Dissertations / Theses on the topic 'Large scale oscillations'
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Xu, Xiaoying. "Robust Measurement of the Cosmic Distance Scale Using Baryon Acoustic Oscillations." Diss., The University of Arizona, 2012. http://hdl.handle.net/10150/241935.
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We present techniques for obtaining precision distance measurements using the baryon acoustic oscillations (BAO) through controlling systematics and reducing statistical uncertainties. Using the resulting distance-redshift relation, we can infer cosmological parameters such as w, the equation of state of dark energy. We introduce a new statistic, ɷ(l)(r(s)), for BAO analysis that affords better control over systematics. It is computed by band-filtering the power spectrum P(k) or the correlation function ξ(r) to extract the BAO signal. This is conducive to several favourable outcomes. We compute ɷ(l)(r(s)) from 44 simulations and compare the results to P(k) and ξ(r). We find that the acoustic scales and theoretical errors we measure are consistent between all three statistics. We demonstrate the first application of reconstruction to a galaxy redshift survey. Reconstruction is designed to partially undo the effects of non-linear structure growth on the BAO, allowing more precise measurements of the acoustic scale. We also present a new method for deriving a smooth covariance matrix based on a Gaussian model. In addition, we develop and perform detailed robustness tests on the ξ(r) model we employ to extract the BAO scale from the data. Using these methods, we obtain spherically-averaged distances to z = 0.35 and z = 0.57 from SDSS DR7 and DR9 with 1.9% and 1.7% precision respectively. Combined with WMAP7 CMB observations, SNLS3 data and BAO measurements from 6dF, we measure w = -1.08 ± 0.08 assuming a wCDM cosmology. This represents a ~8% measurement of w and is consistent with a cosmological constant.The preceding does not capture the expansion history of the universe, H(z), encoded in the line-of-sight distance scale. To disentangle H(z), we exploit the anisotropic BAO signal that arises if we assume the wrong cosmology when calculating the clustering distribution. Since we expect the BAO signal to be isotropic, we can use the magnitude of the anisotropy to separately measure H(z) and D(A)(z). We apply our simple models to SDSS DR7 data and obtain a ~3.6% measurement of D(A)(z=0.35) and a ~8.4% measurement of H(z = 0.35).
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Seo, Hee-Jong. "High-Precision Large-Scale Structure: The Baryon Acoustic Oscillations and Passive Flow." Diss., The University of Arizona, 2007. http://hdl.handle.net/10150/194697.
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We present a precision study of large-scale structure from large galaxy redshift surveys. We focus on two main subjects of large-scale structure: precisioncosmology with baryon acoustic oscillations from large galaxy surveys and the evolution of galaxy clustering for passively flowing galaxies.The baryon acoustic oscillations in galaxy redshift surveys can serve as an efficient standard ruler to measure the cosmological distance scale, i.e., theangular diameter distances and Hubble parameters, as a function of redshift, and therefore dark energy parameters. We use a Fisher matrix formalism to show that such a standard ruler tests can constrain the angular diameter distances and Hubble parameters to a precision of a few percent, thereby providing robust measurements of present-day dark energy density and its time-dependence.We use N-body simulations to investigate possible systematic errors in the recovery of the cosmological distance scale from galaxy redshift surveys. We show that the baryon signature on linear and quasi-linear scales is robust against nonlinear growth, redshift distortions, and halo (or galaxy) bias, albeit partial obscuration of the signature occurs due to nonlinear growth and redshift distortions.We present the improved Fisher matrix formalism which incorporates the Lagrangian displacement field to describe the nonlinear effects on baryon signature as a function of time and scale. We present a physically motivated, reduced 2-dimensional fitting formula for the full Fisher matrix formalism. We show that distance precision from the revised formalism is in excellent agreement with distance precision from N-body simulations.Finally, we present a numerical study of the evolution of galaxy clustering when galaxies flow passively from high redshift to low redshift, that is, without merging or new formations. We show that passive flow evolution induces interesting characteristics in the galaxy distribution at low redshift: we find an asymptotic convergence in galaxy clustering and halo occupation distribution regardless of the initial distribution of galaxies.
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Shi, Guangyu. "Nonlinear static and dynamic analyses of large-scale lattice-type structures and nonlinear active control by piezo actuators." Diss., Georgia Institute of Technology, 1988. http://hdl.handle.net/1853/19176.
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Comparat, Johan. "Baryonic acoustic oscillations with emission line galaxies at intermediate redshift : the large-scale structure of the universe." Thesis, Aix-Marseille, 2013. http://www.theses.fr/2013AIXM4720/document.
