This thesis deals with a procedure for determining the complete processing of aerodynamic flow instabilities (rotating stall and surge) in a centrifugal compressor. At small flows the performance of a compressor system is limited by the surge line, which is caused by flow instabilities. Numerical solution is obtained using the method of transfer matrix. This system is simulated through several models with local resistances that represent the dissipation of pressure energy. Pulses are excitated in these models by the pressure jump placed before the centrifugal compressor. From the frequency-amplitude characteristics for the selected range of frequencies and flow the impedance characteristic of the compressor system is determined. We are looking for problematic frequencies in this characteristic that cause flow instabilities in the compressor system.

This thesis analyses how users' mouse behaviour during a video stall gets influenced by the type of video, either branched or linear, and by the presence of a playback bar. An experiment was conducted with thirty-two participants divided into six groups. Each group was watching a different combination of four videos with stalls, the first two videos belonging to the same type of video and either with or without a playback bar, and the last two videos changed in one of the two aspects. With mouse-tracking, these aspects were studied through the variables of mouse activity, average speed, average distance between the cursor and the playback bar, and the total distance moved on the screen. The participants also filled in questionnaires about their mouse behaviour, after watching each video, and their answers were later analysed through a thematic analysis. The results showed no significant differences between the groups in any of the main dependent variables. In general, within all groups, the participants moved the mouse very scarcely, indicating that the results about mouse movement should be interpreted carefully. During videos with a playback bar, mouse movements appeared to be concentrated to the stalls, focusing the movements to the bottom of the screen where the playback bar is located. Mouse behaviour during videos without a playback bar was more evenly divided between the different parts of the video and of the screen, or the user were not moving the mouse at all. Watching branched or linear videos influenced the mouse behaviour in such a way that branched videos seemed to engage the user to interact with the video player more than the linear videos. However, no difference was noticed between these conditions for active users during stalls when a playback bar was present. The thematic analysis gave clear indications that the playback bar was an important component for understanding a stall. Based on these findings, conclusions are drawn that a stall is a situation of watching videos during which mouse behaviour may be less affected by the type of video, and more influenced by the access to a playback bar. The playback bar was shown to be a source of information about the system and the situation.

It is emerging that the brain’s vasculature consists of a highly spatially heterogeneous network; however, information on how various vascular characteristics differ between brain regions is still lacking. Furthermore, aging studies rarely acknowledge regional differences in the changes of vascular features. The density of the capillary bed is one vascular feature that is important for the adequate delivery of nutrients to brain tissue. Additionally, capillary density may influence regional cerebral blood flow, a parameter that has been repeatedly correlated to cognitive-behavioural performance. Age-related decline in capillary density has been widely reported in various animal models, yet important questions remain concerning whether there are regional vulnerabilities and what mechanisms could account for these regional differences, if they exist. Here we used confocal microscopy combined with a fluorescent dye-filling approach to label the vasculature, and subsequently quantified vessel length, tortuosity and diameter in 15 brain regions in young adult and aged mice. Our data indicate that vessel loss was most pronounced in white matter followed by cortical, then subcortical gray matter regions, while some regions (visual cortex, amygdala, insular cortex) showed little decline with aging. Changes in capillary density are determined by a balance of pruning and sprouting events. Previous research showed that capillaries are naturally prone to plugging and prolonged obstructions often lead to vessel pruning without subsequent compensatory vessel sprouting. We therefore hypothesized that regional susceptibilities to plugging could help predict vessel loss. By mapping the distribution of microsphere-induced capillary obstructions, we discovered that regions with a higher density of persistent obstructions were more likely to show vessel loss with aging and vice versa. Although the relationship between obstruction density and vessel loss was strong, it was clear obstruction rates were insufficient to explain vessel loss on their own. For that reason, we subsequently used in vivo two-photon microscopy to track microsphere-induced capillary obstructions and vascular network changes over 24 days in two areas of cortex that showed different magnitudes of vessel loss and obstruction densities: visual and retrosplenial cortex. Surprisingly, we did not find evidence for differences in vessel pruning rates between areas, as we would have expected. Instead, we observed brain region-specific differences in recanalization times and rates of angiogenesis. These findings indicate that age-related vessel loss is region specific and that regional susceptibilities to capillary plugging and angiogenesis must be considered to explain these differences. Altogether, this work supports the overarching hypothesis that regional differences in vascular structure and function contribute to a regionally heterogeneous phenotype in the aging brain. / Graduate

