Since the identification, in the UK, of the need for further information on the dynamic loading of grandstands in the early 1990s, a number of research projects have investigated the issues relating to dynamic loading of structures due to groups of people participating in synchronised activities. These studies have, to date, largely focused on producing load models to accurately represent the dynamic crowd load and the human-structure interaction. However, whilst the vibrational response of grandstand structures is becoming better understood, the question arises as to what level of dynamic response is acceptable to the users. Currently there is very little experimental data available regarding human perception of vibrations in such crowd loading situations. As a result those producing design standards and design guides have very little information on which to base serviceability requirements. To address this, tests have been carried out at the University of Bath using a section of grandstand, whose dynamic properties could be varied, with the aim of developing acceptability criteria. Groups of participants were subjected to a range of vibrations induced by selected members of the group jumping in synchrony. Both those seated and jumping during the tests were asked to rate their perception and emotion of each vibration as well as the acceptability of the vibration in a real grandstand situation. These ratings were then used to statistically model perception and emotion to find the key vibration characteristics influencing the human response of both the seated and jumping participants prior to developing acceptability curves for each group. It was found that those seated are more sensitive to vibrations than those jumping to create them. The response of the jumpers is relatively simple and can be fairly accurately modelled using just a single variable namely the square of vertical RMS displacement. The seated response is much more complex but can be relatively accurately represented using logarithm of vertical RMS acceleration. The proposed acceptability criteria and load model generally tie in well with previously published guidelines provided that the serviceability criteria are in the same format as the original research upon which they were based (i.e. peak accelerations).

This thesis examines the quantification of uncertainty in real-world engineering systems using the multilevel Monte Carlo method. It is often infeasible to use the traditional Monte Carlo method to investigate the impact of uncertainty because computationally it can be prohibitively expensive for complex systems. Therefore, the newer multilevel method is investigated and the cost of this method is analysed in the finite element framework. The Monte Carlo and multilevel Monte Carlo methods are compared for two prototypical examples: structural vibrations and buoyancy driven flows through porous media. In the first example, the impact of random mass density is quantified for structural vibration problems in several dimensions using the multilevel Monte Carlo method. Comparable eigenvalues and energy density approximations are found for the traditional Monte Carlo method and the multilevel Monte Carlo method, but for certain problems the expectation and variance of the quantities of interest can be computed over 100 times faster using the multilevel Monte Carlo method. It is also tractable to use the multilevel method for three dimensional structures, where the traditional Monte Carlo method is often prohibitively expensive. In the second example, the impact of uncertainty in buoyancy driven flows through porous media is quantified using the multilevel Monte Carlo method. Again, comparable results are obtained from the two methods for diffusion dominated flows and the multilevel method is orders of magnitude cheaper. The finite element models for this investigation are formulated carefully to ensure that spurious numerical artefacts are not added to the solution and are compared to an analytical model describing the long term sequestration of CO2 in the presence of a background flow. Additional cost reductions are achieved by solving the individual independent samples in parallel using the new podS library. This library schedules the Monte Carlo and multilevel Monte Carlo methods in parallel across different computer architectures for the two examples considered in this thesis. Nearly linear cost reductions are obtained as the number of processes is increased.

