1,721,021 research outputs found
Hamiltonian magnetohydrodynamics: Lagrangian, Eulerian, and dynamically accessible stability-Examples with translation symmetry
No full textBecause different constraints are imposed, stability conditions for dissipationless fluids and magnetofluids may take different forms when derived within the Lagrangian, Eulerian (energy-Casimir), or dynamically accessible frameworks. This is in particular the case when flows are present. These differences are explored explicitly by working out in detail two magnetohydrodynamic examples: convection against gravity in a stratified fluid and translationally invariant perturbations of a rotating magnetized plasma pinch. In this second example, we show in explicit form how to perform the time-dependent relabeling introduced in Andreussi et al. [Phys. Plasmas 20, 092104 (2013)] that makes it possible to reformulate Eulerian equilibria with flows as Lagrangian equilibria in the relabeled variables. The procedures detailed in the present article provide a paradigm that can be applied to more general plasma configurations and in addition extended to more general plasma descriptions where dissipation is absent
Internal wave pressure, velocity, and energy flux from density perturbations
Full text linkDetermination of energy transport is crucial for understanding the energy budget and fluid circulation in density varying fluids such as the ocean and the atmosphere. However, it is rarely possible to determine the energy flux field J = pu, which requires simultaneous measurements of the pressure and velocity perturbation fields p and u, respectively. We present a method for obtaining the instantaneous J(x, z, t) from density perturbations alone: A Green's function-based calculation yields p; u is obtained by integrating the continuity equation and the incompressibility condition. We validate our method with results from Navier-Stokes simulations: The Green's function method is applied to the density perturbation field from the simulations and the result for J is found to agree typically to within 1% with J computed directly using p and u from the Navier-Stokes simulation. We also apply the Green's function method to density perturbation data from laboratory schlieren measurements of internal waves in a stratified fluid and the result for J agrees to within 6% with results from Navier-Stokes simulations. Our method for determining the instantaneous velocity, pressure, and energy flux fields applies to any system described by a linear approximation of the density perturbation field, e.g., to small-amplitude lee waves and propagating vertical modes. The method can be applied using our MATLAB graphical user interface EnergyFlux.Office of Naval Research N000141110701U.S. Department of Energy, Office of Science, Office of Fusion Energy SciencesDE-FG02-04ER-54742Physic
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Full text linkWe 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
Recommended from our members
Darwinian evolution: the mutation of a weakly relativistic lagrangian
Full text linktextThe work studies Darwin’s order-(v/c)2 approximation to the relativistic interaction of classical charged particles. The first part presents an in- troduction to the methods of symplectic reduction in the Newtonian two- body problem and then applies these to a two-body Darwin interaction. The momentum-dependent interaction of the Darwin system plays an important role in the ability to reduce to a system of one degree of freedom. Circular orbits are sought, and it is shown that two of the three possible orbits are prohibited by velocity conditions.
The second part of the work derives a self-consistent Darwin particle theory from a Lagrangian for electromagnetic fields coupled to particles. The resulting particle Lagrangian agrees with previous results. A similar procedure is followed to obtain a Low-Darwin system, coupling the self-consistent Darwin theory to the Vlasov equation.Physic
Recommended from our members
Spatially-homogeneous Vlasov-Einstein dynamics
Full text linktextThe influence of matter described by the Vlasov equation, on the evolution of anisotropy in the spatially-homogeneous universes, called the Bianchi cosmologies, is studied. Due to the spatial-homogeneity, the Einstein equations for each Bianchi Type are reduced to a set of coupled ordinary differential equations, which has Hamiltonian form with the metric components being the canonical coordinates. In the vacuum Bianchi cosmologies, it is known that a curvature potential, which comes from the symmetries of the three-dimensional Lie groups, determines the basic properties of the evolution of anisotropy. In this work, matter potentials are constructed for Vlasov matter. They are obtained by first introducing a new matter action principle for the Vlasov equation, in terms of a conjugate pair of functions, and then enforcing the symmetry to obtain a reduction. This yields an expression for the matter potential in terms of the phase space density, which is further reduced by assuming cold streaming matter. Some vacuum Bianchi cosmologies and Type I with Vlasov matter are compared. It is shown that the Vlasov-matter potential for cold streaming matter results in qualitatively distinct dynamics from the well-known vacuum Bianchi cosmologies.Physic
