• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 753
  • 163
  • 104
  • 70
  • 57
  • 37
  • 19
  • 16
  • 15
  • 12
  • 11
  • 9
  • 9
  • 7
  • 6
  • Tagged with
  • 1547
  • 175
  • 142
  • 127
  • 125
  • 123
  • 119
  • 119
  • 117
  • 93
  • 92
  • 92
  • 83
  • 82
  • 78
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
431

Visual feature learning with application to medical image classification

Manivannan, Siyamalan January 2015 (has links)
Various hand-crafted features have been explored for medical image classification, which include SIFT and Local Binary Patterns (LBP). However, hand-crafted features may not be optimally discriminative for classifying images from particular domains (e.g. colonoscopy), as not necessarily tuned to the domain’s characteristics. In this work, I give emphasis on learning highly discriminative local features and image representations to achieve the best possible classification performance for medical images, particularly for colonoscopy and histology (cell) images. I propose approaches to learn local features using unsupervised and weakly-supervised methods, and an approach to improve the feature encoding methods such as bag-of-words. Unlike the existing work, the proposed weakly-supervised approach uses image-level labels to learn the local features. Requiring image-labels instead of region-level labels makes annotations less expensive, and closer to the data normally available from normal clinical practice, hence more feasible in practice. In this thesis, first, I propose a generalised version of the LBP descriptor called the Generalised Local Ternary Patterns (gLTP), which is inspired by the success of LBP and its variants for colonoscopy image classification. gLTP is robust to both noise and illumination changes, and I demonstrate its competitive performance compared to the best performing LBP-based descriptors on two different datasets (colonoscopy and histology). However LBP-based descriptors (including gLTP) lose information due to the binarisation step involved in their construction. Therefore, I then propose a descriptor called the Extended Multi-Resolution Local Patterns (xMRLP), which is real-valued and reduces information loss. I propose unsupervised and weakly-supervised learning approaches to learn the set of parameters in xMRLP. I show that the learned descriptors give competitive or better performance compared to other descriptors such as root-SIFT and Random Projections. Finally, I propose an approach to improve feature encoding methods. The approach captures inter-cluster features, providing context information in the feature as well as in the image spaces, in addition to the intra-cluster features often captured by conventional feature encoding approaches. The proposed approaches have been evaluated on three datasets, 2-class colonoscopy (2, 100 images), 3-class colonoscopy (2, 800 images) and histology (public dataset, containing 13, 596 images). Some experiments on radiology images (IRMA dataset, public) also were given. I show state-of-the-art or superior classification performance on colonoscopy and histology datasets.
432

The rotational evolution of Sun-like stars and the influence of low-mass binary companions / A evolução rotacional de gêmeas solares e a influência de companheiras binárias de baixa massa

