The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
A V Artemyev - One of the best experts on this subject based on the ideXlab platform.
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Scaling Laws for the inner structure of the radiation belts
Geophysical Research Letters, 2017Co-Authors: Didier Mourenas, A V ArtemyevAbstract:Accurately modeling the evolution of the electron radiation belts within the plasmasphere represents both an imperative goal for space weather forecasting and a great challenge. Combining previously developed approximate analytical expressions of electron lifetimes with recent statistical models of plasma density, ULF, whistler-mode, and electromagnetic ion cyclotron waves, we demonstrate that geomagnetic activity and plasma density actually govern the inner structure of the radiation belts through several simple analytical Scaling Laws when Kp < 3. Many of the observed characteristic features of electron fluxes in the energy versus L shell parameter space are straightforwardly explained. In particular, the upper energy limit of significant electron fluxes at L = 1.5 is estimated as ∼1 MeV in agreement with recent satellite observations. This approximate analytical model represents a very simple and powerful tool for exploring and better understanding the complex variations of the inner structure of the radiation belts with geomagnetic activity during relatively quiet times.
Eduardo G Altmann - One of the best experts on this subject based on the ideXlab platform.
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Scaling Laws and fluctuations in the statistics of word frequencies
New Journal of Physics, 2014Co-Authors: Martin Gerlach, Eduardo G AltmannAbstract:In this paper, we combine statistical analysis of written texts and simple stochastic models to explain the appearance of Scaling Laws in the statistics of word frequencies. The average vocabulary of an ensemble of fixed-length texts is known to scale sublinearly with the total number of words (Heaps? law). Analyzing the fluctuations around this average in three large databases (Google-ngram, English Wikipedia, and a collection of scientific articles), we find that the standard deviation scales linearly with the average (Taylor?s law), in contrast to the prediction of decaying fluctuations obtained using simple sampling arguments. We explain both Scaling Laws (Heaps? and Taylor) by modeling the usage of words using a Poisson process with a fat-tailed distribution of word frequencies (Zipf?s law) and topic-dependent frequencies of individual words (as in topic models). Considering topical variations lead to quenched averages, turn the vocabulary size a non-self-averaging quantity, and explain the empirical observations. For the numerous practical applications relying on estimations of vocabulary size, our results show that uncertainties remain large even for long texts. We show how to account for these uncertainties in measurements of lexical richness of texts with different lengths.
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Scaling Laws and fluctuations in the statistics of word frequencies
arXiv: Physics and Society, 2014Co-Authors: Martin Gerlach, Eduardo G AltmannAbstract:In this paper we combine statistical analysis of large text databases and simple stochastic models to explain the appearance of Scaling Laws in the statistics of word frequencies. Besides the sublinear Scaling of the vocabulary size with database size (Heaps' law), here we report a new Scaling of the fluctuations around this average (fluctuation Scaling analysis). We explain both Scaling Laws by modeling the usage of words by simple stochastic processes in which the overall distribution of word-frequencies is fat tailed (Zipf's law) and the frequency of a single word is subject to fluctuations across documents (as in topic models). In this framework, the mean and the variance of the vocabulary size can be expressed as quenched averages, implying that: i) the inhomogeneous dissemination of words cause a reduction of the average vocabulary size in comparison to the homogeneous case, and ii) correlations in the co-occurrence of words lead to an increase in the variance and the vocabulary size becomes a non-self-averaging quantity. We address the implications of these observations to the measurement of lexical richness. We test our results in three large text databases (Google-ngram, Enlgish Wikipedia, and a collection of scientific articles).
L.-x. Liu - One of the best experts on this subject based on the ideXlab platform.
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Scaling Laws for ultra short hydrostatic gas journal bearings
Journal of Vibration and Acoustics, 2005Co-Authors: Zoltán S. Spakovszky, L.-x. LiuAbstract:The journal bearings of the MIT micro-devices are located at the outer periphery of the rotor and are designed to operate at rotational speeds of order two million rpm in order to enable high-power densities with turbomachinery tip speeds near 500 m/s. These journal bearings are very short compared to their relatively large bearing diameters such that the bearing L/D is typically less than 0.1, that is at least one order of magnitude smaller than in conventional gas bearings. Thus, the ultra-short micro gas journal bearings essentially act as short annular seals and operate at Reynolds numbers of order 300, two orders of magnitude lower than conventional annular seals. The concepts that hold for turbulent flow, large scale annular seals do not apply to micro bearings and the laminar flow regime sets new challenges in the design, implementation and operation of ultra-short, high-speed gas bearings. In order to reach the goal of operating the MIT micro devices at full design speed, the micro-bearing design must be improved and engineering solutions need to be found to overcome the challenges of high-speed bearing operation. This paper is the first to derive the Scaling Laws for the dynamics of ultra-short hydrostatic gas journal bearings. The theory is established from first principles and enables a physics based characterization of the dynamic behavior of ultra-short hydrostatic gas bearings. The derived Scaling Laws for natural frequency and damping ratio show good agreement with experimental data. A simple criterion for whirl instability is found that only depends on bearing geometry. The Scaling Laws together with this criterion are used to delineate engineering solutions critical for stable high-speed bearing operation. Design charts are developed which provide the link between fabrication tolerances, bearing performance, and the tolerable level of rotor unbalance for a minimum required whirl ratio.