In this thesis, I report on the investigation of the low frequency vibrational modes in a number of different systems using ultrafast spectroscopic techniques. These consist of biological systems, including the enzyme, morphinone reductase (MR) and the related biomolecules, riboflavin (Rb) and flavin mononucleotide (FMN), as well as non-biological systems, including the semiconductor gallium nitride (GaN) and gold nanoparticles (Au NPs). The term low frequency refers to terahertz (THz) frequencies, where vibrational modes exist at the molecular level, with molecular rotations, lattice vibrations and inter- and intra-molecular vibrations occurring in the THz spectral range. These vibrational modes occur on sub-picosecond timescales and therefore ultrafast techniques utilising femtosecond laser pulses provide a means of studying these modes, and are employed throughout this thesis. The two ultrafast techniques of transient absorption (TA) spectroscopy and terahertz time-domain spectroscopy (THz-TDS) were used. Firstly, a high-repetition rate transient absorption (HRRTA) spectrometer was commissioned to perform pump-probe measurements with an ultraviolet pump and broadband visible probe. The performance of the HRRTA spectrometer was benchmarked using Au NPs and used to investigate the existence of a promoting vibration in MR contributing to the catalysis process, predicted to occur at THz frequencies. Weak oscillations were detected in the charge-transfer absorption band of MR bound to the non-reactive cofactor 1,4,5,6-tetra-hydro-nicotinamide adenine dinucleotide (NADH4), with a frequency of approximately 1.5 THz and provide evidence of the first direct observation of a promoting vibration in an enzyme. To complement the TA measurements, THz-TDS was also used to obtain direct measurements of the absorption at THz frequencies. Due to the challenge of studying water-based biological samples, an initial investigation was performed on a wurtzite GaN wafer, which exhibited optical phonon modes in the THz frequency range that were found to determine the dielectric response of the semi-insulating semiconductor wafer. Use of a non-polar m-plane wafer allowed the anisotropic nature to be observed and values of 9.22 ± 0.02 and 10.32 ± 0.03 for the static dielectric constants were obtained for the THz electric field polarised both perpendicular and parallel to the c-axis of the wurtzite GaN wafer, respectively. Finally, biological studies using THz-TDS were performed with measurements on Rb pellets and films revealing vibrational modes in the THz region. The sharp absorption features were not observed in FMN, despite a small difference in molecular content from Rb, and dehydration was required to reveal small amplitude absorption features. Final measurements on MR and MR-NADH4 films were carried out and evidence of absorption features in the THz frequency range were observed, however further work is required to determine the precise origin of these features.

Pontes são estruturas denominadas obras de arte ou obras especiais, utilizadas em rotas e vias de comunicação, possibilitando que veículos atravessem obstáculos naturais ou artificiais como, por exemplo, rios. Tais estruturas são calculadas e construídas considerando os esforços aos quais serão submetidas em serviço e sua distribuição na estrutura. Além disso, também existe a necessidade de se avaliar as frequências naturais de vibração para garantir a segurança das mesmas. No presente trabalho são analisados alguns problemas relacionados às vibrações de pontes e viadutos em grelha, resultantes das variações de rigidez e massa nessas estruturas. Essa análise foi feita considerando-se que vibrações excessivas não condizem com o conforto humano e durabilidade da ponte. Ademais, para a estrutura é interessante fugir do efeito de ressonância, que acontece quando a frequência de excitação coincide com uma das frequências naturais de oscilação do sistema. Essa condição faz o sistema vibrar em amplitudes cada vez maiores, podendo causar inclusive a ruína. Para estudar formas de sair de uma frequência natural não amortecida indesejada, foi realizada uma pesquisa paramétrica das características dinâmicas deste tipo de estrutura, modificando parâmetros como rigidez (pela distribuição de longarinas e de transversinas), massa (devido à alteração das dimensões) e danos estruturais causados durante a vida útil da estrutura. Com intuito de estudar diversos modelos, foram adotadas três diferentes situações de projeto: superestrutura com variadas alturas de longarinas e transversinas, da qual foi possível concluir que a altura da longarina tem mais impacto na frequência natural não amortecida da estrutura que a transversina; pequenos defeitos ou danos na estrutura e o efeito de uma longarina rompida para os quais foram comparadas suas frequências naturais não amortecidas. As análises foram realizadas pelo Método dos Elementos Finitos, utilizando-se o programa comercial SAP2000. / Bridges are structures sometimes referred as works of art or special works, used on routes and roads that allow vehicles to cross natural or artificial obstacles, such as rivers. Such structures are calculated and constructed considering the efforts to which they will be submitted in service and their distribution in the structure. In addition, there is also a need to evaluate the natural frequencies of vibration to ensure their safety. In the present study some effects were analyzed related to the vibrations of grid bridges and viaducts resulting from changes in stiffness and mass in these structures. This analysis was made considering that excessive vibrations are not consistent with human comfort and bridge durability. For the structure, it is interesting to stay away from the resonance effect, which occurs when the excitation frequency matches one of the natural frequencies of oscillation of the system. This leads to oscillation with increasing amplitudes which may cause even the ruin. In order to study several models, three different design situations were adopted: superstructure with varying heights of beams and crossbeams, from which it was possible to conclude that the height of the beams has more impact on the undamped natural frequency of the structure than the crossbeams; small defects or damage to the structure and the effect of a ruptured beam for which its undamped natural frequencies were compared. The analysis were performed using the Finite Element Method, using the SAP2000 commercial program.

