1,720,961 research outputs found
Il caso Pershing: un esempio emblematico dei fattori di successo della media impresa italiana
No full textDispersive to nondispersive transition in the plane wake and channel flows
No full textBy varying the wavenumber over a large and finely discretized interval of values, we analyse the phase and group velocity of linear three-dimensional travelling waves both in the plane wake and channel flows to get the transition between dispersive and non-dispersive behaviour. The dispersion relation is computed from the Orr-Sommerfeld and Squire eigenvalue problem by observing the least stable mode, see figure 2, panels (a,b) and the comparison with [1, 2, 4–11, 15, 16]. The group velocity vg is also shown. The Reynolds number varies in the 20-100, 1000-8000 ranges for the wake and the channel flow, respectively, while we consider wavenumbers in the range 0.1-10. The wake basic flow consists of the first two orders of the Navier-Stokes matched asymptotic expansion described in [3, 13, 14]. At low wavenumbers we observe a dispersive behaviour where the phase speed and the group velocity substantially differ. The relevant perturbed solution is amenable to the typical solution belonging to the left branch of the eigenvalue spectrum, see the two examples shown in figure 1 (channel flow: Re = 6000; k = 1; wake Re = 100; k = 0:7). By rising the wave number value, we observe a sharp transition from the dispersive to the nondispersive regime. This transition is located at a critical wave number kd which is a function of the Reynolds number Re, the wave angle _, and the wake downstream observation point x0. Precisely, kd increases with Re and decreases with _ for the wake flow, while these trends are reversed for the channel flow, see tables 1,2. Beyond the wavenumber threshold, the observed least-stable mode belongs to the right branch of the spectrum. The asymptotic solutions in the dispersive region are wall modes for the channel flow , and in-wake modes for the wake flow. This means that, for both the flows, the dispersive behaviour is related to perturbations with high momentum variations (high vorticity) in correspondence to the base flow high-shear region. On the contrary, if k > kd the solutions are central modes for the channel case, and out-of-wake modes for the wake flow. In these cases, the disturbance has high variations outside the base flow high-shear region. To understand the physical mechanism of the dispersive-nondispersive transition we focused on time variation of the wave kinetic energy associated to the convective transport. Figure 2 (c,d) shows the convective term as a function of the wavenumber for the two least stable modes. We observe that the dispersive-nondisperive transition allows waves k > kd to keep the lowest possible temporal variation of kinetic energy, i.e. the lowest decay. This remains true also when all the other more stable modes are considered. In practice nondispersive waves maintain their convective energy with k
Wave focusing and related multiple dispersion transitions in plane Poiseuille flows
Get PDFMotivated by the recent discovery of a dispersive-to-nondispersive transition for linear waves in shear flows, we accurately explored the wavenumber-Reynolds number parameter map of the plane Poiseuille flow in the limit of least-damped waves. We have discovered the existence of regions of the map where the dispersion and propagation features vary significantly from their surroundings. These regions are nested in the dispersive, low-wavenumber part of the map. This complex dispersion scenario demonstrates the existence of linear dispersive focusing in wave envelopes evolving out of an initial, spatially localized, three-dimensional perturbation. An asymptotic wave packet's representation, based on the saddle-point method, allows to enlighten the nature of the packet's morphology, in particular, the arrow-shaped structure and spatial spreading rates. A correlation is also highlighted between the regions of largest dispersive focusing and the regions which are most subject to strong nonlinear coupling in observations
Solar wind spectral analysis in heliosheath from Voyager 2 data
No full textThe solar wind is a supersonic flow of magnetized plasma. It is time-dependent on all scales and expands with distance. The flow has fluctuations on a broad range of scales and frequencies. This fluctuations are not just convected outward but show energy cascades among the different scales. The solar wind turbulence peculiar phenomenology has been comprehensively reviewed by Tu and Marsch [11] and Bruno and Carbone [2]. As the distance from the sun increases, the available data on plasma and magnetic field become increasingly scarce. At distance of the order of 1 astronomic unit (AU), several measurement have been performed by various crafts, but, nowadays, only the Voyager spacecrafts can measure data in the heliosheath, the outermost layer in heliosphere where the solar wind is slowed by the pressure of the interstellar gas, and only the Voyager 2 craft can measure both plasma and magnetic fields (Voyager 1 can measure only the magnetic field, and Pioneer 10 and 11 has ceased communications). Taken together, the Voyager 1 and 2 probes have collected over 11 year of data in the heliosheath. The Voyager plasma experiment observes plasma currents in the energy/charge range 10 – 5950 eV /q using four modulated-grid Faraday cup detectors [1]. The observed currents are fit to convected isotropic proton Maxwellian distributions to derive the parameters (velocity, density, and temperature) used in this work. Magnetic field and plasma data are taken the COHO web site and MIT Space Plasma Group repository. Several studies have been done in order to extend the existing models to make them consistent with the energetic particles and magnetic fields measured in the heliosheath, but so far an exhaustive explanation has not yet been obtained. In particular, the differences between the energetic particle intensity variations seen by the two crafts are unexplained. The electron intensity measured by Voyager 2 varies steeper by a factor of 10 in a single year, while the same quantity from Voyager 1 changes gradually over time.[7] A possible explanation can be the presence of bubble of turbulence that travels in the heliosheat. Therefore, a characterization of turbulence and its intermittency is necessary to explain this phenomenology. The aim of this work is to provide the first spectral analysis of heliosheath solar wind, trying to characterize the plasma turbulence in that region by estimating the spectral slopes. A first result is represented in figure 2, where it can be seen that the low frequency spectral slope is lower when the electron intensity is low. In order to compute spectra, signal reconstruction techniques are mandatory: at distance over 80 AU, available data are very spotty. For the plasma velocity, there are 97% of missings due to unsteadiness in the signals, see 1, the most important of which are: tracking gaps due to the V2 location and due to limited deep space network availability; interference from other instruments; possible errors in the measurement chain (from the Faraday cups up to the data acquisition system and the signal shipping to Earth); the temporal sequence of the nuclear propulsion used to control the Voyager trajectory and to assist in several critical repositionings of the craft. For data recovery, we mainly use two different methods. The first method used is based on the correlation computation [8] that allows to reconstruct correlations and use it to compute spectra. Better results can be achieved implementing the maximum likelihood reconstruction by Rybicki and Press [10] based on a minimum-variance recovery with a stochastic component. The second methods comes from the Compress Sensing, a recent technique widely used in telecommunications, that provides the reconstruction of the signal from a sparse dataset [3], by using sparse Fourier matrices [9]. The methods used have been previously tested on 1979 data and on synthetic fluid turbulent fields. Results were in good agreement with the literature, and allows to compute largest spectra of solar wind at 5 AU, with frequencies ranging from 10-7 to 10-2 Hz [4, 5, 6]
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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