O presente trabalho tem como objetivo desenvolver novas alternativas de identificação modal para estruturas sob excitações em condição de operação baseadas em funções de transmissibilidade. Recentes metodologias formuladas sobre conceitos de transmissibilidade têm surgido como alternativa para a identificação de parâmetros modais de estruturas. A identificação nestas metodologias é independente do espectro da excitação, sendo uma vantagem importante com respeito a metodologias anteriores no domínio da frequência que supõem a excitação como ruído branco. Dessa forma, aproveitando os diferentes trabalhos dirigidos a avaliar parâmetros modais com uso da transmissibilidade, são propostas três novas alternativas. A primeira delas propõe a decomposição de valores singulares sobre matrizes de funções de transmissibilidade escalar com densidade espectral para estimar frequências naturais e modos de vibração. A segunda alternativa propõe o conceito de funções de transmissibilidade multivariável com diferente referência para a identificação modal. E a terceira introduz uma melhora na primeira alternativa incluindo a possibilidade da estimação de taxas de amortecimento. Uma ferramenta computacional para a análise modal é desenvolvida como apoio para as simulações numéricas de verificação das metodologias de identificação modal propostas. Diferentes exemplos numéricos com uma viga submetida a excitações de ruído colorido mostram que os métodos propostos são capazes de identificar parâmetros modais sem a introdução das frequências adicionais devido às excitações de ruído colorida utilizadas. Além disso, os dados de um teste de vibrações sobre uma ponte em operação foram utilizados para verificar os métodos. / This research aims to develop new alternatives of modal identification for structures under excitation in operation condition based on transmissibility functions. Latest methodologies based on transmissibility concepts have been arising as alternatives for modal parameter identification of structures. Modal parameter identification in this type methodology is input spectrum independent being an important advantage with respect previous frequency domain methods that assumes white noise excitation. Different alternatives of modal identification based on transmissibility functions are proposed in this work. The first of them proposes singular value decomposition on scalar transmissibility functions matrices with spectral density to estimate natural frequencies and vibration modes (PSDTM-SVD method). A second alternative proposes the concept of multivariable transmissibility functions with different transferring outputs for modal parameter identification. And the third alternative proposes an enhanced PSDTM-SVD method, which permits to identify modal damping. Computational tool for modal analysis is developed as a support for the numerical simulations of verification of modal identification methodologies proposed. Different numerical examples of a beam model subjected to colored noise excitations show that the proposed methods are capable of identifying modal parameters without the introduction of the additional frequencies due to the excitations used. Furthermore, data from an operational vibration bridge test were used to verify the methods.

Much of the recent progress in cosmology has come from studying the power spectrum of the cosmic microwave background (CMB). The latest results from the Planck satellite confirmed that the inflationary paradigm with the $\Lambda$CDM six-parameter model provides a very good description of the observed structures in the Universe. Even so, additional parameters, such as cosmic defects, are still allowed by current observational data. Additionally, many of the inflationary models predict a significant departure from Gaussianity in the distribution of primordial perturbations. Higher order statistics, such as the bispectrum, are required to test and constrain such models. The late-time distribution of matter in the Universe - large-scale structure (LSS) - contains much more information than the CMB that has not yet been used. In this thesis, we look at both problems: the effects of cosmic defects, in particular cosmic strings and domain walls on the CMB power spectrum through numerical simulations, and the dark matter bispectrum of large-scale structure. Topological defects are predicted by most inflationary theories involving symmetry breaking in the early Universe. In this thesis we study the effects of cosmic strings and domain walls on the CMB by determining their power spectrum. We use Nambu-Goto and field theory simulations for cosmic strings and domain walls respectively, and we determine the power spectra they produce with a modified Einstein-Boltzmann solver sourced by unequal time correlators from components of the energy-momentum tensor of the defects. We use these spectra together with CMB likelihoods to obtain constraints on the energy scales of formation of the cosmic defects, finding $G\mu/c^{2} < 1.29 \times 10^{−7}$ and $\eta < 0.93$ MeV (at 95% confidence level) for cosmic strings and domain walls respectively, when using the Planck satellite likelihoods. For the matter bispectrum of LSS, we compare different perturbative and phenomenological models with measurements from $N$-body simulations by using shape and amplitude correlators and we determine on which scales and for which redshifts they are accurate. We propose a phenomenological ‘three-shape’ model, based on the fundamental shapes we have observed by studying the halo model that are also present in the simulations. When calibrated on the simulations, this model accurately describes the bispectrum on all scales and redshifts considered, providing a prototype bispectrum HALOFIT-like methodology that could be used to describe and test parameter dependencies.

