A partir da solução analítica obtida para o campo turbulento gerado por duas grades oscilantes em fluido em repouso, considerados os casos de viscosidade constante e viscosidade variável, foram estudados escoamentos turbulentos complexos. A partir de proposta de equação governante linear ainda não publicada de SCHULZ (2003a), foram obtidas soluções promissoras e exploradas possibilidades de generalização. As equações em questão baseiam-se no modelo &#954-&#949. A ferramenta de trabalho no estudo das soluções teóricas foi o programa Mathematica. Embora essa ferramenta simplifique o trabalho mecânico decorrente da busca de soluções viáveis, vale lembrar que a construção do problema depende da criatividade do estudioso. Os objetivos propostos foram atingidos: reproduzir e ampliar os resultados para grades oscilantes de SCHULZ (2001). Adicionalmente, os equacionamentos governantes lineares mostraram possuir soluções compatíveis com a observação. / From the analytical solution of turbulent field generated by two oscillating grids in fluid in rest, complex turbulent flows have been studied considering the cases of constant and variable viscosities. Promising solutions as well as some turbulence generalization possibilities were obtained considering SCHULZ\'s (2003) non-published proposal of linear governing equations. The solutions are based on the &#954-&#949 model. The Mathematica program was used as a tool in the study of the theoretical solutions. Although this tool simplifies the work derived from the search for viable solutions, it is important to remember that the construction of the problem depends on the researcher\'s skills. The aims of reproducing and extending SCHULZ\'s (2001) model for oscillating grids have been achieved. Additionally, the linear governing equations have shown to have compatible solutions with the observation.

Este trabalho representa o início de uma série de pesquisas que visam o estudo da turbulência e de sua relação com processos de mistura e trocas gasosas entre ar e água, através de experimentos com grades oscilantes. Seu objetivo foi o projeto e a construção de um tanque de grade oscilante, equipamento que gera turbulência com intensidade controlável. Após a construção do tanque, experimentos para medições de velocidade turbulenta foram feitos, com uso de uma técnica de velocimetria a laser. Uma grade de 9x9 barras foi usada na agitação do fluido, com uma amplitude de oscilação de 3 cm, para 4 diferentes freqüências de oscilação. Adquiriu-se 9600 imagens do movimento do fluido, em 6 regiões do tanque, para a obtenção dos campos de velocidade turbulenta, calculados através do software Visiflow e de um programa computacional desenvolvido neste trabalho. Os gráficos criados a partir dos campos possibilitaram a observação do decaimento espacial da turbulência e da região de sua produção. Os campos médios de velocidade mostraram-se bem inferiores aos campos instantâneos, indicando a existência de baixo escoamento médio do fluido. As condições de isotropia e a homogeneidade espacial da turbulência são mais aproximadas à medida que se afasta da grade. A intensidade turbulenta produzida é diretamente relacionada com a freqüência de oscilação. O número de imagens para a obtenção de uma média representativa da velocidade turbulenta neste equipamento parece ser dependente da freqüência de oscilação da grade. Isto aponta para a necessidade de estabelecer corretamente as condições para os cálculos estatísticos em escoamentos turbulentos / This work presents the project and construction of a tank with an oscillating grid, equipment which provides for the experimental studies of turbulence and its relation to mixing processes and gas transfer across fluid interface. Experiments were carried out with the use of digital particle image velocimetry technique, to investigate the properties of the produced turbulence. A grid made of 9x9 square bars was used to stir the water, with a stroke of 3 cm and 4 different oscillation frequencies. A number of 9600 images were acquired, in 6 regions of the tank, for the generation of the turbulent velocity fields through the software Visiflow and a computational program developed in this work. The results showed that the current equipment, with a new concept for the grid oscillation system, can be conveniently useful for studies in this research field. Average velocity fields appeared to be considerably smaller than the instantaneous velocity fields, which leads to the existence of nearly-stationary turbulence in the water volume. Nearly-isotropic turbulence and spatial homogeneity were approximate as the measurements distanced from the grid. The turbulent intensity was directly dependent on the oscillation frequency. The spatial decay of the turbulence and the region near the grid where it is produced could also be observed. The number of images necessary for the calculus of reliable root-mean-square turbulent velocities seems to be dependent on the oscillation frequency of the grid. It results in the necessity of establishing correct statistical analysis of turbulent flows