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J'ai démontrer la faisabilité de la sélection de la cible pour les galaxies en ligne des émissions lumineuses. Je comprends maintenant les principaux mécanismes physiques de conduite de l'efficacité d'une sélection, en particulier le rapport à la photométrie de parent. Une question reste perplexe, je ne pouvais pas encore estimer quantitativement l'impact de la poussière sur l'efficacité de la sélection. J'espère que d'aborder cette question avec l'ensemble des données décrites dans le chapitre 4.En dehors de la ligne de sélection de la cible de la galaxie d'émission, j'ai étudié, au premier ordre, les deux principales erreurs systématiques sur la détermination de l'échelle BAO nous attendent en raison de l'utilisation galaxies en ligne des émissions comme traceurs de la question. J'ai d'abord montré le caractère incomplet de la distribution redshift, en raison de la mesure du décalage spectral avec [Oii], est lié à la résolution instrumentale. Je trouve qu'il ya deux régimes intéressants. Pour une observation des plus brillants [OII] émetteurs, une résolution modérée est suffisante, alors que pour une enquête plus faible, la plus haute de la résolution le meilleur. Deuxièmement, j'ai estimé le biais de la galaxie linéaire des sélections discuté avant et je trouve qu'ils sont très biaisés. D'une part, ce sont d'excellentes nouvelles pour les observateurs, comme le temps nécessaire pour observer à un signal donné au bruit dans le spectre de puissance diminue avec le carré de la partialité. D'autre part, elle constitue un nouveau défi pour les algorithmes de reconstruction et la fabrication de catalogues simulacresIn this PhD, I demonstrate the feasibility of the target selection for bright emission line galaxies. Also I now understand the main physical mechanisms driving the efficiency of a selection, in particular the relation to the parent photometry. A puzzling issue remains, I could not yet estimate quantitatively the impact of the dust on the selection efficiency. I hope to address this question with the data set described in chapter 4.Apart from the emission line galaxy target selection, I investigated, at first order, the two main systematic errors on the determination of the BAO scale we expect due to using emission line galaxies as tracers of the matter. First I showed the incompleteness in the redshift distribution, due to the measurement of the redshift with [Oii], is related to the instrumental resolution. I find there are two interesting regimes. For an observation of the brightest [Oii]emitters, a moderate resolution is sufficient, whereas for a fainter survey, the highest the resolution the best. Secondly, I estimated the linear galaxy bias of the selections discussed before and I find they are highly biased. On one hand, this is great news for the observers, as the time required to observed at a given signal to noise in the power spectrum decreases with the square of the bias. On the other hand, it constitutes a new challenge for reconstruction algorithms and the making of mock catalogs. The work in progress described in the last chapter shows I am starting to try and handle these questions in a robust manner
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Molina, Diogenes. "Intelligent control and system aggregation techniques for improving rotor-angle stability of large-scale power systems." Diss., Georgia Institute of Technology, 2013. http://hdl.handle.net/1853/50291.
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A variety of factors such as increasing electrical energy demand, slow expansion of transmission infrastructures, and electric energy market deregulation, are forcing utilities and system operators to operate power systems closer to their design limits. Operating under stressed regimes can have a detrimental effect on the rotor-angle stability of the system. This stability reduction is often reflected by the emergence or worsening of poorly damped low-frequency electromechanical oscillations. Without appropriate measures these can lead to costly blackouts. To guarantee system security, operators are sometimes forced to limit power transfers that are economically beneficial but that can result in poorly damped oscillations. Controllers that damp these oscillations can improve system reliability by preventing blackouts and provide long term economic gains by enabling more extensive utilization of the transmission infrastructure.
Previous research in the use of artificial neural network-based intelligent controllers for power system damping control has shown promise when tested in small power system models. However, these controllers do not scale-up well enough to be deployed in realistically-sized power systems. The work in this dissertation focuses on improving the scalability of intelligent power system stabilizing controls so that they can significantly improve the rotor-angle stability of large-scale power systems.
A framework for designing effective and robust intelligent controllers capable of scaling-up to large scale power systems is proposed. Extensive simulation results on a large-scale power system simulation model demonstrate the rotor-angle stability improvements attained by controllers designed using this framework.
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Bautista, Julian Ernesto. "Baryon acoustic oscillations in the large scale structures of the universe as seen by the Sloan Digital Sky Survey." Paris 7, 2014. http://www.theses.fr/2014PA077132.