Leonardo Augusto Gonçalves dos Santos 31 July 2017 (has links)
The Solar Twin Planet Search program is an unprecedented eort that aimed to procure extrasolar planets in more than 70 stars extremely similar to the Sun. In the course of this program, hundreds of high-quality optical spectra were obtained for these stars using the HARPS spectrograph, which is fed by the ESO La Silla 3.6 m telescope. Beyond the search for exoplanets, the data are invaluable to study the physical properties of Sun-like stars. Particularly in this dissertation, we are interested in verifying if the Sun possesses a regular rotation for its age among stars that are strictly similar to it, how the rotation of solar twins evolve with age and if the rotation of Sun-like stars is influenced by the presence of stellar mass companions. Previous conclusions on the regularity of the Suns rotation have been conflicting, and this is the first time such a large sample of solar twins with high quality spectroscopic data is used to clarify this puzzle. Our results suggest that the Sun is indeed a regular rotator for its age, which favors the use of the solar rotation to calibrate gyrochronology -- the estimation of stellar ages from their rotation. However, these results also imply a rotational evolution process that saturates after the solar age, constituting a departure from the widely used Skumanich relation and posing a challenge for gyrochronology. We securely identified 18 binary or multiple systems in the solar twin sample, of which only three display enhanced rotation for their ages. I estimated the orbital parameters of the binaries from their radial velocity variations, and the results show that their spectroscopic companions lie at orbital periods varying from a few to several years. I conclude that the presence of red or brown dwarf companions at moderate to long orbital periods do not influence the evolution of rotation in these systems, and therefore the main stars should evolve as single in this regard. The peculiarities in HIP 19911, HIP 67620 and HIP 103983 can be fully explained by spectral contamination from their companions. / O programa The Solar Twin Planet Search é um esforço sem precedentes na procura de planetas extra-solares em mais de 70 estrelas extremamente similares ao Sol. Ao longo desse programa, centenas de espectros ópticos de alta qualidade foram obtidos com o espectrógrafo HARPS, que está instalado no telescópio de 3,6 m do Observatório de La Silla. Além da busca de exoplanetas, estes dados são úteis para estudar as propriedades físicas de estrelas como o Sol. Estamos interessados em verificar se o Sol possui uma rotação regular para sua idade quando comparado com estrelas estritamente similares a ele, como que a rotação de gêmeas solares evolui com o tempo e se a rotação dessas estrelas é influenciada pela presença de companheiras estelares. Conclusões anteriores na regularidade da rotação solar são conflitantes, e esta é a primeira vez que uma amostra grande de gêmeas solares com dados espectroscópicos de alta qualidade é usada para esclarecer essa questão. Nossos resultados sugerem que o Sol de fato rota regularmente para sua idade, o que favorece o uso da rotação solar para calibrar a girocronologia -- a estimativa de idades estelares a partir de sua rotação. No entanto, tais resultados também implicam em um processo de evolução rotacional que satura depois da idade solar, constituindo um desvio da amplamente usada relação de Skumanich e apresentando um desafio para a girocronologia. Nós identificamos 18 sistemas binários na amostra de gêmeas solares, das quais apenas três mostram rotações elevadas para suas idades. Os parâmetros orbitais das binárias foram estimados a partir da variação de suas velocidades radiais, e os resultados mostram que suas companheiras espectroscópicas possuem períodos orbitais variando de alguns poucos até muitos anos. Concluimos que a presença de companheiras do tipo anãs vermelhas ou marrons em períodos orbitais moderados não influenciam a evolução rotacional desses sistemas. As peculiaridades de HIP 19911, HIP 67620 e HIP 103983 podem ser completamente explicadas por contaminação espectral de suas companheiras.
433

Understanding Ancient Math Through Kepler: A Few Geometric Ideas from The Harmony of the World

Arthur, Christopher 08 1900 (has links)
Euclid's geometry is well-known for its theorems concerning triangles and circles. Less popular are the contents of the tenth book, in which geometry is a means to study quantity in general. Commensurability and rational quantities are first principles, and from them are derived at least eight species of irrationals. A recently republished work by Johannes Kepler contains examples using polygons to illustrate these species. In addition, figures having these quantities in their construction form solid shapes (polyhedra) having origins though Platonic philosophy and Archimedean works. Kepler gives two additional polyhedra, and a simple means for constructing the “divine” proportion is given.
434

Atributos que influyen en la decisión de compra de autos híbridos de la marca Toyota y Hyundai en comparación a adquirir autos convencionales en la zona 7 de Lima Metropolitana