No description available.

The fine-scale structure of global (30N-30S) tropical rainfall is characterised using 13 years (1998-2010) of 3-hourly and daily, 0.25-degree Tropical Rainfall Measuring Mission (TRMM) 3B42 rainfall product. At the outset, the dominant timescales present in rainfall are identified. Specifically, the Fourier spectrum (in time) is estimated in two ways (a) spectrum of spatially averaged (SoSA) rainfall; and (b) spatial average of the spectrum (SAoS) of rainfall at each grid point. This procedure is applied on rainfall at the 3-hourly and daily temporal resolutions. Both estimates of the spectrum show the presence of a very strong seasonal cycle. But, at subseasonal timescales, the two methods of estimating spectrum show a marked difference in daily rainfall. Specifically, with SoSA the variability peaks at a subseasonal timescale of around 5 days, with a possible secondary peak around 30-40 days (mostly in the southern tropics). With SAoS, the variability is distributed across a range of timescales, from 2 days to 90 days. However, with finer resolution (3-hourly) observations, it is seen that (besides the seasonal cycle) both methods agree and yield a dominant diurnal scale. Along with other subseasonal scales, the contribution and geographical distribution of diurnal scale variability is estimated and shown to be highly significant. Given its large contribution to the variability of tropical rainfall, the diurnal cycle is extracted by means of a Fourier-based filtering and analysed. The diurnal rainfall anomaly is constructed by eliminating all timescales larger than 1 day. Following this, taking care to avoid spurious peaks associated with Gibbs oscillations, the time of day (called the peak octet) when the diurnal anomaly is largest is identified. The peak octet is estimated for each location in the global tropics. This is repeated for 13 years, and the resulting mode of the time of maximum rainfall is established. It is seen that (i) most land regions receive rainfall during the late afternoon/early evening hours; (ii) rainfall over open oceans lack a dominant diurnal signature with a possible combination of early morning and afternoon showers; (iii) coastal regions show a clear south/southwest propagation in the mode of the peak octet of rainfall. In addition to being a comprehensive documentation of the diurnal cycle at very fine scales, the results serve as a critical test for the validation of theoretical and numerical models of global tropical rainfall.

Se zvyšující se dostupností medicínského CT vyšetření a s rostoucím počtem patologických stavů, pro které je indikováno, se redukce pacientské dávky ionizujícího záření stává stále aktuálnějším tématem. Výrazný pokrok v tomto odvětví představují nové metody rekonstrukce obrazů z projekcí, tzv. moderní iterativní rekonstrukční metody. Zároveň se zavedením těchto metod vzrostla potřeba pro měření obrazové kvality. Kvalita iterativně rekonstruovaných dat byla doposud kvantitativně hodnocena pouze na fantomových datech nebo na malých oblastech zájmu v reálných pacientských datech. Charakter iterativně rekonstruovaných dat však naznačuje, že tyto přístupy nadále nejsou dostatečné a je nutné je nahradit přístupy novými. Hlavním cílem této dizertační práce je navrhnout nové přístupy k měření kvality CT obrazových dat, které budou respektovat specifika iterativně rekonstruovaných obrazů a budou počítána plně automaticky přímo z reálných pacientských dat.