A retificação representa o processo mais utilizado na usinagem de peças para obtenção de altos níveis de qualidade superficial, dimensional e produtividade. Uma importante etapa da retificação consiste na preparação e condicionamento do rebolo para o corte, a dressagem. Durante a dressagem, a superfície da ferramenta de corte é alterada para obtenção de arestas cortantes de acordo com as especificações desejadas. O objetivo deste trabalho é desenvolver um método para inscrição de texturas na superfície do rebolo que proporcionem melhores condições de corte durante a retificação e, também, possibilite utilizar a retificação como processo para texturização superficial de peças. O método desenvolvido consiste em alterar controladamente a profundidade de dressagem utilizando um excitador eletromecânico fixado na base do dressador de uma retificadora cilíndrica CNC. A movimentação é controlada por um software desenvolvido para sincronizar o deslocamento longitudinal do dressador com ondas senoidais enviadas ao amplificador operacional do excitador. Desta forma, o dressador proporciona inscrição de texturas no rebolo durante a dressagem. O método foi experimentalmente avaliado e sua funcionalidade foi comprovada. Texturas diversas foram inscritas na superfície do rebolo, com até 15 \'mü\'m de profundidade. Com o uso da texturização de rebolos foi possível obter redução de até 20% da potência específica de corte, sem perda representativa da qualidade final da peça (rugosidade e cilindricidade) e mantendo a vida da ferramenta de corte, em comparação com retificação convencional. Em complemento, o método possibilitou criação de diversos padrões de texturas nas superfícies de peças retificadas, possibilitando utilizar a retificação como um processo de usinagem para este fim. / Grinding represents the most applied machining process to achieve best combination of surface and dimentional quality, also productivity. One important stage of the grinding process is to prepare and condition the grinding wheel surface for the cutting, the dressing operation. During the dressing, wheel surface is changed to acquire cutting specification required for the operation. The objective of this work is develop a method to texturing the grinding wheel surface for better grinding performance and, also, texturing of surfaces through normal grinding cutting. The method controlled changes the dressing depth using an electro-mechanical actuator fixed to the dressing tool base of a CNC grinding machine. The displacement of the dressing tool is controlled using the developed software. It is able to synchronize the longitudinal movement with generated sine waves sent to the operational amplifier of the actuator. Through this movement and control, the dressing tool is able to texture the grinding wheel. The method was experimentally evaluated and its functionally proved. It is possible to inscribe different texturing patterns in the wheel surface, with depth up to 15 \'mü\'m. Using texturized wheels, is possible to reduce up to 20% the grinding power, without representative losses with the machined workpiece quality (roughness and roundness) and tool life, compared to regular grinding. In addition, it is possible to transfer different textures patterns to the workpiece surface while cutting, enabling grinding s a machining process to produce workpiece patterns.