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Ce manuscrit décrit la mesure des oscillations acoustiques des baryons (BAO) en utilisant des forêts Lyman-alpha et des galaxies, et son interprétation cosmologique. Entre 2011 et 2014, le télescope du Sloan Digital Sky Survey III a observé le spectre de plus de 1,3 millions de galaxies et plus de 150 000 quasars lointains dans le cadre du projet Baryon Oscillation Spectroscopic Survey (BOSS), en visant la mesure la plus précise des BAO dans la distribution de matière à grande échelle. Nous décrivons l'analyse des donnés, l'interprétation cosmologique et des tests avec des données simulées. Nous présentons une nouvelle méthode pour générer des données simulées, utilisées pour étudier des effets systématiques qui pourraient affecter la mesure. Aucun de ces effets ne change significativement nos résultats. Avec les données disponibles en 2014, le pic BAO est mesuré avec une précision de 3% dans la direction radiale et 5% dans la direction transverse à = 2,34. Nous comparons nos résultats avec des mesures précédentes, en particulier celles du satellite Planck, et nous trouvons un accord au niveau de 1,8ơThis thesis describes the measurement of the baryon acoustic oscillations using Lyman-alpha forests and galaxies and its cosmological interprétation. From 2011 to 2014, the Sloan Digital Sky Survey III telescope observed spectra of — 1. 3 million galaxies and ∽ 150 000 quasars in the context of the Baryon Oscillation Spectroscopic Survey (BOSS). This project aimed the most accurate BAO measurement in the large scale distribution of matter. We describe the data analysis, the cosmological interprétation and tests using mock data. We present a new method to generate mock data, that was used to study systematic effects potentially affecting our measurement. No evidence for a systematic bias comming from these tests could be found. Using data available in 2014, the BAO peak is measured at 3% precision in the radial direction and 5% in the transverse direction, at z = 2. 34. We compare our results with previous measurements, in particular those from the Planck satellite, and we find an agreement at 1. 8ơ level
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Ding, Zhejie. "Systematics Study and Detection of Baryon Acoustic Oscillations from Future Galaxy Survey and Weak Lensing Survey." Ohio University / OhioLINK, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1554330484538948.
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Arico, Giovanni. "Testing the methods to reconstruct and model the Baryonic Acoustic Oscillations of different tracers using N-body simulations." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2017. http://amslaurea.unibo.it/13167/.
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The accelerated expansion of the Universe and the nature of the Dark Energy are still open questions in cosmology. One of the most powerful ways to investigate these issues is to map the large-scale structure of the Universe, to constrain its expansion history and growth of structures.
In particular, baryon acoustic oscillations (BAO) occurred at recombination make a peak in the correlation function of galaxies at the characteristic scale of the sound horizon (a sufficiently large scale to “protect” the signal from strong non-linearities), or alternatively a series of oscillations in the power spectrum. Since the sound horizon can be estimated with a great precision from the position of the first peak in the angular power spectrum of the Cosmic Microwave Background (which has the same physical origin of BAO, oscillations of the baryons-photons plasma), the BAO peak in the correlation function can be used as a standard ruler, providing paramount cosmological information.
The aim of this thesis is to systematically test and possibly improve the state-of- the-art statistical methods to model the BAO peak, taking into account the non-linear evolution of matter overdensities, redshift-space distortions and the bias of cosmic tracers. To do that, we analyse mock samples of galaxies, quasars and galaxy clusters extracted from one of the largest available cosmological hydrodynamical simulations. We extract cosmological constraints from the BAO peak through different statistical tools in the redshift range 0.2 < z < 2.
Although the BAO peak is at large scales, non-linear growth and galaxy peculiar velocities make the BAO signal smoothed and broader with respect to linear predictions, especially at low redshifts. A possible method to overcome these issues is the so-called reconstruction of the density field: one of the primary goals of this work is to implement a reconstruction method, to check its performances as a function of sample selections and redshift.
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Disotell, Kevin James. "Low-Frequency Flow Oscillations on Stalled Wings Exhibiting Cellular Separation Topology." The Ohio State University, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=osu1449162356.
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Pockberger, Moritz [Verfasser]. "Abiotic and biotic impacts on fish in the Wadden Sea - evaluating the effect of large scale climate oscillations, local ecosystem characteristics and invasive species / Moritz Pockberger." Kiel : Universitätsbibliothek Kiel, 2016. http://d-nb.info/1094661953/34.
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Labatie, Antoine. "Etude statistique des corrélations dans la distribution des galaxies - application aux oscillations baryoniques acoustiques." Phd thesis, Université Paris Sud - Paris XI, 2012. http://tel.archives-ouvertes.fr/tel-00785091.