The running of gas turbines with low emissions causes high pressure pulsations to arise in the combustion chamber. These pulsations are high amplitude sound vibrations. At some frequencies these are harmful to the gas turbine. Hence these pulsations are monitored to avoid operating conditions where pulsations are specifically harmful. It is necessary to expose the system of known pulsations to verify the functionality of the monitoring process. This report describes the development of a generator of pressure pulsations to verify the monitoring system. The output should be of harmonic form with a frequency of greater than 160 Hz and 15 mbar in amplitude. To begin with, a few alternatives to a pulsation generator are described. In light of studies of these possibilities the alternative based on a roller bearing is selected. This is considered as one of the easier alternatives to design. An existing generator of pressure pulsations has been analyzed. The calculations from this analysis supported the construction of the new device. The new generator far exceeded the specification regarding frequency, amplitude and harmonics of the pulsations. With a few modifications, such as encapsulation, the pulsation generator will be ready for usage. It could then serve as a mobile device for the inspection of the pulsation monitoring system. Possibly the device could be equipped with a faster and stronger motor. This would permit the device to be used for the calibration of pulsation monitoring sensors.

Le projet porte sur le développement d'une pompe à chaleur magnétocalorique pour l'entreprise NextPAC. Une étude a été réalisée sur les transferts thermiques se déroulant dans le régénérateur de la pompe. Cette étude a permis de développer un modèle numérique permettant de simuler les échanges thermiques au sein du régénérateur et de développer un démonstrateur afin de pouvoir réaliser des mesures thermiques sur un matériau magnétocalorique échangeant avec un fluide caloporteur en écoulement alterné. De plus, une étude critique et approfondie a été réalisée sur le premier prototype de NextPAC afin d'évaluer son bon fonctionnement. De nombreuses propositions d'amélioration ont été proposées pour la réalisation d'un second prototype. / The project involves the development of a magnetocaloric heat pump for the NextPAC company. A study has been realized on the thermal transfers in the pump's regenerator. This study allowed us to develop a digital model that simulates the thermal exchanges in the regeneratot and to develop a demonstrator in order to collect thermal measurements of a magnetocaloric material. This last exchanges heat transfers with an oscillating fluid flow. Moreover, a study on a first industrial prototype hads been carry out in order to check and to show improvements that have to be carry out fo a second planned prototype.

The Stability And Control CONfiguration (SACCON) model represents an emerging trend in airplane design where the classical tube, wing and empennage are replaced by a single tailless configuration. The challenge is to assure that these designs are stable and controllable. Nonlinear aerodynamic behavior is observed on the SACCON at higher incidence angles due to leading edge vortex structures. Active Flow Control (AFC) used in preliminary design represents a promising solution to the longitudinal stability problems and this was demonstrated experimentally on a semi span model. AFC can be used to trim the SACCON in pitch and it alters forces and moments comparable to common control surface deflections. A combination of AFC and control surface deflection may increase the overall efficiency and opens up a variety of maneuvering possibilities. This implies that AFC should be treated concomitantly with other design parameters and should be considered in the preliminary design process already and not as an add-on tool. Integral force and moment data was supplemented by observations using Pressure Sensitive Paint (PSP) and flow visualization. Two arrays of individually controlled sweeping jets, one located along the leading edge and the other along the flap hinge provided the AFC input needed to alter the flow. The array positioned over the flap-hinge of the model was most effective in stabilizing the wing by decreasing the pitching moment at lower and intermediate angles of incidence. This effect was achieved by reducing the spanwise flow on the swept back portion of the wing through jet-entrainment that also affected the leading edge vortex. Leading edge actuation showed some beneficial effects by inhibiting the formation of the leading edge vortex near the wing tip. A preliminary study using suction was carried out. The tests were carried out at Mach numbers smaller than 0.2 and Reynolds numbers based on the root chord of the model that approached 10⁶.

Commercial viability of Wave Energy Converters (WEC) depends on addressing not only the energetic effciency, but also in solving the practical issues related to manufacturing methods, access to technology, handling, transportation and installation, operation and maintenance, impact on marine life and most importantly the cost per kW-h. The UFCAP WEC is one concept which has the potential to facilitate handling, manufacturing, and installation activities as well as to be able to lower the current wave energy cost per kW-h, however its feasibility had not been properly assessed nor proved. It consists of multiple interconnected Oscillating Water Columns (OWC) chambers, it is modular, and simple, with no-moving parts in contact with the water and can use a simpler one-direction turbine which is more economic, and more effcient than self-rectifying turbines used in most of the OWCs devices. Testing of the device to fully assess its feasibility required a low pressure check-valve, and a customized turbine which were developed during the present work. Check-valves are widely used in the industry for medium or high-pressures, but were not available at all for large-flows with low-pressure-differences. A novel check-valve was devised for this application, along with the scaled UFCAP prototypes developed to be tested in a wave-flume and in the ocean to validate UFCAPs concept feasibility, and identify critical design parameters and features such as the conduit/air-chamber ratio. Ocean tests allowed to observe performance at component and assembly levels, learning new failure-modes and stablishing best-practices for future deployments. Testing confirmed the UFCAP WEC is not only an idea, but a concept which works and can generateing electricity at a competitive cost. / Graduate