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Scaling Laws for ultra short hydrostatic gas journal bearings
ASME 2003 International Design Engineering Technical Conferences and Computers and Information in Engineering Conference, 2003Co-Authors: Zoltán S. Spakovszky, L.-x. LiuAbstract:The journal bearings of the MIT micro-devices are located at the outer periphery of the rotor and are designed to operate at rotational speeds of order 2 million rpm in order to enable high-power densities with turbomachinery tip speeds near 500 m/s. These journal bearings are very short compared to their relatively large bearing diameters such that the bearing L/D is typically less than 0.1, that is at least one order of magnitude smaller than in conventional gas bearings. Thus, the ultra-short micro gas journal bearings essentially act as short annular seals and operate at Reynolds numbers of order 300, two orders of magnitude lower than conventional annular seals. The concepts that hold for turbulent flow, large scale annular seals do not apply to micro bearings and the laminar flow regime sets new challenges in the design, implementation and operation of ultra-short, high-speed gas bearings. In order to reach the goal of operating the MIT micro devices at full design speed, the micro-bearing design must be improved and engineering solutions need to be found to overcome the challenges of high-speed bearing operation. This paper is the first to derive the Scaling Laws for the dynamics of ultrashort hydrostatic gas journal bearings. The theory is established from first principles and enables a physics based characterization of the dynamic behavior of ultra-short hydrostatic gas bearings. The derived Scaling Laws for natural frequency and damping ratio show good agreement with experimental data. A simple criterion for whirl instability is found that only depends on bearing geometry. The Scaling Laws together with this criterion are used to delineate engineering solutions critical for stable high-speed bearing operation. Design charts are developed which provide the link between fabrication tolerances, bearing performance, and the tolerable level of rotor unbalance for a minimum required whirl ratio.Copyright © 2003 by ASME
Didier Mourenas - One of the best experts on this subject based on the ideXlab platform.
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Scaling Laws for the inner structure of the radiation belts
Geophysical Research Letters, 2017Co-Authors: Didier Mourenas, A V ArtemyevAbstract:Accurately modeling the evolution of the electron radiation belts within the plasmasphere represents both an imperative goal for space weather forecasting and a great challenge. Combining previously developed approximate analytical expressions of electron lifetimes with recent statistical models of plasma density, ULF, whistler-mode, and electromagnetic ion cyclotron waves, we demonstrate that geomagnetic activity and plasma density actually govern the inner structure of the radiation belts through several simple analytical Scaling Laws when Kp < 3. Many of the observed characteristic features of electron fluxes in the energy versus L shell parameter space are straightforwardly explained. In particular, the upper energy limit of significant electron fluxes at L = 1.5 is estimated as ∼1 MeV in agreement with recent satellite observations. This approximate analytical model represents a very simple and powerful tool for exploring and better understanding the complex variations of the inner structure of the radiation belts with geomagnetic activity during relatively quiet times.
Andrea Rinaldo - One of the best experts on this subject based on the ideXlab platform.
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covariations in ecological Scaling Laws fostered by community dynamics
Proceedings of the National Academy of Sciences of the United States of America, 2017Co-Authors: Silvia Zaoli, Andrea Giometto, Amos Maritan, Andrea RinaldoAbstract:Scaling Laws in ecology, intended both as functional relationships among ecologically relevant quantities and the probability distributions that characterize their occurrence, have long attracted the interest of empiricists and theoreticians. Empirical evidence exists of power Laws associated with the number of species inhabiting an ecosystem, their abundances, and traits. Although their functional form appears to be ubiquitous, empirical Scaling exponents vary with ecosystem type and resource supply rate. The idea that ecological Scaling Laws are linked has been entertained before, but the full extent of macroecological pattern covariations, the role of the constraints imposed by finite resource supply, and a comprehensive empirical verification are still unexplored. Here, we propose a theoretical Scaling framework that predicts the linkages of several macroecological patterns related to species’ abundances and body sizes. We show that such a framework is consistent with the stationary-state statistics of a broad class of resource-limited community dynamics models, regardless of parameterization and model assumptions. We verify predicted theoretical covariations by contrasting empirical data and provide testable hypotheses for yet unexplored patterns. We thus place the observed variability of ecological Scaling exponents into a coherent statistical framework where patterns in ecology embed constrained fluctuations.
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covariations in ecological Scaling Laws fostered by community dynamics
arXiv: Populations and Evolution, 2017Co-Authors: Silvia Zaoli, Andrea Giometto, Amos Maritan, Andrea RinaldoAbstract:Scaling Laws in ecology, intended both as functional relationships among ecologically-relevant quantities and the probability distributions that characterize their occurrence, have long attracted the interest of empiricists and theoreticians. Empirical evidence exists of power Laws associated with the number of species inhabiting an ecosystem, their abundances and traits. Although their functional form appears to be ubiquitous, empirical Scaling exponents vary with ecosystem type and resource supply rate. The idea that ecological Scaling Laws are linked had been entertained before, but the full extent of macroecological pattern covariations, the role of the constraints imposed by finite resource supply and a comprehensive empirical verification are still unexplored. Here, we propose a theoretical Scaling framework that predicts the linkages of several macroecological patterns related to species' abundances and body sizes. We show that such framework is consistent with the stationary state statistics of a broad class of resource-limited community dynamics models, regardless of parametrization and model assumptions. We verify predicted theoretical covariations by contrasting empirical data and provide testable hypotheses for yet unexplored patterns. We thus place the observed variability of ecological Scaling exponents into a coherent statistical framework where patterns in ecology embed constrained fluctuations.