Le crissement des freins des Trains à Grande Vitesse (TGV) est un bruit aigu particulièrement gênant émis lors de l'arrivée du train en gare. Ce travail a pour but de comprendre le phénomène en vue de réduire voire de supprimer l'émission du bruit. Ce phénomène acoustique est d'origine vibratoire : le crissement des freins à disque résulte des vibrations auto-entretenues des différents constituants induites par le frottement des garnitures sur le disque. Le phénomène de frottement étant lié aux aspérités de surface, il met en évidence un problème à deux échelles où un phénomène apparaissant à l'échelle microscopique crée des vibrations dans un disque à l'échelle macroscopique (amplitude de vibration du disque de l'ordre de quelques micromètres). Les phénomènes de bruits de freinage dont le crissement fait partie sont complexes, variés et ont des origines diverses selon les points de fonctionnement du mécanisme. Un état de l'art sur les différents travaux de modélisation montre qu'une instabilité de l'équilibre stationnaire dite géométrique ou par couplage de mode permet de mobiliser les modes de vibration responsables du crissement et ceci à coefficient de frottement constant. D'un point de vue modélisation, ces constatations nous orientent vers un modèle de mécanique des milieux continus déformables dans l'hypothèse de petites transformations. Beaucoup de travaux sur les vibrations auto-entretenues s'intéressent à la stabilité de l'équilibre mais font un raccourci rapide entre fréquences des modes instables et solution transitoire. Les travaux numériques traitant de la caractérisation de ces évolutions transitoires mettent en évidence des événements fortement non linéaires sous la zone de contact (comme de 'adhérence ou du décollement à cause de l'unilatéralité du contact). Un premier objectif de ce travail est l'étude de l'évolution des perturbations proches de l'équilibre vers la solution transitoire. Cette étude permet de mettre en évidence les liens entre les fréquences caractéristiques de cette solution transitoire (fréquences de crissement) et les fréquences des modes de stabilité. Le cas particulier d'un mécanisme de frein à disque TGV simplifié tridimensionnel est choisi. La transition entre l'évolution des perturbations proches de l'équilibre et l'évolution transitoire est étudiée grâce à une méthode originale de projection sur les modes de stabilité. Elle permet une étude spatiale et temporelle des champs vibratoires solutions. Son application sur le modèle simplifié de frein à disque est rendue possible grâce à une formulation eulérienne du problème. D'un point de vue calcul de structures, les solutions transitoires ont été déterminées grâce à des algorithmes et des stratégies numériques adaptés permettant de tenir compte du caractère non régulier du comportement du contact unilatéral frottant dans le cadre dynamique (tel que les situations de choc). L'instabilité de l'équilibre par couplage de mode est le mécanisme générateur de vibrations auto-entretenues privilégié dans la littérature. Qu'en est-il sur le mécanisme de freinage TGV? Cette question nous a conduit dans un premier temps à observer le comportement du mécanisme réel grâce à des campagnes d'essais. Dans un deuxième temps, elle nous a conduit à proposer un modèle de frein TGV réaliste. L'étude de la stabilité de l'équilibre a permis de prédire les fréquences de crissement mesurées. Elle a aussi permis de préciser le mécanisme physique de couplage de mode ainsi que sa dépendance vis à vis des caractéristiques physiques des garnitures. La connaissance de ce mécanisme physique est un fil conducteur robuste dans la recherche de solutions de réduction du bruit.