<p>This thesis reports on a numerical and experimental investigation of the unsteady loading of high solidity vertical axis wind turbines (VAWTs). Two-dimensional, unsteady Reynolds averaged Navier-Stokes simulations of a small scale, high solidity, H-type Darrieus vertical axis wind turbine revealed the dominant effect of dynamic stall on the power production and vibration excitation of the turbine. Operation of the turbine at low blade speed ratios resulted in complex flow-blade interaction mechanisms. These include; dynamic stall resulting in large scale vortex production, vortex impingement on the source blade, and significant flow momentum extraction.</p> <p>To validate the numerical model, a series of full-scale experimental wind tunnel tests were performed to determine the aerodynamic loading on the turbine airfoils, vibration response behaviour, and wake velocity. In order to accomplish this, a complex force measurement and wireless telemetry system was developed. During the course of this investigation, high vibration response of the turbine was observed. This resulted in conditions that made it difficult or impossible to measure the underlying aerodynamic loading. A vibration mitigation methodology was developed to remove the effect of vibration from the measured aerodynamic forces. In doing so, an accurate and complete measurement of the aerodynamic loading on the turbine blades was obtained.</p> <p>Comparison of the two-dimensional numerical model results to the experimental measurements revealed a considerable over-prediction of the turbine aerodynamic force and power coefficients, and wake velocity. From this research, it was determined that the three-dimensional flow effects due to the finite aspect ratio of the turbine and blades, as well as parasitic losses, could be accounted for through the application of inlet velocity and turbine height correction factors. In doing so, the two-dimensional numerical model results could be properly scaled to represent the three-dimensional flow behaviour of the turbine prototype. Ultimately, a validated VAWT design tool was developed.</p> / Doctor of Philosophy (PhD)

Die in dieser Broschüre vorgestellten Beispiele aus dem Freistaat Sachsen zeigen, dass Holz ein sehr schöner, flexibler, effizienter und zukunftsweisender Baustoff ist. Wir hoffen, dass damit das Interesse von Bauherren, Architekten und Planern am modernen Holzbau geweckt wird und sie dazu inspiriert, Holz bei künftigen Bauvorhaben verstärkt einzusetzen. Redaktionsschluss: 01.04.2014

In this dissertation, various VAT parameters are investigated to determine the effect of the overall efficiency of the turbine at a high Reynolds number. To increase the efficiency of the vertical axis turbines, 2D CFD simulations are completed in an effort to better understand the physics behind the operation of these turbines. Specifically, the effect of advance ratio, solidity, and wake interactions were investigated. Simulations were completed in OpenFOAM using the k-ω SST turbulence model at a nominal Reynolds number of 500,000 using a NACA 0015 airfoil. To simulate the motion of the turbine, Arbitrary Mesh Interfacing (AMI) was used. For all of the parameters tested, it was found that the geometric effective angle of attack seen by the turbine blades had a significant impact on the power extracted from the flow. The range of effective angles of attack was found to decrease as the advance ratio increased. In spite of this, a severe loss in the power coefficient occurred at an advance ratio of 2.5 during which the blade experienced dynamic stall. This effect was also seen when the number of turbine blades was changed to four, at a solidity of 1.08. This negative impact on performance was found to be due to the increase in the drag component of the tangential force when dynamic stall occurs. Results indicate that wake interactions between subsequent blades have a large impact on performance especially when the wake interaction alters the flow direction sufficiently to create conditions for dynamic stall. To improve the performance of the VAT in the presence of dynamic stall, calculations were completed of a static twisted blade profile using GenIDLEST and OpenFOAM. There was found to be no improvement in the lift coefficient when comparing the twisted blade profile with a 2D blade at the same median angle of attack as the twisted blade. To further see the effects of the twisted blade, an effective VAT pitching motion was given to the blade and again compared to a 2D blade with the same motion. In this case there was significant improvement seen in the performance of the twisted blade. / Master of Science

In this work, a proposal for BLDC motor control, which will be used as a drive linear actuator. Control with microcontroller focuses mainly aspects such as motor mode, regenerative mode, measuring rotor position sensors and measuring the motor current, which corresponds to the moment. The result of the work is focused on upgrading the existing Honeywell actuator, where the processor-controlled BLDC motor to replace the existing system and take over the actuator working function and emergency function.

