2,221 research outputs found

    Biharmonic Split Ring Resonator Metamaterial: Artificially dispersive effective density in thin periodically perforated plates

    Full text link
    We present in this paper a theoretical and numerical analysis of bending waves localized on the boundary of a platonic crystal whose building blocks are split ring resonators (SRR). We first derive the homogenized parameters of the structured plate using a three-scale asymptotic expansion in the linearized biharmonic equation. In the limit when the wavelength of the bending wave is much larger than the typical heterogeneity size of the platonic crystal, we show that it behaves as an artificial plate with an anisotropic effective Young modulus and a dispersive effective mass density. We then analyze dispersion diagrams associated with bending waves propagating within an infinite array of SRR, for which eigen-solutions are sought in the form of Floquet-Bloch waves. We finally demonstrate that this structure displays the hallmarks of All-Angle-Negative-Refraction(AANR) and it leads to superlensing and ultrarefraction effects, interpreted thanks to our homogenization model as a consequence of negative and vanishing effective density, respectively.Comment: 17 pages, 6 figure

    Practical lessons for winning support for radical transport projects

    Get PDF
    This paper proposes that while many plans and solutions to the transport problems of the 21st Century have been mooted, very few have succeeded in significantly improving the situation within Europe. It is suggested that many schemes face problems at the project implementation stage due to adverse public and/or political reaction. This paper incorporates a series of vignettes, several of which are based on indepth interviews with practitioners directly involved in the implementation of 'radical'transport schemes from around the world in an attempt to draw lessons as to how they overcame this, not least in terms of how the implementation of alternative strategies by European policy-makers could be shaped and adopted world-wide

    The Gas Temperature of Starless Cores in Perseus

    Get PDF
    In this paper we study the determinants of starless core temperatures in the Perseus molecular cloud. We use NH3 (1,1) and (2,2) observations to derive core temperatures (T_kin) and data from the COMPLETE Survey of Star Forming Regions and the c2d Spitzer Legacy Survey for observations of the other core and molecular cloud properties. The kinetic temperature distribution probed by NH3 is in the fairly narrow range of 9 - 15 K. We find that cores within the clusters IC348 and NGC1333 are significantly warmer than "field" starless cores, and T_kin is higher within regions of larger extinction-derived column density. Starless cores in the field are warmer when they are closer to class O/I protostars, but this effect is not seen for those cores in clusters. For field starless cores, T_kin is higher in regions in which the 13CO linewidth and the 1.1mm flux from the core are larger, and T_kin is lower when the the peak column density within the core and average volume density of the core are larger. There is no correlation between T_kin and 13CO linewidth, 1.1mm flux, density or peak column density for those cores in clusters. The temperature of the cloud material along the line of sight to the core, as measured by CO or far-infrared emission from dust, is positively correlated with core temperature when considering the collection of cores in the field and in clusters, but this effect is not apparent when the two subsamples of cores are considered separately.Comment: Accepted to ApJ; 13 pages, including 3 tables and three figure

    Quasi-TEM modes in rectangular waveguides: a study based on the properties of PMC and hard surfaces

    Full text link
    Hard surfaces or magnetic surfaces can be used to propagate quasi-TEM modes inside closed waveguides. The interesting feature of these modes is an almost uniform field distribution inside the waveguide. But the mechanisms governing how these surfaces act, how they can be characterized, and further how the modes propagate are not detailed in the literature. In this paper, we try to answer these questions. We give some basic rules that govern the propagation of the quasi-TEM modes, and show that many of their characteristics (i.e. their dispersion curves) can be deduced from the simple analysis of the reflection properties of the involved surfaces

    Marketing and the British bus industry

    Get PDF

    The Protostellar Mass Function

    Full text link
    The protostellar mass function (PMF) is the Present-Day Mass Function of the protostars in a region of star formation. It is determined by the initial mass function weighted by the accretion time. The PMF thus depends on the accretion history of protostars and in principle provides a powerful tool for observationally distinguishing different protostellar accretion models. We consider three basic models here: the Isothermal Sphere model (Shu 1977), the Turbulent Core model (McKee & Tan 2003), and an approximate representation of the Competitive Accretion model (Bonnell et al. 1997, 2001a). We also consider modified versions of these accretion models, in which the accretion rate tapers off linearly in time. Finally, we allow for an overall acceleration in the rate of star formation. At present, it is not possible to directly determine the PMF since protostellar masses are not currently measurable. We carry out an approximate comparison of predicted PMFs with observation by using the theory to infer the conditions in the ambient medium in several star-forming regions. Tapered and accelerating models generally agree better with observed star-formation times than models without tapering or acceleration, but uncertainties in the accretion models and in the observations do not allow one to rule out any of the proposed models at present. The PMF is essential for the calculation of the Protostellar Luminosity Function, however, and this enables stronger conclusions to be drawn (Offner & McKee 2010).Comment: 16 pages, 8 figures, published in Ap

    A NEW GEOMETRIC MODEL AND METHODOLOGY FOR UNDERSTANDING PARSIMONIOUS SEVENTH-SONORITY PITCH-CLASS SPACE

    Get PDF
    Parsimonious voice leading is a term, first used by Richard Cohn, to describe non-diatonic motion among triads that will preserve as many common tones as possible, while limiting the distance traveled by the voice that does move to a tone or, better yet, a semitone. Some scholars have applied these principles to seventh chords, laying the groundwork for this study, which strives toward a reasonably comprehensive, usable model for musical analysis. Rather than emphasizing mathematical proofs, as a number of approaches have done, this study relies on two- and three-dimensional geometric visualizations and spatial analogies to describe pitch-class and harmonic relationships. These geometric realizations are based on the organization of the neo-Riemannian Tonnetz, but they expand and apply the organizational principles of the Tonnetz to seventh sonorities. It allows for the descriptive “mapping” or prescriptive “navigation” of harmonic paths through a defined space. The viability of the theoretical model is examined in analyses of passages from the repertoire of Frédéric Chopin. These passages exhibit a harmonic syntax that is often difficult to analyze as anything other than “tonally unstable” or “transitional.” This study seeks to analyze these passages in terms of what they are, rather than what they are not
    • …
    corecore