Obregon Corbella, Andres, Condor Sotil, Trilce 03 September 2019 (has links)
Esta investigación tiene como propósito identificar cuáles son los factores más determinantes para un posible comprador de un auto híbrido en comparación a un auto convencional en Lima Metropolitana, es por esto que se han utilizado herramientas cualitativas y cuantitativas para obtener un resultado; esto permitirá a las marcas de autos saber que factor es más importante para el consumidor y potenciarlo a través de estrategias de marketing directo e indirecto. En el análisis cualitativo desarrollado a través de un focus group se encontraron diferentes afirmaciones y negaciones que son respaldadas a través del análisis cuantitativo con el modelo estadístico logístico binario; en este caso se demostró que todas las variables son importantes para un posible comprador de un auto híbrido; pero unas tienen mayor importancia que otras y esto determinaría que se deben llevar a cabo estrategias que estén relacionadas al precio y diseño del auto; ya que son las variables que obtuvieron mayor importancia en el análisis. / This investigation has as purpose identify which are the factors more important for a possible buyer of a hybrid car in comparison of a conventional car in Lima Metropolitana, that is why qualitative and quantitative tools have been used to get a result; this will allow car brands to know which factor is more important for the consumer and to empower it through direct and indirect marketing strategies. In the qualitative analysis developed through a focus group, different affirmations and negations were found that are supported through quantitative analysis with the statistical binary logistic model; in this case it was shown that all the variables are important for a possible buyer of a hybrid car; but some are more important than others and this would determine that price and design should be the ones that have to have the focus of the strategies because they are the variables that obtained more importance in the statistical analysis. / Tesis
435

Gender Beyond Binary

Zubairi, Sidra 01 January 2018 (has links)
Transgender individuals challenge the binary definition of gender accepted in Pakistani society and as a result, Pakistan’s transgender community is highly marginalized. Pakistani society regards transgender individuals as “abnormal,” because their physical appearance and behavior fail to conform with conventional expectations. Based on contextual research and interviews conducted with transgender individuals in Pakistan, my thesis explores the obstacles transgender individuals face in the course of everyday survival. My research responds to the physical realities of being transgender in Pakistani society and analyzes cultural norms associated with gender, which trigger harassment. By designing transformable apparel for these individuals, informed by primary and secondary research, my goal is to help them cope with the everyday struggles of being transgender in Pakistan.
436

Adsorption and imbibition of binary liquids in nanoporous solids / Adsorption et imbibition de liquides binaires dans des solides nanoporeux

Dutta, Sujeet 02 December 2016 (has links)
Les mélanges de toluène et de tert-butanol sont complètement miscibles dans toutes les compositions à l'échelle macroscopique. Toutefois tert-butanol forme un réseau de liaisons hydrogène à l'échelle nanométrique qui persistent même dans les mélanges liquides binaires tert-butanol/toluène. Des expériences de diffusion de neutrons ont révélé la séparation des phases du mélange dans une structure cœur-gaine sous confinement dans des solides nanoporeux hydrophiles. Le travail effectué dans cette thèse vise à comprendre le rôle joué par la concurrence des interactions intermoléculaires (liaisons hydrogène, Van der Waals) lors de la séparation de phase sous nanoconfinement. Des expériences de RMN révèlent la persistance d'un réseau de liaisons hydrogène dans ces liquides binaires confinés dans des nanopores de silice à des concentrations faibles de tert -butanol, fournissant la preuves d'un autre type de réseau de liaisons hydrogène sous confinement. Des expériences d'adsorption du mélange gazeux dans des nanopores de silice ont aidé à expliquer l'affinité plus élevée des parois de silice polaires pour tert-butanol par une modèle thermodynamique. Le remplacement de la matrice hôte par un analogue hydrophobe est associé à une inversion de la sélectivité, montrant cette fois une plus grande affinité du toluène pour la surface des pores. L'effet des interactions spécifiques avec la surface sur la dynamique d'imbibition spontanée de ces liquides binaires à travers le réseau de silice nanoporeux a également été étudié. Des expériences de radiographie de neutrons ont révélé la séparation des flux à deux composants, au sein d'une dynamique toujours gouvernée par une loi de Lucas-Washburn. / Toluene and tert-butanol mixtures are completely miscible for all compositions at the macroscopic scale. However tert-butanol forms a network of hydrogen-bonded clusters at the nanoscale which persist even in the tert-butanol-toluene binary liquid mixtures. Interpretation of neutron scattering experiments revealed phase separation of the mixture into a core-shell structure inside hydrophilic nanoporous solids, with a tert-butanol shell and a toluene core. The work carried out in this thesis is aimed at understanding the role played by competing intermolecular interactions (hydrogen-bonding, van der Waals) in driving phase separation in confinement. NMR experiments reveal the persistence of a hydrogen-bonding network in these binary liquids confined in silica nanopores even at very low concentrations of tert-butanol, providing evidence of a new kind of hydrogen bonded network under confinement. Vapour sorption isotherms of tert-butanol-toluene binary gas mixtures in silica nanopores helped explain higher affinity of polar silica walls for tert-butanol by a thermodynamic model. Replacing the host matrix by a hydrophobic analogue was found to reverse the selectivity, with toluene showing greater affinity for the pore surface. Effect of surface specific interactions was studied on spontaneous imbibition dynamics of these binary liquids through nanoporous silica network. Neutron radiography experiments revealed the separation of fluxes into a twocomponent flow, generally obeying the Lucas-Washburn law.
437