Hybrid stepper motors are a common type of electric motor used throughout industry thanks to its low-cost, high torque at low speed and open loop positioning capabilities. However, a closed loop control is often required for industrial applications with high precision requirements. The closed loop control can also be used to lower the power consumption of the motor and ensure that stalls are avoided. It is quite common to utilise a large and costly position encoder or resolver to feedback the position signal to the control logic. This thesis has explored the possibility of using a low-cost position sensor based on Hall elements. Additionally, a sensorless estimation algorithm, using only stator winding measurements, has been investigated both as a competitive alternative and as a possible complement to the position sensor. The thesis work summarises and discusses previous research attempts to adequately measure or estimate and control the hybrid stepper motors position and load angle without using a typical encoder or resolver. Qualitative results have been produced through simulations prior to implementation and experimental testing. The readings from the position sensor is subject to noise, owing to its resolution and construction. The position signal has been successfully filtered, improving its accuracy from 0.56° to 0.25°. The output from the sensorless estimation algorithm is subject to non-linear errors caused by errors in phase voltage measurements and processing of velocity changes. However, the dynamics are reliable at constant speeds and could be used for position control.

L’étude effectuée au cours de cette thèse a permis de caractériser numériquement les instabilités d’origine aérodynamique rencontrées dans un compresseur centrifuge dessiné par Turbomeca. Ce compresseur est composé d’une roue directrice d’entrée, d’un rouet centrifuge, d’un diffuseur radial et de redresseurs axiaux. Le module expérimental, dénommé Turbocel, sera accueilli au LMFA courant 2016. Le contenu de cette étude repose donc exclusivement sur des résultats numériques dont certains sont cependant comparés à des résultats expérimentaux partiels obtenus par Turbomeca sur une configuration proche. _ Le fonctionnement du compresseur est analysé à différentes vitesses de rotation, à partir de simulations RANS et URANS menées avec le code elsA. Du point de vue de la méthodologie, deux points importants sont à retenir :- Du fait du caractère transsonique de l’écoulement dans le rouet et le diffuseur radial à haut régime de rotation, les simulations RANS stationnaires ne permettent pas d’accéder à une description satisfaisante des phénomènes physiques. Cela est dû à l’utilisation d’un plan de mélange aux différentes interfaces rotor-stator qui a pour effet d’empêcher les ondes de choc de remonter à l’amont, et qui affecte tant la physique de l’écoulement que l’étendue de la plage de fonctionnement stable.- En-dessous d’un certain débit, les calculs URANS sur période machine révèlent que le comportement de l’étage n’obéit plus à la périodicité spatio-temporelle mono-canal. Une plage instable est alors obtenue à toutes les iso-vitesses simulées. A bas régime de rotation, une autre plage stable existe lorsque le compresseur est suffisamment vanné. L’étage retrouve alors une périodicité spatio-temporelle, à condition d’étendre le domaine de calcul dans le stator à deux canaux inter-aubes. En ce qui concerne les limites de stabilité de Turbocel, différentes évolutions sont décrites selon la vitesse de rotation considérée :- A haut régime de rotation, une basse fréquence commence à émerger près du point de rendement maximal et son intensité ne fait qu’augmenter jusqu.au pompage.