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Les Oscillations Baryoniques Acoustiques (BAOs) correspondent au phénomène d'ondes acoustiques dans le plasma baryon-photon avant la recombinaison. Ces oscillations impriment une échelle standard, correspondant à l'horizon sonique, qui peut être observée dans les grandes structures de l'Univers. De par cette propriété d'étalon standard, les BAOs permettent d'étudier la relation distance-redshift dans les catalogues de galaxies et fournissent un outil très prometteur pour étudier les propriétés de l'énergie sombre. Les BAOs peuvent être étudiés à partir des statistiques du second ordre (la fonction de corrélation ou le spectre de puissance) dans la distribution des galaxies. Dans cette thèse, on se restreint au cas de la fonction de corrélation. Les BAOs apparaissent dans la fonction de corrélation comme un faible pic à l'échelle de l'horizon sonique. Il y a deux applications principales des BAOs: la détection des BAOs et la contraintes des paramètres cosmologiques grâce à la propriété d'étalon standard. La détection du pic BAO à une échelle proche de l'échelle attendue permet de confirmer le modèle cosmologique actuel. Quant à la contrainte des paramètres cosmologiques, permettant d'étudier l'énergie sombre, il s'agit d'un objectif majeur de la cosmologie moderne. Dans cette thèse, on s'intéresse à différents problèmes statistiques concernant l'étude de la fonction de corrélation dans la distribution des galaxies, avec un accent particulier sur l'étude des BAOs. Dans la première partie, on fait une étude à la fois théorique et pratique du biais dû à la contrainte intégrale dans les estimateurs de la fonction de corrélation. On montre que ce biais est très faible pour les relevés de galaxies actuels. Dans la seconde partie, on s'intéresse à la détection des BAOs. On montre les limitations de la méthode de détection classique, et on propose une nouvelle méthode plus rigoureuse. Notre méthode permet notamment de prendre en compte la dépendance de la matrice de la covariance de l'estimateur en fonction du modèle. Enfin dans la troisième partie, on étudie à nouveau la question la dépendance de la matrice de la covariance, cette fois pour la contrainte des paramètres cosmologiques. On estime une matrice de covariance dépendant du modèle et l'on compare nos contraintes avec les contraintes obtenues en faisant l'approximation habituelle d'une matrice de covariance constante. On montre que l'effet de cette approximation est relativement faible et diminue lorsque les relevés de galaxies grandissent.
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Du, Mas des Bourboux Hélion. "Mesure de l'échelle des oscillations acoustiques de baryons dans la fonction de corrélation des forêts Lyman-α avec la distribution des quasars observés dans le relevé SDSS." Thesis, Université Paris-Saclay (ComUE), 2017. http://www.theses.fr/2017SACLS157/document.
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La propagation des ondes acoustiques dans le plasma primordial a laissé son empreinte sous la forme d'un pic dans la fonction de corrélation à deux points de la densité de matière. Ce pic d'oscillations acoustiques de baryons (BAO) constitue une échelle standard permettant de déterminer certains paramètres des différents modèles cosmologiques.Dans ce manuscrit de thèse, nous présentons une mise à jour de la mesure de BAO à un redshift z=2.40, à l'aide de la fonction de corrélation croisée entre deux traceurs des fluctuations primordiales de densité de matière: les quasars de SDSS-III (BOSS) et leurs fluctuations d'absorption du flux des forêts Lyman-α. Ces fluctuations tracent la distribution d'hydrogène neutre dans le milieu intergalactique (IGM).Cette étude constitue le premier développement d'un ajustement entièrement physique de la fonction de corrélation croisée; il prend notamment en compte la physique des quasars et la présence d'éléments plus lourds que l'hydrogène dans l'IGM. Nous y présentons également les premières simulations de notre analyse. Celles-ci nous permettent de valider l'ensemble de la procédure de mesure de l'échelle BAO.Cette étude mesure la distance de Hubble et la distance de diamètre angulaire avec respectivement une précision de 2% et 3% (intervalle à 1 σ). Nous combinons nos résultats avec d'autres mesures de BAO à des redshifts plus faibles et trouvons la densité de matière noire et d'énergie noire dans le cadre de deux différents modèles cosmologiques: ΛCDM et oΛCDMThe acoustic wave propagation in the primordial plasma left its imprint in the two-point correlation function of the matter density field. This baryonic acoustic oscillation (BAO) peak builds up a standard ladder allowing us to infer some parameters of the different cosmological models.In this thesis manuscript we present an update of the BAO measurement at a redshift z=2.40, from the cross-correlation function between two tracers of the primordial matter density fluctuations: quasars of SDSS-III (BOSS) and their Lyman-α-forest absorption fluctuations. These fluctuations trace the neutral hydrogen distribution in the intergalactic medium (IGM).This study gives the first developpment of the full physical fit of the cross-correlation. Among other effects, it takes into account quasar physics and the distribution of IGM elements heavier than hydrogen. We also present the first simulations of our analysis. They allow us to validate the overall data analysis leading to the BAO measurement.This study measures the Hubble distance and the angular diameter distance at the 2%$ and 3%$ precision level respectivelly (1 σ interval). We combine our results with other BAO measurements at lower redshifts and find the dark matter density and dark energy density in the framework of two different cosmological models: ΛCDM et oΛCDM
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Wallisch, Benjamin. "Cosmological probes of light relics." Thesis, University of Cambridge, 2018. https://www.repository.cam.ac.uk/handle/1810/283003.
