A prototype is produced, the band gap confirmed in the laboratory, and active control is applied to create a wideband region of attenuation. Next an acoustic metamaterial consisting of an array of Helmholtz resonators is presented and it is shown that such a metamaterial has a resonant band gap and dispersive density and bulk modulus. The acoustic metamaterial is produced using 3D printing techniques and the predicted behaviour confirmed in the laboratory. Finally an active Helmholtz resonator is introduced as a pathway to creating an active acoustic metamaterial and the potential for band gap and material parameter manipulation is investigated before a prototype resonator is produced and feedback controllers applied, enhancing the band gap attenuation
An active viscoelastic metamaterial for isolation applications
No full textMetamaterials are of interest due to their ability to produce novel acoustic behaviour beyond that seen in naturally occurring media. Of particular interest is the appearance of band gaps which lead to very high levels of attenuation within narrow frequency ranges. Resonant elements within metamaterials allow band gaps to form within the long wavelength limit at low frequencies where traditional passive isolation solutions suffer poor performance. Hence metamaterials may provide a path to high performance, low frequency isolation. Two metamaterials are presented here. An acoustic material consisting of an array of split hollow spheres is developed, and its performance is validated experimentally. The application of an acoustic/mechanical analogy allows the development of an elastodynamic metamaterial that could be employed as a high performance vibration isolator at low frequencies. A prototype isolator is manufactured, and its performance is measured. The passively occurring band gap is enhanced using an active control architecture. The use of the active control system in conjunction with the natural passive behaviour of the metamaterial enables high levels of isolation across a broad frequency range. An eventual goal of the work is to produce such materials on a small scale, and as such the metamaterials developed are designed for, and produced using, additive layer manufacturing technique
Performance and stability constraints of an active acoustic metamaterial
No full textMetamaterials are the subject of significant interest due to their ability to reproduce behaviour beyond what is possible with naturally occurring media such as the appearance of high levels of wave attenuation at specified frequencies, known as band gaps. These band gaps can be tuned to appear at low frequencies, providing isolation performance at long wavelengths where the performance of traditional passive isolation techniques is generally poor. However, due to the resonant nature of the band gaps they tend to only extend over a very narrow frequency range. The application of active control within a metamaterial structure can provide an effective solution to this since the natural material response can be modified to enable attenuation over a much broader frequency region. This paper presents an active acoustic metamaterial, consisting of an array of active Helmholtz resonators. An actuator embedded within the resonators is used with a feedback controller to regulate the velocity of the air within the neck of the resonator elements, thereby enhancing their natural performance. The attainable performance of such an active metamaterial, however, is constrained by the requirement to maintain closed loop stability and formal constraints are presented and illustrated in the paper. Constrained non-linear optimisation is then employed to design a practical feedback controller consisting of an FIR filter, and its performance analysed
Enhancing the band gap of an active metamaterial
No full textMetamaterials have been the subject of significant interest over the past decade due to their ability to produce novel acoustic behaviour beyond that seen in naturally occurring media. As well as their potential in acoustic cloaks and lenses, of particular interest is the appearance of band gaps which lead to very high levels of attenuation across the material within narrow frequency ranges. Unlike traditional periodic materials which have been employed at high frequencies, the resonant elements within metamaterials allow band gaps to form within the long wavelength limit; at low frequencies where it is most difficult to design satisfactory passive isolation solutions. Hence, metamaterials may provide a path to high-performance isolation at low frequencies. Passively these band gaps occur over a narrow bandwidth, however the inclusion of active elements provide a method for enhancing this behaviour and producing attenuation over a broad band. A new type of active viscoelastic metamaterial is presented that achieves double negativity and could be employed as a high-performance vibration isolator at low frequencies. A mathematical method for manipulating the band gap profile is developed and a prototype is produced. The passive band gap is confirmed in the laboratory, and then by applying active control using optimised feedback filters it is shown that the region at which attenuation occurs around the band gap could be greatly enhanced whilst retaining the peak passive band gap performance. The active metamaterial demonstrates that a unified design philosophy matching the best features of active and passive functionality can achieve high levels of attenuation over wide frequency bands