- A bas régime, une signature basse fréquence comparable se manifeste près du point de rendement maximal mais disparaît passé un certain vannage, et n’est donc présente que sur une plage de débit délimitée. La seconde zone stable peut alors être numériquement parcourue jusqu.au pompage proprement dit. La signature basse fréquence est imputée à l’instauration d’une recirculation dans l’inducteur qui une fois établie est quasi-stationnaire. Les résultats numériques mettent en évidence que la source d’instabilité sévère sur Turbocel provient du diffuseur aubé. En fonction du point de fonctionnement, ce composant adopte des comportements différents, entre lesquels une certaine continuité existe, et ses performances chutent progressivement lorsque le débit diminue. Au final, les domaines de stabilité de l’étage de compression peuvent être reliés au type d’écoulement qui se développe dans le diffuseur radial, et apparaissent dictés par le diffuseur semi-lisse à haut régime de rotation. Enfin, afin d’étendre les plages de fonctionnement stable, une stratégie de contrôle basée sur l’aspiration de couche limite dans le diffuseur aubé a également été déterminée dans le cadre de cette thèse. Son évaluation fera l’objet d’études ultérieures sur Turbocel. / The present study aims at characterizing the aerodynamic instabilities involved in a centrifugal compressor designed by Turbomeca, by means of numerical simulation. This compressor is composed of inlet guide vanes, a centrifugal impeller, a radial vaned diffuser and axial outlet guide vanes. The test module, named Turbocel, will be delivered to the LMFA in 2016. Thus, the results presented in this manuscript are only based on CFD, although some of them are compared to experimental results obtained by Turbomeca on a close configuration.RANS and URANS simulations are performed for several rotational speeds, using the elsA software.Two methodological key points are to be emphasized:- As the flow in both the impeller and the radial diffuser is transonic at high rotational speed, steady RANS simulations cannot provide a satisfactory description of the physical phenomena taking place. This can be explained by the use of the mixing plane approach which prevents shock waves to extend upstream the rotor-stator interfaces, and which impacts the flow field predicted as well as the prediction of the stable operating range.- Below a given massflow rate, URANS simulations covering the spatial period of the compressor prove that the stage behavior does not obey to the single passage spatio-temporal periodicity anymore. An unstable operating range then appears at all the simulated rotational speeds. At low rotational speed, another stable range is however obtained if the compressor is further throttled’ A new periodicity arises on this massflow range, provided that the stator domain is extended to two neighboring blade passages. Concerning the stability domains of Turbocel, different evolutions are obtained depending on the rotational speed:- At high rotational speed, a low frequency phenomenon starts to develop near the peak efficiency point and its intensity keeps increasing until surge happens.- At low rotational speed, a low frequency signature also appears near the peak efficiency point, but it then vanishes when the compressor is further throttled, so that only a restricted operating range exhibits this instability. It then gives rise to a second stable operating range which can be described numerically, ending with surge itself. The low frequency signature is attributed to the enhancement of a flow recirculation in the inducer which, once fully established, is quasi-steady. The numerical results underline that the source of severe instability in the compressor comes from the vaned diffuser. Depending on the operating point, this component can adopt different behaviors, between which a relative continuity exists, and its performances decrease when the massflow rate decresases. The overall stage performances prove that at high rotational speed, the global stability is driven by the semi-vaneless diffuser and depends on the flow developing in the radial diffuser. Finally, in order to extend the stable operating range of the compressor, a flow control strategy based on boundary layer suction has also been determined in the diffuser. Its impact on the performances of Turbocel will be deeply studied later on.