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One of the primary targets of current and especially future cosmological observations are light thermal relics of the hot big bang. Within the Standard Model of particle physics, an important thermal relic are cosmic neutrinos, while many interesting extensions of the Standard Model predict new light particles which are even more weakly coupled to ordinary matter and therefore hard to detect in terrestrial experiments. On the other hand, these elusive particles may be produced efficiently in the early universe and their gravitational influence could be detectable in cosmological observables. In this thesis, we describe how measurements of the cosmic microwave background (CMB) and the large-scale structure (LSS) of the universe can shed new light on the properties of neutrinos and on the possible existence of other light relics. These cosmological observations are remarkably sensitive to the amount of radiation in the early universe, partly because free-streaming species such as neutrinos imprint a small phase shift in the baryon acoustic oscillations (BAO) which we study in detail in the CMB and LSS power spectra. Building on this analytic understanding, we provide further evidence for the cosmic neutrino background by independently confirming its free-streaming nature in different, currently available datasets. In particular, we propose and establish a new analysis of the BAO spectrum beyond its use as a standard ruler, resulting in the first measurement of this imprint of neutrinos in the clustering of galaxies. Future cosmological surveys, such as the next generation of CMB experiments (CMB-S4), have the potential to measure the energy density of relativistic species at the sub-percent level and will therefore be capable of probing physics beyond the Standard Model. We demonstrate how this improvement in sensitivity can indeed be achieved and present an observational target which would allow the detection of any extra light particle that has ever been in thermal equilibrium. Interestingly, even the absence of a detection would result in new insights by providing constraints on the couplings to the Standard Model. As an example, we show that existing bounds on additional scalar particles, such as axions, may be surpassed by orders of magnitude.
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Burton, Kenneth R. Jr. "Influence of Antarctic oscillation on intraseasonal variability of large-scale circulations over the Western North Pacific." Thesis, Monterey California. Naval Postgraduate School, 2005. http://hdl.handle.net/10945/2310.
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Approved for public release, distribution is unlimitedThis study examines Southern Hemisphere mid-latitude wave variations connected to the Antarctic Oscillation (AAO) to establish connections with the 15- to 25-day wave activity in the western North Pacific monsoon trough region. The AAO index defined from the leading empirical orthogonal functions of 700 hPa height anomalies led to seven distinct circulation patterns that vary in conjunction with the 15- to 25-day monsoon trough mode. For nearly one half of the significant events the onset of 15- to 25-day monsoon trough convective activity coincided with a peak negative AAO index and the peak in monsoon trough convection coincided with a peak positive index. The remaining events either occur when the AAO is not significantly varying or when the AAO-related Southern Hemisphere mid-latitude circulations do not match 15- to 25-day transitions. When a significant connection occurs between the Southern Hemisphere mid-latitude circulations related to the AAO and the 15- to 25-day wave activity in the western North Pacific monsoon trough, the mechanism is via equatorward Rossby-wave dispersion. When wave energy flux in the Southern Hemisphere is directed zonally, no connection is established between the AAO and the alternating periods of enhanced and reduced convection in the western North Pacific monsoon trough.
Captain, United States Air Force
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Burton, Kenneth R. "Influence of Antarctic oscillation on intraseasonal variability of large-scale circulations over the Western North Pacific /." Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 2005. http://library.nps.navy.mil/uhtbin/hyperion/05Mar%5FBurton.pdf.
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Magazzini, Lorenzo. "Quality control of visual gamma oscillation frequency in studies of pharmacology, cognitive neuroscience and large-scale multi-site collaborations." Thesis, Cardiff University, 2016. http://orca.cf.ac.uk/98613/.