Coiled-coil motifs are part of 5–10 % of the eukaryotic proteome and are involved in important cellular processes such as membrane trafficking, chromosome segregation or mechanosensing. Their canonical form is well understood and based on a heptad repeat with hydrophobic amino acids at positions 1 and 4. A sequence of these peptides folds into an α-helix and two, or more, of these helices bind together by winding around each other, covering up the hydrophobic residues and giving rise to the coiled-coil structure. Coiled-coil proteins appearing in nature do, however, deviate from this form by introducing discontinuities into the heptad repeat. The effect of these imperfections on the structure is only known for few cases and not generally understood or predictable. The additional impact of these discontinuities on the dynamic function of coiled-coil domains is unknown altogether. Here, in order to tackle these questions, the adhesive forces between the α-helices are studied in single-molecule experiments. To measure these small forces (∼ pN) with a high spatial and temporal resolution, a dual-trap optical tweezers setup was constructed. Special emphasis was put on realizing the required high resolution, a large degree of automation and versatility during the building process. The instrument’s performance was assessed by recording force-extension curves of DNA yielding results for the molecular parameters persistence length and stretch modulus in good agreement with those found in the literature. Additionally, the Allan deviation was computed for different configurations of beads and a high stability and resolution of the instrument was found with optimal performance on the time scale of 1–10 s. Optical tweezers require calibration to accurately measure forces. To this end, generally a scheme is used that leverages the Brownian motion of a trapped object in the harmonic potential, created by the laser focus, to determine the parameters required to convert the analog voltage signal to distances and forces. However, this approach requires prior knowledge of the bead’s drag coefficient. A method was suggested previously that allows to measure this parameter by exciting the trapped bead through an external fluid flow and observing its response. Yet, this scheme was proposed for single-trap devices only. The precision and versatility of the new instrument was increased by extending this technique to work with two traps and implementing it in the apparatus. To this aim, the underlying equations of a trapped bead’s motion were modified to include hydrodynamic interactions between the objects resulting from the external fluid flow. It was found that a single multiplicative factor is sufficient to correct the calibration results for the hydrodynamic effects and ensure precise calibration. The drag coefficient of several beads yielded the same result for a single and two traps within the measurement error thus confirming the validity of the method. The newly built instrument was then used to study the coiled-coil protein early endosome antigen 1 (EEA1). This 200 nm long homodimer was shown to undergo an entropic collapse upon binding a small GTPase at the N-terminus. For further investigations of this effect and the adhesives forces at play, an experiment was designed here to unzip the two α-helices of the protein. To this end, DNA handles were attached to each of the two helices using a sortase A based ligation reaction as force moderators and first optical tweezers experiments were performed with the protein-DNA chimera. Thus, the necessary tools for unzipping assays of EEA1 are now at hand to further research the entropic collapse process. To summarize, a dual-trap optical tweezers setup was built, the calibration routine extended and realized in a more precise way and the instrument was used to investigate binding energies of EEA1 α-helices. / Coiled-Coil Strukturmotive sind in 5–10 % aller Proteine von Eukaryoten vertreten und wichtiger Teil zellulärer Prozesse wie Membrantransport, Segregation von Chromosomen oder Mechanoperzeption. Ihre grundlegende Struktur besteht aus dem sogenannten Heptadenmuster, einer Sequenz aus sieben Aminosäuren mit hydrophoben Molekülen an Position eins und vier. Eine Reihe dieser Muster kann sich zu einer α-Helix falten und zwei, oder mehr, solcher Helices sich umeinander winden, um die hydrophoben Moleküle abzuschirmen. Das Ergebnis ist eine Coiled-Coil- oder Doppelwendelstruktur. Natürlich vorkommende Coiled-Coil Proteine weichen jedoch durch Fehlstellen im Heptadenmuster von dieser kanonischen Form ab. Die Auswirkung dieser Störstellen auf die Struktur des gesamten Moleküls ist bisher nur für einige wenige Fälle untersucht und nicht allgemein vorstanden oder vorhersagbar. Der zusätzliche Einfluss dieser Fehlstellen auf die Funktion und dynamischen Prozesse solcher Proteine ist gänzlich unbekannt. Um diesen Fragen nachzugehen werden hier die Bindungskräfte zwischen den α-Helices in Einzelmolekülstudien untersucht. Um diese winzigen Kräfte (∼ pN) mit hoher räumlicher und zeitlicher Auflösung untersuchen zu können, wurde im Rahmen der vorliegenden Arbeit eine optische Doppelfalle konstruiert. Besonderes Augenmerk lag dabei auf dem Erreichen der erforderlichen Auflösung, einem hohen Grad an Automatisierung und der vielfälting Einsatzfähigkeit des Instruments. Die Leistungsfähigkeit dieses Kraftmikroskops wurde besonders durch zwei Experimente überprüft und sichergestellt. Zum einen wurden DNA Moleküle gedehnt und die Polymerparameter Persistenzlänge und Zugmodul gemessen, welche sehr gut mit veröffentlichten Referenzwerten übereinstimmten. Zum anderen wurde die Allan Schwankung für verschiedene experimentelle Konfigurationen von mikroskopischen Kugeln ermittelt, was eine hohe Stabilität und Auflösung des Gerätes, mit optimaler Leistung bei Mittelung auf Zeitskalen von 1–10 s, bestätigte. Optische Fallen müssen kalibriert werden, um Kräfte exakt messen zu können. Im Allgemeinen kommt dafür ein Verfahren zum Einsatz, welches die brownsche Bewegung eines gefangenen Objektes im harmonischen Potential des Laserfokus ausnutzt. Aus diesen Fluktuationen werden die benötigten Parameter ermittelt, um das gemessene analoge Spannungssignal in Abstände und Kräfte umzuwandeln. Dieser Ansatz erfordert jedoch die Kenntnis des Reibungskoeffizienten des gehaltenen Objektes, meist einer mikroskopischen Kugel. Daher wurde eine Methode vorgeschlagen, die durch ein oszillierendes Flussfeld eine zusätzliche Bewegung der Kugel anregt aus welcher der Reibungskoeffizient bestimmt werden kann. Dieses Vorgehen reduziert die im vornherein benötigten Informationen, wurde jedoch nur für eine einzelne optische Falle entwickelt. Der Ansatz wurde in dieser Arbeit erweitert, indem die zu zugrundeliegenden Bewegungsgleichungen einer gefangenen Kugel um hydrodynamische Wechselwirkungen zwischen mehreren Objekten ergänzt und die Kalibrationparameter basierend darauf hergeleitet wurden. Im Ergebnis konnte gezeigt werden, dass ein einzelner multiplikativer Faktor ausreicht, um die Hydrodynamik zu berücksichtigen und die exakte Kalibration des Instruments sicherzustellen. Dieses Vorgehen wurde überprüft, indem der Reibungskoeffizient einer einzelnen oder mehrerer mikroskopischer Kugeln gleichzeitig durch Anlegen eines externen Flussfeldes gemessen wurde. Die Ergebnisse stimmen im Rahmen der Messgenauigkeit überein und bestätigen damit den gewählten Ansatz. Das neu implementierte Kraftmikroskop wurde im Folgenden eingesetzt, um das Coiled-Coil Protein Early Endosome Antigen 1 (EEA1) zu erforschen. Dieser 200 nm lange Homodimer kollabiert aufgrund entropischer Kräfte sobald eine kleine GTPase an seinen N-Terminus bindet. Um diesen Effekt und die wirkenden Bindungskräfte besser zu verstehen, wurde hier ein Experiment entwickelt bei dem die beiden α-Helicen auseinandergezogen werden. Dazu wurde mittels einer Sortase A basierten Ligationsreaktion an jede Helix ein DNA-Stück gebunden, über welches Kräfte auf das Molekül übertragen werden können. Erste Experimente wurden mit der optischen Doppelfalle und dieser Protein-DNA Chimäre durchgeführt. Somit sind alle benötigten Werkzeuge zum weiteren Studium des entropischen Kollapses von EEA1 verfügbar, indem die Bindungskräfte der α-Helicen untersucht werden. Zusammenfassend wurde eine hoch auflösende Doppelfalle konstruiert, die Kalibrationsmethode weiterentwickelt und verfeinert und das Kraftmikroskop zur Erforschung der Bindungskräfte der α-Helicen von EEA1 eingesetzt.