Experimental investigation of free dendritic growth of succinonitrile-acetone alloys

Melendez Ramirez, Antonio Jose 01 December 2009 (has links)
Measurements are carried out for dendrite tip growth of succinonitrile-acetone alloys solidifying freely in an undercooled melt. The current experimental investigation is conducted using the equiaxed dendritic solidification experiment (EDSE). This setup allows for precise measurements of the dendrite tip velocity, radius and shape for a range of undercoolings and solute concentrations. The collected data are compared to available theories of free dendritic growth, such as the Lipton-Glicksman-Kurz and Li-Beckermann models. It is found that for dilute succinonitrile-acetone alloys, the measured dendrite tip Péclet numbers agree well with previous theories of free dendritic growth, if the effects of melt convection are taken into account. The tip selection parameter deviates significantly from the pure succinonitrile value and is inversely related to the applied undercooling. Besides, the selection parameter shows no dependence on the solute concentration. These results are consistent with phase-field simulations and preceding experimental investigations. In addition, scaling relationships for the sidebranching shape were obtained in terms of the dendritic envelope, projection area and contour length. These new scaling relations agree well with previous measurements in pure succinonitrile dendrites by Li and Beckermann.
438

A Hybrid Dynamic Modeling of Time-to-event Processes and Applications

Appiah, Emmanuel A. 31 May 2018 (has links)
In the survival and reliability data analysis, parametric and nonparametric methods are used to estimate the hazard/risk rate and survival functions. A parametric approach is based on the assumption that the underlying survival distribution belongs to some specific family of closed form distributions (normal, Weibull, exponential, etc.). On the other hand, a nonparametric approach is centered around the best-fitting member of a class of survival distribution functions. Moreover, the Kaplan-Meier and Nelson-Aalen type nonparametric approach do not assume either distribution class or closed-form distributions. Historically, well-known time-to-event processes are death of living specie in populations and failure of component in engineering systems. Recently, the human mobility, electronic communications, technological changes, advancements in engineering, medical, and social sciences have further diversified the role and scope of time-to-event processes in cultural, epidemiological, financial, military, and social sciences. To incorporate extensions, generalizations and minimize scope of existing methods, we initiate an innovative alternative modeling approach for time-to-event dynamic processes. The innovative approach is composed of the following basic components: (1) development of continuous-time state of dynamic process, (2) introduction of discrete-time dynamic intervention process, (3) formulation of continuous and discrete-time interconnected dynamic system, (4) utilizing Euler-type discretized schemes, developing theoretical dynamic algorithms, and (5) introduction of conceptual and computational state and parameter estimation procedures. The presented approach is motivated by state and parameter estimation of time-to-event processes in biological, chemical, engineering, epidemiological, medical, military, multiple-markets and social dynamic processes under the influence of discrete-time intervention processes. We initiate (1) a time-to-event process to be a probabilistic dynamic process with unitary state. Action, normal, operational, radical, survival, susceptible, etc. and its complementary states, reaction, abnormal, nonoperational, non-radical, failure, infective and so on (quantitative and qualitative variables), are considered to be illustrations of a unitary state of time-to-event dynamic processes. A unitary state is measured by a probability distribution function. Employing Newtonian dynamic modeling approach and observing the definition of hazard rate as a specific rate, survival or failure probabilistic state dynamic model is developed. This dynamic model is further extended to incorporate internal or external discrete-time dynamic intervention processes acting on unitary state time-to-event processes (2). This further demanded a formulation and development of an interconnected continuous-discrete-time hybrid, and totally discrete-time dynamic models for time-to-event processes (3). Employing the developed hybrid model, Euler-type discretized schemes, a very general fundamental conceptual analytic algorithm is outlined (4). Using the developed theoretical computational procedure in (4), a general conceptual computational data organizational and simulation schemes are presented (5) for state and parameter estimation problems in unitary state time-to-event dynamic processes. The well-known theoretical existing results in the literature are exhibited as special cases in a systematic and unified manner (6). In fact, the Kaplan-Meier and Nelson-Aalen type nonparametric estimation approaches are systematically analyzed by the developed totally discrete-time hybrid dynamic modeling process. The developed approach is applied to two data sets. Moreover, this approach does not require a knowledge of either a closed-form solution distribution or a class of distributions functions. A hazard rate need not be constant. The procedure is dynamic. In the existing literature, the failure and survival distribution functions are treated to be evolving/progressing mutually exclusively with respect to corresponding to two mutually exclusive time varying events. We refer to these two functions (failure