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In visual cortex, high-contrast grating stimuli induce neurons to oscillate synchronously with a centre frequency in the gamma range (~30–80 Hz). The peak frequency of visual gamma oscillations is modulated by numerous factors, including stimulus properties, cortical architecture and genetics, however, it can be measured reliably over time. As demonstrated by both animal models and human pharmacological studies, the gamma peak frequency is determined by the excitation/inhibition balance and the time constants of GABAergic processes. This oscillatory parameter could thus reflect inter-individual differences in cortical function/physiology, representing a possible biomarker for pharmacological treatment in conditions such as epilepsy, autism and schizophrenia. This thesis demonstrates the importance of measuring the gamma peak frequency accurately and reliably in magnetoencephalographic (MEG) recordings. In Chapter 2, a novel quality-control (QC) approach was validated for peak frequency estimation and identification of poor-quality data. In Chapter 3, QC of a previous pharmacological MEG study of visual gamma with tiagabine revealed a marked drug-induced reduction of peak frequency. Although contrasting with the null finding originally reported (Muthukumaraswamy et al., 2013), the result is supported by both animal models and recent human studies, demonstrating the potentialities of appropriate QC routines. In Chapter 4, testing for the effect of spatial attention on the gamma peak frequency in primary visual cortex resulted in no evidence of a change. However, the modulation of gamma amplitude by attention was consistent with a role in feed-forward signal propagation across the visual hierarchy. In Chapter 5, the QC approach was used to compare visual gamma data recorded at different sites of the UK MEG Partnership, demonstrating the feasibility of combining data from different MEG systems. These results have implications particularly for pharmacological and large-scale multi-site studies, both of which are emerging as promising approaches for the study of brain function with MEG.
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Herzberg, Wiebke [Verfasser], and Markus [Akademischer Betreuer] Roth. "Effects of large-scale poloidal flows on oscillation modes of subgiant stars and the Sun / Wiebke Herzberg ; Betreuer: Markus Roth." Freiburg : Albert-Ludwigs-Universität Freiburg, 2016. http://d-nb.info/1126920746/34.
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Kuznetsova, Daria. "Modélisation de l'oscillation Madden-Julian lors de son passage sur l'océan Indien et le continent maritime." Thesis, Toulouse 3, 2018. http://www.theses.fr/2018TOU30220/document.
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L'oscillation de Madden-Julian (MJO) est la composante dominante de la variabilité intrasaisonnière dans l'atmosphère tropicale, se propageant vers l'est dans la bande équatoriale. Elle se compose d'un centre convectif (phase active) accompagné de la convergence des anomalies du vent zonal de bas niveau et de la divergence de niveau supérieur, et de zones de convection faible (phases supprimées). Trois périodes de l'activité MJO sur l'océan Indien et le continent maritime ont été choisies : 6-14 avril 2009, 23-30 novembre 2011 et 9-28 février 2013. Les simulations avec et sans paramétrisation de la convection ont été réalisées pour un grand domaine avec le modèle atmosphérique Méso-NH. Il a été obtenu que les simulations avec convection paramétrée n'étaient pas capables de reproduire un signal MJO. Pour 2009 et 2011, lorsque le couplage entre la convection et la circulation de grande échelle était fort, les simulations avec convection explicite ont montré une propagation visible de la MJO, ce qui n'a pas été le cas pour 2013. Pour 2011, les processus contribuant à la suppression de la convection ont été étudiés avec une analyse isentropique pour séparer les masses d'air ascendantes ayant une température potentielle équivalente élevée des masses d'air subsidentes ayant une température potentielle équivalente faible. Trois circulations de grande échelle ont été trouvées : une circulation troposphérique, une circulation de percées nuageuses dans la couche de tropopause tropicale, et une circulation de masses d'air à faible température potentielle équivalente dans la basse troposphère. Cette dernière correspond aux intrusions d'air sec de grande échelle des zones subtropicales dans la bande équatoriale, trouvées principalement pendant la phase supprimée de la MJO sur l'océan IndienThe Madden-Julian Oscillation (MJO) is the dominant component of the intraseasonal variability in the tropical atmosphere, propagating eastward in the equatorial band. It consists of a convective center (active phase) accompanied by the low-level zonal wind anomaly convergence and the upper-level zonal wind anomaly divergence, and zones of weak convection (suppressed phases). Three time periods of the MJO activity over the Indian Ocean and the Maritime Continent were chosen: 6-14 April 2009, 23-30 November 2011, and 9-28 February 2013. The simulations with and without convective parameterizations were performed for a large domain with the atmospheric model Méso-NH. It was obtained that the simulations with parameterized convection were not able to reproduce an MJO signal. For 2009 and 2011 when the coupling between convection and large-scale circulation was strong, the convection-permitting simulations showed a visible MJO propagation, which was not the case for 2013. For the 2011 episode, the processes contributing to the suppression of the convection were studied using an isentropic analysis to separate the ascending air masses with high equivalent potential temperature from the subsiding air masses with low equivalent potential temperature. Three large-scale circulations were found: a tropospheric circulation, an overshoot circulation within the tropical tropopause layer, and a circulation of air masses with low equivalent potential temperature in the lower troposphere. The latter corresponds to the large-scale dry air intrusions from the subtropical zones into the equatorial band, mostly found during the suppressed MJO phase over the Indian Ocean
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Schmeling, Daniel. "Experimental Acquisition and Characterisation of Large-Scale Flow Structures in Turbulent Mixed Convection." Thesis, 2014. http://hdl.handle.net/11858/00-1735-0000-0022-5F32-8.