Multilevel Random Pulse Width Modulation (RPWM) schemes have drawn increasing attention in the past few years. Multilevel topologies provide high voltage and high power capabilities and random PWM schemes offer reduction in discrete harmonics spectral. This dissertation provides a generalized theory and analysis methods of the standard five-level Weighted RPWM (WRPWM). Equations have been derived to analyze the spectral performance and average switching frequency of the WRPWM output waveform using statistical approach. A modified WRPWM scheme has been proposed. The modified WRPWM scheme is then analyzed with the equations derived from the same approach. The analyzed theoretical spectrum of the standard five-level WRPWM is then compared with the three-level WRPWM scheme and the conventional carrier based PWM scheme. A scaled laboratory prototype diode clamping five-level inverter has been built for verification of the standard and the proposed modified WRPWM schemes. It can be seen that the experimental measurements and the theoretical analyzed results are all in good agreement. Results show the two five-level WRPWM schemes offers significant improvements on the spectrum content than the conventional carrier based PWM scheme. It was found that the five-level WRPWM schemes have successfully suppress the magnitude of third harmonic below 5% of the magnitude of fundamental component and even less for the higher order harmonic components. Research contributions made by the dissertation are: - The proposed modified multilevel WRPWM scheme which utilizing the switching decision redundancy of multilevel inverter to manipulate the harmonic content of the output signal. - The derived mathematical equations of the standard and modified five-level WRPWM scheme for analytical purposes. / Dissertation (MEng (Electrical Engineering))--University of Pretoria, 2005. / Electrical, Electronic and Computer Engineering / unrestricted