In the present work, we experimentally study thrust generation from sinusoidally pitched rigid and flexible foils immersed in a uniform flow. The flexible foils are made by attaching a flexible flap of known flexural rigidity and flap length to the trailing edge of a rigid foil. For such thrust generating systems, a propulsive efficiency (η) may be defined as the ratio of the useful work done to the input energy requirement. In the present experiments, the propulsive efficiency (η) of the flapping foil can be determined from direct measurement of the unsteady forces and torque on the foil. The effects of systematic variation of the flexural rigidity of the foil, from highly flexible to rigid, on the thrust and efficiency characteristics of the foil are investigated. Studying such oscillating foils helps one to understand and mimic the efficient thrust generating mechanism in fishes and other creatures that use flapping to locomote themselves. A strain guage based loadcell is used to measure the forces normal to the foil (N) and forces along the chord of the foil (A). With a potentiometer, the instantaneous angular position (θ) is also measured, so that instantaneous lift (L) and thrust (T ) can be calculated. The measured moment (M) is used to calculate the instantaneous power input (P = Mθ˙). The foil is immersed in a uniform flow (u) set in a water tunnel, and the sinusoidal pitching (θ = θmaxsinωt) is provided by a servo motor. The Reynolds number (Re = uc/ν) in the present study is in the range of 103 to 104 . For the case of the rigid foil, the thrust and efficiency characteristics are presented for variation of the non-dimensional flapping frequency called the ‘reduced frequency’ (k = πfc/u), which is varied in the range of 1 to 10. At small reduced frequency (k < 3), the foil experiences a mean drag, while at k > 3, the foil experiences a mean thrust that grows rapidly as the reduced frequency (k) is increased. The thrust characteristics of the rigid foil are decided mainly by the normal force’s phase with respect to θ (φCN ) and its magnitude ([CN ]), as the chord-wise force is very small compared to the normal force (A << N). The measurements show that the non-dimensional mean thrust coefficient (CT ) scales as k2 and non-dimensional mean power (CP ) scales as k3 for k Ҳ 4. The maximum efficiency for rigid foils is found to be 8 % and it occurs at k 6. For the flexible foil case, the effect of making a portion of the total foil flexible by means of attaching a flexible flap of known flexural rigidity (EI) and flap length (cF ) to a rigid foil of length (cR) is studied. Unlike the rigid foils, the chordwise force (A) becomes an important factor in determining the thrust and efficiency characteristics of the flexible foils, due to the bending of the flap. We present results for a broad range of flexural rigidities from highly flexible flaps to stiff flaps, with the extent of flexibility fixed at cF /cR =0.8. We find that there is an optimal flexural rigidity for which the efficiency (η) reaches a maximum of 28 %. This represents a 250 % improvement compared to the rigid foil. The flexible foils with stiff flaps show a strange behavior with all the mean thrust coming from chordwise forces (A), unlike other flexible foils where the contribution to mean thrust come from both normal and chordwise forces. The effect of varying the extent of flexibility (cF/cR) with fixed flexural rigidity has also been studied. We define a non-dimensional flexibility parameter, R∗ = EI/(0.5ρu2sc3F ), which can combine the effect of variations in EI and cF /cR. Using this non-dimensional flexibility parameter (R∗), we find out that mean thrust and efficiency data for both the EI and cF/cR variation study collapse onto a single curve, indicating that R∗ can indeed be a single parameter characterizing flexibility. The present work shows that flexible foils can improve efficiency over rigid foils. Efficiency improvements can come in two ways depending on the R∗ of the flexible foil. Flexible foils with R∗ in the range of 10−2 to 100 show nearly 250% improvement in efficiency, accompanied by nearly 70 % loss in thrust compared to an entirely rigid foil of the same total chord. Flexible foils with R∗ in the range of 100 to 101 show nearly 50 % improvement in efficiency accompanied by nearly 100% increase in thrust.

