2,284 research outputs found
Biharmonic Split Ring Resonator Metamaterial: Artificially dispersive effective density in thin periodically perforated plates
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
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
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
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
The Protostellar Mass Function
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
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
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