Les études expérimentales et numériques suivantes visent à la compréhension profonde de l’écoulement se développant dans le compresseur haute-vitesse axial de 3.5 étages CREATE, étudié sur un banc d’essai de 2 MW au Laboratoire de Mécanique des Fluides et Acoustique (LMFA) à Lyon, France. Ce travail a trois objectifs principaux : D’abord, une description globale de l’écoulement avec une identification des limites aux méthodes d’exploration utilisées ; Ensuite, la caractérisation de l’effet du clocking stator-stator dans un compresseur à haute-vitesse ; Troisièmement, l’identification des instabilités à faibles débits pour confirmer les études sur les compresseurs à basse-vitesse et contribuer à plus de compréhension.Il est montré qu’une mauvaise interprétation des données de performance stationnaire se fait facilement en raison des contraintes de mesure et des coefficients de correction sont proposés. À certains endroits dans le compresseur, des limites aux méthodes d’exploration (expérimentales et numériques) de l’écoulement sont identifiées. Cette identification va permettre la poursuite du développement des méthodes. Les principales erreurs de prédiction des simulations concernent la surestimation du blocage induit par l’écoulement de jeu et l’augmentation de pression. En outre, les mesures fournies par les sondes de pression pneumatique surestiment la pression statique en amont des stators. Cette erreur est probablement provoquée par l’interaction entre le champ potentiel du stator et la sonde elle-même. De plus, l’anémométrie Doppler laser surestime la vitesse en aval des stators. Le transport des sillages du rotor à travers des stators n’est pas correctement capturé avec les particules d’ensemencement.Le clocking a seulement un petit effet global dans la bande d’incertitude de mesure dans ce compresseur. Plusieurs contributions à ce faible effet de clocking sont identifiées par l’analyse du transport des structures d’écoulement : Le mélange circonférentiel du sillage de stator et la déformation des sillages le long de leur trajet dans l’écoulement. L’effet local du clocking dépend de la hauteur de veine en raison de la variation de la forme des aubages et du transport des sillages. Des effets positifs et négatifs sont présentés, qui globalement se compensent dans ce compresseur. Les instabilités dans ce compresseur dépendent du point de fonctionnement et des méthodes d’exploration de l’écoulement. Aux points de fonctionnement stables et à la vitesse nominale du compresseur, les résultats numériques montrent une perturbation tournante dans les rotors 2 et 3, alors que les mesures montrent une perturbation tournante que dans le premier rotor et seulement à basse vitesse du compresseur. Dans les deux cas, les perturbations montrent des caractéristiques semblables. Une étude numérique permet d’exclure l’influence des interactions rotor-stator sur la perturbation tournante et met en évidence sa source. Des nouvelles connaissances sur le comportement stable et la périodicité du rotating instability (mesuré) sont dérivées contrairement au comportement instable suggéré par la dénomination et la littérature. Il est montré que cette perturbation évolue en cellule de décrochage tournante à l’approche de la limite de stabilité. A la vitesse nominale du compresseur, une entrée en instabilités de type spike est identifiée expérimentalement. Une description précise de l’apparition brutale du spike et sa différence par rapport à une cellule de décollement tournant sont présentées. / The following experimental and numerical investigations aim at the deep understanding of the flow field in the 3.5 stages high-speed axial compressor CREATE, studied on a 2 MW test rig at the Laboratory of Fluid Mechanics and Acoustics (LMFA) in Lyon, France. This work focuses on three major objectives: Firstly, a global description of the flow field with an identification of limitations to the used exploration methods; Secondly, the characterization of the effect of stator-stator clocking in a high-speed compressor; Thirdly, the identification of instabilities arising at low mass flow rates for confirming studies on low-speed compressors and giving new insights.This work demonstrates that a mis-interpretation of steady performance data occurs easily due to measurement constraints and correction coefficients are proposed. At certain locations in the compressor, the flow field exploration (experimental and numerical) methods are identified to be challenged. This identification will initiate further development of the methods. The main mis-predictions of the simulations concern the over-prediction of the blockage induced by the tip leakage flow and eventually an over-predicted pressure rise. Furthermore, the measurements provided by the pneumatic pressure probes over-estimate the static pressure upstream of the stators. This error is induced by the interaction between the stator potential field and the probe it-self. In addition, the laser Doppler anemometry method over-estimates the velocity downstream the stators. The transport of the rotor wakes through the stators might not be correctly captured with the seeding particles in this high-speed compressor.The investigation of the stator clocking reveals only a small global effect within the measurement uncertainty band. Several contributions to the weak effect of clocking are identified by analysis of the flow structure transport, namely the time-mean mixing out of the stator wakes and the deformation of wakes along their flow path. The local effect of clocking depends on the span-height because of the variation of the circumferential position of the stator wakes and the stator blade shape over the span-height. Local possible positive and negative effects of clocking are identified and are shown to be almost in balance in this compressor. Furthermore, this work demonstrates that the unsteadiness in the flow field is not linked conclusively to the stator clocking.In this compressor, the arising instabilities depend on the operating point and flow field exploration methods. At stable operating points and nominal compressor speed, the numerical results reveal a rotating disturbance in the rotors 2 and 3, whereas the measurements show a rotating disturbance only in the first rotor and only at part speed. In both cases the disturbance exhibits rotating instability like characteristics. An exhaustive numerical study allows to exclude the commonly assumed influence of rotor-stator interactions on the rotating disturbance and pinpoints its source. New insights into the stable behavior and periodicity of the measured rotating instability are derived contrary to the unstable behavior suggested by the naming and literature. This disturbance is shown to evolve into rotating stall cells when approaching the stability limit. At nominal compressor speed, a spike type surge inception is identified I n the measured field. A precise description of the abrupt onset of the spike cell and its difference to a rotating stall cell are presented.