and survival) as cumulative distributions of two mutually disjoint state output processes with respect to two mutually exclusive time-varying complementary unitary states of a time-to-event processes in any discipline of interest (7). This kind of time-to-event process can be thought of as a Bernoulli-type of deterministic/stochastic process. Corresponding to these two complementary output processes of the Bernoulli-type of stochastic process, we associate two unitary dynamic states corresponding to a binary choice options/actions (8), namely, ({action, reaction}, {normal, abnormal}, {survival, failure}, {susceptible, infective}, {operational, nonoperational}, {radical, non-radical}, and so on.) Under this consideration, we extend unitary state time-to-event dynamic model to binary state time-to-event dynamic model. Using basic tools in mathematical sciences, we initiate a Newtonian-type dynamic approach for binary state time-to-event processes in the sciences, technologies, and engineering (9). Introducing an innovative concept of “survival state dynamic principle”, an innovative interconnected nonlinear non-stationary large-scale hybrid dynamic model for number of units/species and its unitary survival state corresponding to binary state time-to-event process is formulated (10). The developed model in (10) includes dynamic model (3) as a special case. The developed approach is directly applicable to binary state time-to-event dynamic processes in biological, chemical, engineering, financial, medical, physical, military, and social sciences in a coherent manner. A by-product of this is a transformed interconnected nonlinear hybrid dynamic model with a theoretical discrete-time conceptual computational dynamic process (11). Employing the transformed discrete-time conceptual computational dynamic process, we introduce notions of data coordination, state data decomposition and aggregation, theoretical conceptual iterative processes, conceptual and computational parameter estimation and simulation schemes, conceptual and computational state simulation schemes in a systematic way (12). The usefulness of the developed interconnected algorithm is validated by using three real world data sets (13). We note that the presented algorithm does not need a closed-form representation of distribution/likelihood function. In fact, it is free from any required assumptions of the “Classical Maximum Likelihood Function Approach” in the “Survival and Reliability Analysis.” The rapid electronic communication and human mobility processes have facilitated to transform information, knowledge, and ideas almost instantly around the globe. This indeed generates heterogeneity, and it causes to form nonlinear and non-stationary dynamic processes. Moreover, the heterogeneity, non-linearity, non-stationarity, further generates two types of uncertainties, namely, deterministic, and stochastic. In view of this, it is obvious that nothing is deterministic. In short, the 21st century problems are highly nonlinear, non-stationary and under the influence of internal and external random perturbations. Using tools in stochastic analysis, interconnected deterministic models in (3) and (10) are extended to interconnected stochastic hybrid dynamic model for binary state time-to-event processes (14). The developed model is described by a large-scale nonlinear and non-stationary stochastic differential equations. Moreover, a stochastic version of a survival function is also introduced (15). Analytical, computational, statistical, and simulation algorithms/procedures are also extended and analyzed in a systematic and unified way (16). The presented interconnected stochastic model is motivated to initiate conceptual computational parameter and state estimation schemes for time-to-event statistical data (17). Again, stochastic version of computational algorithms are validated in the context of three real world data sets. The obtained parameter and state estimates show that the algorithm is independent of the choice of nonlinear transformation (18). Utilizing the developed alternative innovative procedure and the recently modified deterministic version of Local Lagged Adapted Generalized Method of Moments (LLGMM) is also extended to stochastic version in a natural way (19). This approach provides a degree of measure of confidence, prediction, and planning assessments (20). In addition, it initiates a conceptual computational parameter and state estimation and simulation schemes that is suitable for the usage of mean square sub-optimal procedure (21). The usefulness and the significance of the approach is illustrated by applying to three data sets (22). The approach provides insight for investigating various type of invariant sets, namely, sustainable/unsustainable, survival/failure, reliable/unreliable (23), and qualitative properties such as sustainability versus unsustainability, reliability versus unreliability, etc. (24) Once again, the presented algorithm is independent of any form of survival distribution functions or data sets. Moreover, it does not require a closed form survival function distribution. We also note that the introduction of intervention processes provides a measure of influence and confidence for the usage of new tools/procedures/approaches in continuous-time binary state time-to-event dynamic process (25). Moreover, the presented dynamic modeling is more feasible for its usage of investigating a more complex time-to-event dynamic process (26). The developed procedure is dynamic and indeed non-parametric (27). The dynamic approach adapts with current changes and updates statistic process (28). The dynamic nature is natural rather than the existing static and single-shot techniques (29).
439