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Riese, Michael. "Study of large-scale coherent structures in the near field and transition regions of a mechanically oscillated planar jet." 2009. http://hdl.handle.net/2440/54150.
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Enhancing the performance of mixing and fluid entrainment by excitation of quasi-steady jets has been a subject of research for more than three decades. During the 1980s a special emphasis was placed on mechanically oscillating planar jets and the possibility to augment thrust of V/STOL aircraft. However, during this time, little attention was paid to the classification of flow regimes, the development of coherent structures or the existence of different regions in the flow within the jet near field. For the present study, a large aspect ratio nozzle was oscillated in the direction transverse to the width of the nozzle in simple harmonic motion. For a constant nozzle height, the stroke length, oscillation frequency and jet velocity were systematically varied. Over 240 flow cases were examined using a novel method of phase-locked flow visualisation. Following an initial analysis of the acquired data, a small subset of flow conditions was selected for further quantitative investigation using Particle Image Velocimetry (PIV). The phase-locked flow visualisation led to the identification and classification of three separate flow regimes, the Base Flow, the Resonant Flow and the Bifurcation Flow Regimes. Each regime is linked to the other regimes by the presence of a small number of repetitive coherent structures in the form of starting and stopping vortices. The analysis revealed a relationship between the stroke-to-nozzle height ratio and the ratio of the forcing frequency to the natural vortex shedding frequency in the planar jet. This directly contradicts the relationship between the Strouhal and Reynolds numbers of the jet that was proposed by previous investigators. Comparison of phase-locked PIV and flow visualisation data confirms both, the validity of the new regime classification and the identification of relevant large-scale structures. Time-averaged vorticity data are also used to further illustrate the differences between the three flow regimes. Investigation of the time-averaged qualitative data for the Base and Resonant Flow Regimes show that three distinct flow regions exist within both regimes. Adjacent to the nozzle is the initial formation region, where all large-scale structures form. This is followed by a coherent near-field region in which the jet exhibits very little spread for both the Base and Resonant Flow Regimes. Within this region no pairing of the large-scale vortices from the opposing sides of the flow can be found. This region is followed by a transition region that is marked by the sudden breakup and dissipation of all visible large-scale coherent structures. The vortex formation distance is then investigated using the available PIV data and compared with the results of previous investigations. The data show that the formation distance depends on the jet velocity, oscillation frequency and the stroke length. The agreement with previous data is poor due to differences in the method of measurement. Quantitative data are also used to investigate the centreline velocity decay in relation to changes of the jet Reynolds number and stroke-to-nozzle height ratio. The results show that the velocity decay rate increases with increasing stroke length as is expected from findings of earlier studies. In addition the centreline velocity decay rates in the mean jet transition region appear to be constant for each stroke length in the cases examined. Finally, conclusions are drawn and recommendations for future work are presented.http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1349701
Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 2009
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Maity, Rajib. "Impact Of Large-Scale Coupled Atmospheric-Oceanic Circulation On Hydrologic Variability And Uncertainty Through Hydroclimatic Teleconnection." Thesis, 2007. http://hdl.handle.net/2005/721.
Full textAbstract:
In the recent scenario of climate change, the natural variability and uncertainty associated with the hydrologic variables is of great concern to the community. This thesis opens up a new area of multi-disciplinary research. It is a promising field of research in hydrology and water resources that uses the information from the field of atmospheric science. A new way to identify and capture the variability and uncertainty associated with the hydrologic variables is established through this thesis. Assessment of hydroclimatic teleconnection for Indian subcontinent and its use in basin-scale hydrologic time series analysis and forecasting is the broad aim of this PhD thesis.