The genesis of Clough's poetry
Full text linkThis thesis examines the early poetry of Clough, written between 1830 and 1841, in the light of the information about his personal and intellectual life provided by published and unpublished manuscripts, essays, letters and diaries. More specifically, it sets out to determine the degree to which the seeds (thematic and formal) of Clough’s more mature work can be discerned in the earlier. Chapter One discusses the influence of Clough’s childhood reading, and particularly the heroic ideal as encouraged by his mother. It traces the way this developed, particularly under the historical ideas of Thomas Arnold and the Liberal Anglicans, and the fatalistic moral problems this created. Chapter Three considers Clough’s responses to the Oxford Movement. It teases out those elements that attracted Clough and those he came to reject, particularly in the light of Tractarian ideas about reserve, in relation to poetry, truth and personal behaviour. Chapters Two and Four provide chronological, text-by-text accounts of the Rugby and Balliol poems respectively, offering judgments about influences, dates and sources, and interpretations in the light of Chapters One and Three respectively. Chapter Two argues that much of the Rugby poetry reflects an escapist lament for the past and a failure of will to restore it. Chapter Three argues that Clough’s engagement with Tractarian ideas about reserved truth provides the key context for many of these poems. Chapter Five traces the way in which Clough’s early poetics, derived from Wordsworth via Thomas Arnold, were gradually replaced by his more mature, ambiguous approach which also emerged from his encounters with Tractarian reserve. Two appendices collect ten poems and poetic fragments omitted from Mulhauser’s standard edition; three additional variant texts for poems included by Mulhauser; and four previously unpublished letters to Clough from his friend WilliamTylden
Babel: curse or blessing?
Full text link‘Babel’ is a paradoxical word. It is a name that was given to a city at the moment of its abandonment, and to a tower at the moment of its failure to be built. It signifies the variety of languages and yet is itself to be found unchanged in very many languages of the world. People can understand the word ‘Babel’; but what it means is the disintegration of understanding. Its meaning is the confusion of meaning
An active viscoelastic metamaterial with enhanced band gap properties
No full textMetamaterials have been the subject of significant interest over the past decade due to their ability to produce novel acoustic behaviour beyond that seen in naturally occurring media. Of particular interest is the appearance of band gaps which lead to very high levels of attenuation across the material within narrow frequency ranges. Unlike traditional periodic materials which have been employed at high frequencies, the resonant elements within metamaterials allow band gaps to form within the long wavelength limit. It is at low frequencies where it is most difficult to design satisfactory passive isolation solutions, and hence metamaterials may provide a useful path to high performance, low frequency isolation. A locally resonant, periodic metamaterial is presented that could be employed as a high performance vibration isolator at low frequencies. The passively occurring band gap is enhanced using an active control architecture. The use of the active control system in conjunction with the natural passive behaviour of the metamaterial enables high levels of isolation across a broad frequency range. An eventual goal of the work is to produce such materials on a small scale, and as such the metamaterial developed has been designed for, and produced using, additive layer manufacturing techniques
Controller architectures for optimum performance in practical active acoustic metamaterials
No full textOver the last decade there has been significant interest in the design and production of acoustic metamaterials with physical qualities not seen in naturally occurring media. Progress in this area has been stimulated by the desire to create materials that exhibit novel behaviour when subject to acoustic waves,such as negative refraction or the appearance of band gaps in the frequency response of the material. Proposed designs range from locally resonant phononic crystals to arrays of Helmholtz resonators within ducts and past research has investigated both passive and active materials. Much of the research into active acoustic metamaterials remains theoretical, therefore to determine whether such materials are physically realisable and of potentially practical use it is important to understand the physical constraints that may arise in a produced active metamaterial. In this paper a 1-dimensional active acoustic metamaterial derived from a passive, Helmholtz resonator based design is considered where the applied control forces produce controllable double negative behaviour. The physical dimensions and active forces required to achieve the desired novel behaviour are explored for different architectures and any trade-offs that might have to be considered when producing a practically useful active metamaterial are identifie
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