Similarity Based Large Scale Malware Analysis: Techniques and Implications

Li, Yuping 07 June 2018 (has links)
Malware analysis and detection continues to be one of the central battlefields for cybersecurity industry. For the desktop malware domain, we observed multiple significant ransomware attacks in the past several years, e.g., it was estimated that in 2017 the WannaCry ransomware attack affected more than 200,000 computers across 150 countries with hundreds of millions damages. Similarly, we witnessed the increased impacts of Android malware on global individuals due to the popular smartphone and IoT devices worldwide. In this dissertation, we describe similarity comparison based novel techniques that can be applied to achieve large scale desktop and Android malware analysis, and the practical implications of machine learning based approaches for malware detection. First, we propose a generic and effective solution for accurate and efficient binary similarity analysis of desktop malware. Binary similarity analysis is an essential technique for a variety of security analysis tasks, including malware detection and malware clustering. Even though various solutions have been developed, existing binary similarity analysis methods still suffer from limited efficiency, accuracy, and usability. In this work, we propose a novel graphical fuzzy hashing scheme for accurate and efficient binary similarity analysis. We first abstract control flow graphs (CFGs) of binary codes to extract blended n-gram graphical features of the CFGs, and then encode the graphical features into numeric vectors (called graph signatures) to measure similarity by comparing the graph signatures. We further leverage a fuzzy hashing technique to convert the numeric graph signatures into smaller fixed size fuzzy hash outputs for efficient comparisons. Our comprehensive evaluation demonstrates that our blended n-gram graphical feature based CFG comparison is more effective and efficient compared to existing CFG comparison techniques. Based on our CFG comparison method, we develop BingSim, a binary similarity analysis tool, and show that BingSim outperforms existing binary similarity analysis tools while conducting similarity analysis based malware detection and malware clustering. Second, we identify the challenges faced by overall similarity based Android malware clustering and design a specialized system for solving the problems. Clustering has been well studied for desktop malware analysis as an effective triage method. Conventional similarity-based clustering techniques, however, cannot be immediately applied to Android malware analysis due to the excessive use of third-party libraries in Android application development and the widespread use of repackaging in malware development. We design and implement an Android malware clustering system through iterative mining of malicious payloads and checking whether malware samples share the same version of malicious payloads. Our system utilizes a hierarchical clustering technique and an efficient bit-vector format to represent Android apps. Experimental results demonstrate that our clustering approach achieves precision of 0.90 and recall of 0.75 for the Android Genome mal- ware dataset, and average precision of 0.98 and recall of 0.96 with respect to manually verified ground-truth. Third, we study the fundamental issues faced by traditional machine learning (ML) based Android malware detection systems, and examine the role of ML for Android malware detection in practice, which leads to a revised evaluation strategy that evaluates an ML based malware detection system by checking their zero-day detection capabilities. Existing machine learning based Android malware research obtains the ground truth by consulting AV products, and uses the same label set for training and testing. However, there is a mismatch between how the ML system has been evaluated, and the true purpose of using ML system in practice. The goal of applying ML is not to reproduce or verify the same potentially imperfect knowledge, but rather to produce something that is better — closer to the ultimate ground truth about the apps’ maliciousness. Therefore, it will be more meaningful to check their zero-day detection capabilities than detection accuracy for known malware. This evaluation strategy is aligned with how an ML algorithm can potentially benefit malware detection in practice, by acknowledging that any ML classifier has to be trained on imperfect knowledge, and such knowledge evolves over time. Besides the traditional malware prediction approaches, we also examine the mislabel identification approaches. Through extensive experiments, we demonstrate that: (a) it is feasible to evaluate ML based Android malware detection systems with regard to their zero-day malware detection capabilities; (b) both malware prediction and mislabel identification approaches can be used to achieve verifiable zero-day malware detection, even when trained with an old and noisy ground truth dataset.
440