Maintaining postural control in standing requires the center of mass (COM) to be kept within the boundaries of the base of support (BOS). As the COM moves to a region outside the BOS, a step is often required, increasing the risk of falling, and therefore injury. All too often, humans are subjected to different perturbations while maintaining upright stance, and so to control the COM in these situations, postural activity through musculature at the ankle, knee, and hip are recruited according to demand associated with the level of perturbation. It is also believed that control of the head allows components of the visual and vestibular systems to contribute stable reference values. In order to understand human response to, and in preparation for perturbation, researchers have employed a number of experimental techniques, including i) loading and subsequent unloading of body segments such as the arm or leg; ii) single discrete platform movements; and iii) continuous platform movements. While many studies have focussed on examining postural control mechanisms during discrete perturbations in children and adolescents, these mechanisms in response to continual perturbation have not been well documented in observable developmental stages of youth, nor have they been well documented in special populations. Therefore, the primary aim of this doctoral dissertation was to characterize the reactive and anticipatory postural control mechanisms in children and adolescents, as well as to examine and characterize them in the Cerebral Palsy population. To achieve this overarching goal, three separate studies were performed. The first study characterized the anticipatory and reactive mechanisms of postural control in typically developing children aged 7 to 17 years. Specifically, postural muscle activity in the lower limbs, an index of head anchoring strategy, and body segment cross-correlations were characterized in response to, and in anticipation of, continuous platform oscillation movement at various frequencies. The second experiment characterized these same mechanisms in children of similar ages with cerebral palsy, and compared the results to those found in the first study. The results of these studies indicated a slight shift to anticipatory measures after repeated exposure to a specific frequency of platform oscillation, however, each transition to a greater platform speed resulted in more conservative measures of postural control. Furthermore, youth with cerebral palsy tended to control their posture much in the same way of typically developing children and adolescents under less challenging conditions however, with the increased difficulty associated with higher frequency oscillation were not able to generate adequate postural responses. The final study in this dissertation investigated the effects of a one-week intensive virtual reality-based exercise programme. In this study, postural control mechanisms were observed before and after a one-week time period, during which half of the participants received the enhanced exercise while the remaining participants received no treatment at all. The results from this study indicated there was no effect of a 5-day VR-based intervention on postural control mechanisms used in response to oscillating platform perturbations. Taken together, the results from these studies extend the current understanding of reactive and anticipatory mechanisms of postural control in children and adolescents, both typically developing and those with cerebral palsy.

Laplacian differential operator -- Vibrations of plucked strings and Hollow cylinders.

Son efficacité, son faible impact environnemental et sa fiabilité font de la réfrigération thermoacoustique solaire une alternative intéressante aux systèmes solaires de production de froid. L'adaptation des technologies solaire et thermoacoustique requiert une conception thermique précise de l'élément d'interface, le récepteur solaire, constitué d'une cavité et d'un échangeur irradié par le rayonnement solaire. L'objectif de cet élément est de collecter et de transmettre efficacement l'énergie solaire incidente au fluide de travail du système thermoacoustique. Comme les ondes acoustiques sont très sensibles aux perturbations thermiques, la conception du récepteur doit favoriser l'homogénéité thermique, spatiale et temporelle, à l'intérieur de l'échangeur. Pour cette raison, une étude complète incluant le développement d'outils numériques de simulation pour modéliser le processus thermique, du transfert solaire au transfert thermoacoustique est nécessaire afin d’optimiser les dimensions du prototype de la campagne expérimentale. Cette étude décrit les outils de simulation ainsi que les dispositifs expérimentaux comme les résultats obtenus sur les aspects spatiaux et temporels. / Its efficiency, its low environmental impact and its reliability makes thermoacoustic solar refrigeration an interesting alternative to the existing solar systems for the cooling production. The solar adaptation of a thermoacoustic system requires an appropriate thermal design of the interface element, the solar receiver, which consists in a hot heat exchanger placed in a cavity that surrounds the focused image of the sun. The objective of this element is to efficiently collect and transfer the concentrated solar incident energy to the working fluid of the thermoacoustic system. Since acoustic waves characteristics are very sensitive to thermal disturbances, one challenge in the design of the receiver is that the temperature field within the heat exchanger must be as homogeneous as possible in space and time. Hence, a complete study, including the development of simulations tools which model the whole heat transfer processes, from solar to thermoacoustics, is necessary to optimize the prototype’s dimensions for the experimental campaign. This study describes the simulation tools and the experimental apparatus developed and the results obtained over space and time.