Le contrôle semi-actif est un domaine de recherche en plein essor vu qu'il joint la stabilité des systèmes contrôlés passivement à la possibilité de contrôle en temps réel permise par les stratégies actives. Dans cette thèse on s'intéresse à un assemblage de poutres modélisées sous l'hypothèse de linéarité et représentant un modèle de pont à hauban en cours de construction. Cette structure a été contrôlée précédemment par un dispositif formé d'un pendule à masse accordée couplé à un alternateur et à une résistance électrique sous l'hypothèse de petites rotations du pendule. Dans cette thèse on a pour objectif de rendre robuste cet actionneur pour les grandes rotations du pendule. Afin d'y parvenir le travail effectué peut être divisé en quatre points: Tout d'abord, une étude paramétrique est effectuée afin de mieux comprendre le comportement de l'actionneur en grandes rotations du pendule. Cette étude a permis d'identifier la dépendance du contrôle aux variables et aux paramètres du système. Puis une étude du contrôle passif du système non linéaire virtuel est effectuée et une loi basée sur le contrôle par séquencement de gain est définie et appliquée à la structure. Ces deux lois montrent la nécessité de la conception d'un contrôle en temps réel. Ensuite, on définit des lois semi-actives dans l'esprit de la poursuite d'un modèle de référence pour les deux systèmes pendule seul et pont-pendule. Leurs limites et leurs apports sont établis. Enfin par une stratégie de poursuite on conçoit une loi de contrôle actif et sa version semi-active pour le système non linéaire réel. Cette dernière, permet enfin de réaliser les objectifs de cette thèse i.e avoir un niveau d'amortissement de la structure pont-actionneur électromécanique, pour les grandes rotations du pendule, comparable à celui d'un modèle linéaire.

The research reported in this thesis builds on a novel model developed at the University of Minnesota to analyze the self-excited vibrations that occur when drilling with polycrystalline diamond cutter bits. The lumped parameter model of the drilling system takes into consideration the axial and the torsional vibrations of the bit. These vibrations are coupled through a bit-rock interaction law. At the bit-rock interface, the cutting process combined with the quasihelical motion of the bit leads to a regenerative effect that introduces a coupling between the axial and torsional modes of vibrations and a state-dependent delay in the governing equations, while the frictional contact process is associated with discontinuities in the boundary conditions when the bit sticks in its axial and angular motion. The response of this complex system is characterized by a fast axial dynamics superposed to the slow torsional dynamics. A two time scales analysis that uses a combination of averaging methods and a singular perturbation approach is proposed to study the dynamical response of the system. An approximate model of the decoupled axial dynamics permits to derive a pseudo analytical expression of the solution of the axial equation. Its averaged behavior influences the slow torsional dynamics by generating an apparent velocity weakening friction law that has been proposed empirically in earlier works. The analytical expression of the solution of the axial dynamics is used to derive an approximate analytical expression of the velocity weakening friction law related to the physical parameters of the system. This expression can be used to provide recommendations on the operating parameters and the drillstring or the bit design in order to reduce the amplitude of the torsional vibrations. Moreover, it is an appropriate candidate model to replace empirical friction laws encountered in torsional models used for control. In this thesis, we also analyze the axial and torsional vibrations by basing the model on a continuum representation of the drillstring rather than on the low dimensional lumped parameter model. The dynamic response of the drilling structure is computed using the finite element method. While the general tendencies of the system response predicted by the discrete model are confirmed by this computational model (for example that the occurrence of stick-slip vibrations as well as the risk of bit bouncing are enhanced with an increase of the weight-on-bit or a decrease of the rotational speed), new features in the self-excited response of the drillstring are detected. In particular, stick-slip vibrations are predicted to occur at natural frequencies of the drillstring different from the fundamental one (as sometimes observed in field operations), depending on the operating parameters. Finally, we describe the experimental strategy chosen for the validation of the model and discuss results of tests conducted with DIVA, an analog experimental set-up of the lumped parameter model. Some results of the experiments conducted in an artificial rock seem to validate the model studied here although the same experiments obtained with natural rocks were unsuccessful. Different problems with the design of the experimental setup were identified. By using the outcome of the analysis of the uncoupled dynamics, we could provide critical recommendation to elaborate and to design a simpler and stiffer analog experiment (TAZ) used to study the self excitation of the axial dynamics that ultimately lead to the excitation of the torsional dynamics.

