Windowed Green Function Method for Nonuniform Open-Waveguide Problems

This contribution presents a novel Windowed Green Function (WGF) method for the solution of problems of wave propagation, scattering and radiation for structures which include open (dielectric) waveguides, waveguide junctions, as well as launching and/or termination sites and other nonuniformities. Based on use of a "slow-rise" smooth-windowing technique in conjunction with free-space Green functions and associated integral representations, the proposed approach produces numerical solutions with errors that decrease faster than any negative power of the window size. The proposed methodology bypasses some of the most significant challenges associated with waveguide simulation. In particular the WGF approach handles spatially-infinite dielectric waveguide structures without recourse to absorbing boundary conditions, it facilitates proper treatment of complex geometries, and it seamlessly incorporates the open-waveguide character and associated radiation conditions inherent in the problem under consideration. The overall WGF approach is demonstrated in this paper by means of a variety of numerical results for two-dimensional open-waveguide termination, launching and junction problems.Comment: 16 Page

Windowed Green Function method for layered-media scattering

This paper introduces a new Windowed Green Function (WGF) method for the numerical integral-equation solution of problems of electromagnetic scattering by obstacles in presence of dielectric or conducting half-planes. The WGF method, which is based on use of smooth windowing functions and integral kernels that can be expressed directly in terms of the free-space Green function, does not require evaluation of expensive Sommerfeld integrals. The proposed approach is fast, accurate, flexible and easy to implement. In particular, straightforward modifications of existing (accelerated or unaccelerated) solvers suffice to incorporate the WGF capability. The mathematical basis of the method is simple: the method relies on a certain integral equation posed on the union of the boundary of the obstacle and a small flat section of the interface between the penetrable media. Numerical experiments demonstrate that both the near- and far-field errors resulting from the proposed approach decrease faster than any negative power of the window size. In the examples considered in this paper the proposed method is up to thousands of times faster, for a given accuracy, than a corresponding method based on the layer-Green-function.Comment: 17 page

Approximating the ground state of gapped quantum spin systems

We consider quantum spin systems defined on finite sets $V$ equipped with a metric. In typical examples, $V$ is a large, but finite subset of Z^d. For finite range Hamiltonians with uniformly bounded interaction terms and a unique, gapped ground state, we demonstrate a locality property of the corresponding ground state projector. In such systems, this ground state projector can be approximated by the product of observables with quantifiable supports. In fact, given any subset, X, of V the ground state projector can be approximated by the product of two projections, one supported on X and one supported on X^c, and a bounded observable supported on a boundary region in such a way that as the boundary region increases, the approximation becomes better. Such an approximation was useful in proving an area law in one dimension, and this result corresponds to a multi-dimensional analogue

Magnetic Discontinuities in Magnetohydrodynamic Turbulence and in the Solar Wind

Recent measurements of solar wind turbulence report the presence of intermittent, exponentially distributed angular discontinuities in the magnetic field. In this Letter, we study whether such discontinuities can be produced by magnetohydrodynamic (MHD) turbulence. We detect the discontinuities by measuring the fluctuations of the magnetic field direction, Delta theta, across fixed spatial increments Delta x in direct numerical simulations of MHD turbulence with an imposed uniform guide field B_0. A large region of the probability density function (pdf) for Delta theta is found to follow an exponential decay, proportional to exp(-Delta theta/theta_*), with characteristic angle theta_* ~ (14 deg) (b_rms/B_0)^0.65 for a broad range of guide-field strengths. We find that discontinuities observed in the solar wind can be reproduced by MHD turbulence with reasonable ratios of b_rms/B_0. We also observe an excess of small angular discontinuities when Delta x becomes small, possibly indicating an increasing statistical significance of dissipation-scale structures. The structure of the pdf in this case closely resembles the two-population pdf seen in the solar wind. We thus propose that strong discontinuities are associated with inertial-range MHD turbulence, while weak discontinuities emerge from near-dissipation-range turbulence. In addition, we find that the structure functions of the magnetic field direction exhibit anomalous scaling exponents, which indicates the existence of intermittent structures.Comment: To appear in Physical Review Letter

Where does the lactate come from? A rare cause of reversible inhibition of mitochondrial respiration

Biguanide poisoning is associated with lactic acidosis. The exact mechanism of biguanide-induced lactic acidosis is not well understood. In the previous issue of Critical Care, Protti and colleagues demonstrated that biguanide-induced lactic acidosis may be due in part to a reversible inhibition of mitochondrial respiration. Thus, in the absence of an antidote, increased drug elimination through dialysis is logical

University Collectives and the Detachment Discourse away from the Parliamentary Politics

O primeiro objetivo do trabalho é apresentar dados e o discurso dos coletivos universitários. Para tanto, foram realizadas dezesseis entrevistas qualitativas com todos os coletivos universitários da cidade de Teresina/PI, bem como análises das postagens de 170 coletivos universitários brasileiros com páginas na rede social digital Facebook. Após a análise crítica desses dados, problematizam-se a autonomia e as novidades dos coletivos, por vezes reclamadas por eles, por vezes atribuídas pela literatura. Os resultados empíricos mostram um distanciamento discursivo dos coletivos em relação à política partidária e parlamentar. Para entender tais discursos, o trabalho retoma dados do Latinobarômetro acerca da confiança no Congresso e nos partidos entre jovens estudantes brasileiros. A fim de se entender o aumento da desconfiança na política estatal, foram retomadas reflexões acerca das consequências do capitalismo neoliberal. As reflexões apresentadas permitem avançar na compreensão dos discursos de jovens universitários envolvidos em organizações políticas, sem perder de vista a relação desses posicionamentos com contextos sociais e teóricos mais amplos.The first objective of this work is to present data and the discourse of university collectives. For such, sixteen qualitative interviews were carried out with all university collectives from the city of Teresina, state of Piauí, Brazil, as well as analyses of Facebook posts of 170 Brazilian university collectives. After critical analysis of these data, autonomy and novelty issues of these collectives were discussed, sometimes claimed by them, other times assigned to them by the literature. Empirical results show a discoursive distancing of collectives as regards parliamentary and party politics. For understanding such discourses, this work recalls data from Latinobarómetro on the trust in the Congress and political parties among Brazilian young students. In order to understand the increase of distrust in state politics, reflections were made on the consequences of neoliberal capitalism. These allow us to move forward in comprehending the discourses of young university students involved in political organizations, not losing sight of these positions as regards broader social and theoretical contexts

Impact of observational uncertainties on universal scaling of MHD turbulence

Scaling exponents are the central quantitative prediction of theories of turbulence and in-situ satellite observations of the high Reynolds number solar wind flow have provided an extensive testbed of these. We propose a general, instrument independent method to estimate the uncertainty of velocity field fluctuations. We obtain the systematic shift that this uncertainty introduces into the observed spectral exponent. This shift is essential for the correct interpretation of observed scaling exponents. It is sufficient to explain the contradiction between spectral features of the Elsasser fields observed in the solar wind with both theoretical models and numerical simulations of Magnetohydrodynamic turbulence