2,268 research outputs found
Fluid-loaded metasurfaces
We consider wave propagation along fluid-loaded structures which take the
form of an elastic plate augmented by an array of resonators forming a
metasurface, that is, a surface structured with sub-wavelength resonators. Such
surfaces have had considerable recent success for the control of wave
propagation in electromagnetism and acoustics, by combining the vision of
sub-wavelength wave manipulation, with the design, fabrication and size
advantages associated with surface excitation. We explore one aspect of recent
interest in this field: graded metasurfaces, but within the context of
fluid-loaded structures.
Graded metasurfaces allow for selective spatial frequency separation and are
often referred to as exhibiting rainbow trapping. Experiments, and theory, have
been developed for acoustic, electromagnetic, and even elastic, rainbow devices
but this has not been approached for fluid-loaded structures that support
surface waves coupled with the acoustic field in a bulk fluid. This surface
wave, coupled with the fluid, can be used to create an additional effect by
designing a metasurface to mode convert from surface to bulk waves. We
demonstrate that sub-wavelength control is possible and that one can create
both rainbow trapping and mode conversion phenomena for a fluid-loaded elastic
plate model.Comment: 13 pages, 10 figure
Linearized dynamical model for the NASA/IEEE SCOLE configuration
The linearized equation of motion for the NASA/IEEE SCOLE configuration are developed. The derivation is based on the method of Lagrange and the equations are assembled into matrix second order form
Model reduction for discrete bilinear systems
A model reduction method for discrete bilinear systems is developed which matches q sets of Volterra and covariance parameters. These parameters are shown to represent both deterministic and stochastic attributes of the discrete bilinear system. A reduced order model which matches these q sets of parameters is defined to be a q-Volterra covariance equivalent realization (q-Volterra COVER). An algorithm is presented which constructs a class of q-Volterra COVERs parameterized by solutions to a Hermitian, quadratic, matrix equation. The algorithm is applied to a bilinear model of a robot manipulator
Childhood and the politics of scale: Descaling children's geographies?
This is the post-print version of the final published paper that is available from the link below. Copyright @ 2008 SAGE Publications.The past decade has witnessed a resurgence of interest in the geographies of children's lives, and particularly in engaging the voices and activities of young people in geographical research. Much of this growing body of scholarship is characterized by a very parochial locus of interest — the neighbourhood, playground, shopping mall or journey to school. In this paper I explore some of the roots of children's geographies' preoccupation with the micro-scale and argue that it limits the relevance of research, both politically and to other areas of geography. In order to widen the scope of children's geographies, some scholars have engaged with developments in the theorization of scale. I present these arguments but also point to their limitations. As an alternative, I propose that the notion of a flat ontology might help overcome some difficulties around scalar thinking, and provide a useful means of conceptualizing sociospatiality in material and non-hierarchical terms. Bringing together flat ontology and work in children's geographies on embodied subjectivity, I argue that it is important to examine the nature and limits of children's spaces of perception and action. While these spaces are not simply `local', they seldom afford children opportunities to comment on, or intervene in, the events, processes and decisions that shape their own lives. The implications for the substance and method of children's geographies and for geographical work on scale are considered
A new binding geometry for an ortho-xylylene-linked bis(NHC)cyclophane: a ruthenium(II) complex with a chelating (g1-NHC)2:g6-arene ligand
Using two different reaction procedures, a Ru(II) complex has been isolated that contains an ortho-xylylene-linked bis(NHC)cyclophane (NHC = N-heterocyclic carbene) that binds to the Ru centre through two carbene carbons and one of the arene rings in an η6-mode
Gamma ray monitoring of a AGN and galactic black hole candidates by the Compton Gamma Ray Observatory
The Compton Gamma-Ray Observatory's Burst and Transient Source Experiment (BATSE) has a powerful capability to provide nearly uninterrupted monitoring in the 25 keV-10 MeV range of both active galactic nuclei (AGN) and galactic black hole candidates (GBHC) such as Cygnus X-1, using the occultation of cosmic sources by the Earth. Since the Crab is detected by the BATSE Large Area Detectors with roughly 25(sigma) significance in the 15-125 keV range in a single rise or set, a variation by a factor of two of a source having one-tenth the strength of Cygnus X-1 should be detectable within a day. Methods of modeling the background are discussed which will increase the accuracy, sensitivity, and reliability of the results beyond those obtainable from a linear background fit with a single rise or set discontinuity
Lattice dynamics and vibrational spectra of the orthorhombic, tetragonal and cubic phases of methylammonium lead iodide
The hybrid halide perovskite CH3NH3PbI3 exhibits a complex structural
behaviour, with successive transitions between orthorhombic, tetragonal and
cubic polymorphs at ca. 165 K and 327 K. Herein we report first-principles
lattice dynamics (phonon spectrum) for each phase of CH3NH3PbI3. The
equilibrium structures compare well to solutions of temperature-dependent
powder neutron diffraction. By following the normal modes we calculate infrared
and Raman intensities of the vibrations, and compare them to the measurement of
a single crystal where the Raman laser is controlled to avoid degradation of
the sample. Despite a clear separation in energy between low frequency modes
associated with the inorganic PbI3 network and high-frequency modes of the
organic CH3NH3+ cation, significant coupling between them is found, which
emphasises the interplay between molecular orientation and the corner-sharing
octahedral networks in the structural transformations. Soft modes are found at
the boundary of the Brillouin zone of the cubic phase, consistent with
displacive instabilities and anharmonicity involving tilting of the PbI6
octahedra around room temperature.Comment: 9 pages, 4 figure
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