The present thesis is based on three studies that research different aspects of fine texture perception. The goal is to better understand the mechanisms involved in haptic perception of textures below 200 µm, also known as microtextures. Study I was conducted to establish a friction measurement model and relating the friction measurements to perceived coarseness of fine textures. A set of printing papers was used as stimulus material. In Study II an expanded set, including the set of Study I, was used as stimuli in a multidimensional scaling (MDS) experiment of haptic fine texture perception. Through scaling of perceptual attributes and similarities, a three dimensional space was found to best describe the data and the dimensions were interpreted as rough-smooth, thick-thin and distinct-indistinct. In Study III a series of model surfaces were manufactured with a systematically varied sinusoidal pattern, spanning from 300 nm to 80 µm. As in Study II, a similarity experiment was conducted and a two dimensional space was chosen, the dimensions of which were explained well through friction and the wavelength. Together these three studies form a better picture of fine texture perception. The dimensionality found with paper stimuli was very similar to the corresponding spaces for marcrotextures of everyday materials, even though a different perceptual system is used for fine texture perception. Regardless if the information is coded through the spatial or the vibratory sense, the perception does not seem to differ in dimensionality. Further, the largest among the microtextures seem to have been perceived as carrying spatial information. On the systematically varied, rigid, textures, the MDS space did not come out in a similar fashion to those of everyday materials but instead similar to the physical properties that characterizes the change in the textures. It was further found that the participants in Study III successfully discriminated textures with an amplitude of 13 nm from the unwrinkled surfaces. From these studies the main conclusions are (a) haptically measured friction and surface roughness are important contributors to fine texture perception, (b) even at microscales, spatial information is retrieved haptically, probably through vibrations, and (c) persons can haptically discriminate textures at a nanoscale.

Tuned mass dampers (TMDs) are auxiliary damping devices installed within tall structures to reduce undesirable wind-induced vibrations and to enhance the overall system damping and hence, the dissipative capacity. The design of TMDs involves the selection of optimal auxiliary mass, frequency, and damping, based on the main structure's mass, natural frequency and damping properties. TMDs are inherently susceptible to detuning, where the auxiliary parameters are no longer optimal due to deterioration or changes within the system, resulting in a degradation in their performance. In order to correct for this detuning, it is necessary to perform a condition assessment while the TMDs are in service. The main goal of this thesis is to present a methodology to conduct condition assessment while the TMDs are in service. The proposed methodology does not involve either restraining the TMD or providing controlled external excitation to the structure, and relies on ambient measurements only. The first phase in the condition assessment is to estimate the bare structure's modal properties using acceleration measurements obtained from the structure while the TMDs are unrestrained. The present work accomplishes this goal within the framework of parametric identification using Kalman filtering, where the unknown parameters (bare modal properties) are appended to the state vector and estimated. Unlike most of the literature on this subject, the noise statistics for the filter are not assumed to be known a priori. They are estimated from the measurements and incorporated into the filter equations. This filter involves direct feedthrough of the process noise in the measurement equation and the appropriate filter is derived and used following the noise covariance estimation step. In the next phase, criteria to assess the condition of the TMD are developed. They include optimal tuning parameters established using simulated experiments and measured equivalent viscous damping. The research considered pendulum tuned mass dampers (PTMDs), which presently account for a large fraction of full-scale applications. Results were demonstrated using numerical investigations, a bench-scale model equipped with an adaptive mechanism for adjusting auxiliary damper parameters, and a full-scale PTMD-equipped structure. The main contributions of this thesis are: (a) a broader understanding of the coupled biaxial behaviour of PTMDs has been developed; (b) a systematic procedure for estimating the underlying modal characteristics of the structure from ambient vibration measurements within the framework of Kalman filtering has been achieved; (c) a comprehensive framework to undertake condition assessment of TMDs has been presented, integrating parametric identification from measured response data and performance prediction for design period wind events using boundary layer wind tunnel studies. The work provided new insight into the design and behaviour of PTMDs and presented a comprehensive approach to quantify their performance. The Kalman filtering framework also provides an efficient platform to build adaptive passive tuned mass dampers that can be tuned in place and adjusted to correct for detuning and accommodate various operating conditions.