Oceanic microwave remote sensing provides the data necessary for the estimation of significant geophysical parameters such as the near-surface vector wind. To obtain accurate estimates, a precise understanding of the measurements is critical. This work clarifies and quantifies specific uncertainties in the scattered power measured by an active radar instrument. While there are many sources of uncertainty in remote sensing measurements, this work concentrates on three significant, yet largely unstudied effects. With a theoretical derivation of the backscatter from an ocean-like surface, results from this dissertation demonstrate that the backscatter decays with surface roughness with two distinct modes of behavior, affected by the size of the footprint. A technique is developed and scatterometer data analyzed to quantify the variability of spaceborne backscatter measurements for given wind conditions; the impact on wind retrieval is described in terms of bias and the Cramer-Rao lower bound. The probability density function of modified periodogram averages (a spectral estimation technique) is derived in generality and for the specific case of power estimates made by the NASA scatterometer. The impact on wind retrieval is quantified.

No description available.

No / In a visual search experiment, the subject must find a target item hidden in a display of other items, and their performance is measured by their reaction time (RT). Here I look at how visual search reaction times are correlated with past reaction times. Target-absent RTs (i.e. RTs to displays that have no target) are strongly correlated with past target-absent RTs and, treated as a time series, have a 1/f power spectrum. Target-present RTs, on the other hand, are effectively uncorrelated with past RTs. A model for visual search is presented which generates search RTs with this pattern of correlations and power spectra. In the model, search is conducted by matching search items up with "categorizers," which take a certain time to categorize each item as target or distractor; the RT is the sum of categorization times. The categorizers are drawn at random from a pool of active categorizers. After each search, some of the categorizers in the active pool are replaced with categorizers drawn from a larger population of unused categorizers. The categorizers that are not replaced are responsible for the RT correlations and the 1/f power spectrum.