Estudo numérico da transição laminar-turbulenta de um jato planar binário / Numerical study of the laminar-turbulent transition of a coaxial binary jet

Chiumento, Vinícius Hagemeyer 08 August 2019 (has links)
A eficiência de motores a combustão está diretamente relacionada a mistura dos reagentes. O que é muito desejado em todos os sistemas, inclusive em sistemas aeroespaciais onde a combustão ocorre de forma contínua como nos motores a jato, motores de foguete, ramjet e scramjet. No caso do scramjet a combustão ocorre em regime supersônico e conhecer como os dois fluidos se misturam na câmera de combustão é muito importante pois o tempo de residência na câmera é muito reduzido, aumentando a importância de uma mistura homogênea para a eficiência da combustão. Em determinados casos pequenas pertubações em um jato vão se amplificar podendo ocasionar a transição do escoamento laminar para turbulento. O que é desejado visto que escoamentos turbulentos são caracterizados pela grande capacidade de mistura. No presente trabalho estudamos a estabilidade de jatos coaxiais composto por dois fluídos com pequenas pertubações na base para escoamentos supersônicos para casos bidimensionais e tridimensionais. O escoamento foi investigado utilizando simulação numérica e a teoria de estabilidade linear, os resultados de ambos os métodos foram comparados, casos com pertubações bidimencionais e tridimencionais foram analizados. As simulações numéricas foram realizadas utilizando diferenças finitas de alta ordem de precisão para a discretização espacial. A integração temporal foi feita utilizando o método de Runge-Kutta de quarta ordem. Os resultados de ambos os métodos mostraram uma boa concordância. / The efficiency of combustion engines are strict related to the mixing between reagents. That is very desire in every aerospace propulsion system, when the combustion is continuous such as rocket engines, ramjets and scramjets. The combustion in scramjet occurs in supersonic speed ith a very small resilience time, know how two fluids are mixed in this case is very important because are direct related to the efficiency of the combustion. In such cases small disturbances in a jet flow can be amplified until occur the transition from a laminar flow to a turbulent flow, that is desired because the great capacity of mixing of turbulent flows. In this work are studied the jet flow stability when the jet are composed by two fluids with small disturbances in the base for supersonic flows in bidimensional and tridimensional cases. The numerical results are obtained by numerical simulation and linear stability theory (LST). High order finite difference schemes are adopted for spatial derivatives. The integration in time are caried out by a fouth order Runge-Kutta scheme. The results obtained by numerical simulation and linear stability theory show good agreement.

Page generated in 0.109 seconds