Recommended from our members
Topics in Lagrangian and Hamiltonian fluid dynamics : relabeling symmetry and ion-acoustic wave stability
Full text linkRelabeling symmetries of the Lagrangian action are found for the ideal, compressible fluid and magnetohydrodynamics (MHD). These give rise to conservation laws of potential vorticity (Ertel's theorem) and helicity in the ideal fluid, cross helicity in MHD, and a conservation law for an ideal fluid with three thermodynamic variables. The symmetry that gives rise to Ertel's theorem is generated by an infinite parameter group, and leads to a generalized Bianchi identity. The existence of a more general symmetry is also shown, with dependence on time and space derivatives of the fields, and corresponds to a family of conservation laws associated with the potential vorticity. In the Hamiltonian formalism, Casimir invariants of the noncanonical formulation are directly constructed from the symmetries of the reduction map from Lagrangian to Eulerian variables. Casimir invariants of MHD include a gauge-dependent family of invariants that incorporates magnetic helicity as a special case. Novel examples of finite dimensional, noncanonical Hamiltonian dynamics are also presented: the equations for a magnetic field line flow with a symmetry direction, and Frenet formulas that describe a curve in 3-space. In the study of Lyapunov stability of ion-acoustic waves, existence of negative energy perturbations is found at short wavelengths. The effect of adiabatic, ionic pressure on ion-acoustic waves is investigated, leading to explicit solitary and nonlinear periodic wave solutions for the adiabatic exponent r = 3. In particular, solitary waves are found to exist at any wave speed above Mach number one, without an upper cutoff speed. Negative energy perturbations are found to exist despite the addition of pressure, which prevents the establishment of Lyapunov stability; however the stability of ion-acoustic waves is established in the KdV limit, in a manner far simpler than the proof of KdV soliton stability. It is also shown that the KdV free energy (Benjamin, 1972) is recovered upon evaluating (the negative of) the ion-acoustic free energy at the critical point, in the KdV approximation. Numerical study of an ion-acoustic solitary wave with a negative energy perturbation shows transients with increased perturbation amplitude. The localized perturbation moves to the left in the wave-frame, leaving the solitary wave peak intact, thus indicating that the wave may be stable.Physic
Recommended from our members
Statistical mechanics of two-dimensional fluids
No full text"Two dimensional fluid flows are realized in situations where one dimension is much smaller than the other two or if symmetry allows the neglect of one of the dimensions. Other factors, too, can contribute to result in effectively two dimensional (henceforth 2-D) flows. An example is the planetary atmospheric and oceanic flow where rotation of the fluid about the planet's axis locks it into 2-D motion (Taylor-Proudman theorem). See Greenspan (1968) for the dynamics of rotating fluids. A constant magnetic field perpendicular to a layer of plasma has a similar locking effect (Kraichnan and Montgomery, 1980) which is why there is some interest in 2-D magnetohydrodynamics. One of the interesting features of 2-D flows is the formation of coherent structures, an example of which is the Great Red Spot of Jupiter. An experiment by Sommeria, Meyers and Swinney (1988) demonstrated the formation of coherent structures in 2-D turbulent shear by creating a "Great Red Spot" in a rotating tank of fluid. The problem of 2-D flows is also interesting purely from the point of view of studying the dynamics, hence the wish to study 2-D fluid flows. Various approaches to study the turbulent relaxation of 2-D flows are discussed in this thesis. We begin by setting up the basic equations and by reviewing the Hamiltonian formulation of the dynamics in Chapter 1. In Chapter 2 we discuss statistical approaches to solve the problem of turbulent relaxation. These approaches include the point vortex approximation and the method of maximizing entropy. While the fluid is approximated by a collection of point vortices in the former case, the latter approach allows us to deal with continuous vorticity distributions, although, in practice, one discretizes the vorticity in order to compute the results numerically. Chapter 3 is devoted to the study of a selective decay hypothesis based on arguments of cascade of energy and enstrophy to different scales. After having discussed these three different approaches, we then compare their predictions to the observations in an experiment on electrons in a magnetized column in Chapter 4. The possibility of a simple monotonic restacking of the vorticity is also discussed in Chapter 4"--Introduction.Physic
- …