The initial part of the thesis is devoted to investigate and establish the dependence of Indian summer monsoon rainfall (ISMR) on large-scale Oceanic-atmospheric circulation phenomena from tropical Pacific Ocean and Indian Ocean regions. El Niño-Southern Oscillation (ENSO) is the well established coupled Ocean-atmosphere mode of tropical Pacific Ocean whereas Indian Ocean Dipole (IOD) mode is the recently identified coupled Ocean-atmosphere mode of tropical Indian Ocean. Equatorial Indian Ocean Oscillation (EQUINOO) is known as the atmospheric component of IOD mode. The potential of ENSO and EQUINOO for predicting ISMR is investigated by Bayesian dynamic linear model (BDLM). A major advantage of this method is that, it is able to capture the dynamic nature of the cause-effect relationship between large-scale circulation information and hydrologic variables, which is quite expected in the climate change scenario. Another new method, proposed to capture the dependence between the teleconnected hydroclimatic variables is based on the theory of copula, which itself is quite new to the field of hydrology. The dependence of ISMR on ENSO and EQUINOO is captured and investigated for its potential use to predict the monthly variation of ISMR using the proposed method.
The association of monthly variation of ISMR with the combined information of ENSO and EQUINOO, denoted by monthly composite index (MCI), is also investigated and established. The spatial variability of such association is also investigated. It is observed that MCI is significantly associated with monthly rainfall variation all over India, except over North-East (NE) India, where it is poor.
Having established the hydroclimatic teleconnection at a comparatively larger scale, the hydroclimatic teleconnection for basin-scale hydrologic variables is then investigated and established. The association of large-scale atmospheric circulation with inflow during monsoon season into Hirakud reservoir, located in the state of Orissa in India, has been investigated. The strong predictive potential of the composite index of ENSO and EQUINOO is established for extreme inflow conditions. So the methodology of inflow prediction using the information of hydroclimatic teleconnection would be very suitable even for ungauged or poorly gauged watersheds as this approach does not use any information about the rainfall in the catchment.
Recognizing the basin-scale hydroclimatic association with both ENSO and EQUINOO at seasonal scale, the information of hydroclimatic teleconnection is used for streamflow forecasting for the Mahanadi River basin in the state of Orissa, India, both at seasonal and monthly scale. It is established that the basin-scale streamflow is influenced by the large-scale atmospheric circulation phenomena. Information of streamflow from previous month(s) alone, as used in most of the traditional modeling approaches, is shown to be inadequate. It is successfully established that incorporation of large-scale atmospheric circulation information significantly improves the performance of prediction at monthly scale. Again, the prevailing conditions/characteristics of watershed are also important. Thus, consideration of both the information of previous streamflow and large-scale atmospheric circulations are important for basin-scale streamflow prediction at monthly time-scale.
Adopting the developed approach of using the information of hydroclimatic teleconnection, hydrologic variables can be predicted with better accuracy which will be a very useful input for better management of water resources.
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Reaume, Jonathan Daniel. "Wave-Cavity Resonator: Experimental Investigation of an Alternative Energy Device." Thesis, 2015. http://hdl.handle.net/1828/6960.
Full textAbstract:
A wave cavity resonator (WCR) is investigated to determine the suitability of the
device as an energy harvester in rivers or tidal flows. The WCR consists of coupling
between self-excited oscillations of turbulent flow of water in an open channel along the
opening of a rectangular cavity and the standing gravity wave in the cavity. The device
was investigated experimentally for a range of inflow velocities, cavity opening lengths,
and characteristic depths of the water. Determining appropriate models and empirical
relations for the system over a range of depths allows for accuracy when designing
prototypes and tools for determining the suitability of a particular river or tidal flow as a
potential WCR site. The performance of the system when coupled with a wave
absorber/generator is also evaluated for a range piston strokes in reference to cavity wave
height. Video recording of the oscillating free-surface inside the resonator cavity in
conjunction with free-surface elevation measurements using a capacitive wave gauge
provides representation of the resonant wave modes of the cavity as well as the degree of
the flow-wave coupling in terms of the amplitude and the quality factor of the associated
spectral peak. Moreover, application of digital particle image velocimetry (PIV) provides
insight into the evolution of the vortical structures that form across the cavity opening.
Coherent oscillations were attainable for a wide range of water depths. Variation of the
water depth affected the degree of coupling between the shear layer oscillations and the
gravity wave as well as the three-dimensionality of the flow structure. In terms of the
power investigation, conducted with the addition of a load cell and linear table-driven
piston, the device is likely limited to running low power instrumentation unless it can be
up-scaled. Up-scaling of the system, while requiring additional design considerations, is
not unreasonable; large-scale systems of resonant water waves and the generation of large
scale vortical structures due to tidal or river flows are even observed naturally.Graduate
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reaumejd@uvic.ca