1,416 research outputs found
Loss Compensation in Time-Dependent Elastic Metamaterials
Materials with properties that are modulated in time are known to display
wave phenomena showing energy increasing with time, with the rate mediated by
the modulation. Until now there has been no accounting for material
dissipation, which clearly counteracts energy growth. This paper provides an
exact expression for the amplitude of elastic or acoustic waves propagating in
lossy materials with properties that are periodically modulated in time. It is
found that these materials can support a special propagation regime in which
waves travel at constant amplitude, with temporal modulation compensating for
the normal energy dissipation. We derive a general condition under which
amplification due to time-dependent properties offsets the material
dissipation. This identity relates band-gap properties associated with the
temporal modulation and the average of the viscosity coefficient, thereby
providing a simple recipe for the design of loss-compensated mechanical
metamaterials
Employing pre-stress to generate finite cloaks for antiplane elastic waves
It is shown that nonlinear elastic pre-stress of neo-Hookean hyperelastic
materials can be used as a mechanism to generate finite cloaks and thus render
objects near-invisible to incoming antiplane elastic waves. This approach
appears to negate the requirement for special cloaking metamaterials with
inhomogeneous and anisotropic material properties in this case. These
properties are induced naturally by virtue of the pre-stress. This appears to
provide a mechanism for broadband cloaking since dispersive effects due to
metamaterial microstructure will not arise.Comment: 4 pages, 2 figure
Asphaltene detection using Surface Enhanced Raman Scattering (SERS)
Peer reviewedPostprin
The role of passive surveillance and citizen science in plant health
The early detection of plant pests and diseases is vital to the success of any eradication or control programme, but the resources for surveillance are often limited. Plant health authorities can however make use of observations from individuals and stakeholder groups who are monitoring for signs of ill health. Volunteered data is most often discussed in relation to citizen science groups, however these groups are only part of a wider network of professional agents, land-users and owners who can all contribute to significantly increase surveillance efforts through “passive surveillance”. These ad-hoc reports represent chance observations by individuals who may not necessarily be looking for signs of pests and diseases when they are discovered. Passive surveillance contributes vital observations in support of national and international surveillance programs, detecting potentially unknown issues in the wider landscape, beyond points of entry and the plant trade. This review sets out to describe various forms of passive surveillance, identify analytical methods that can be applied to these “messy” unstructured data, and indicate how new programs can be established and maintained. Case studies discuss two tree health projects from Great Britain (TreeAlert and Observatree) to illustrate the challenges and successes of existing passive surveillance programmes. When analysing passive surveillance reports it is important to understand the observers’ probability to detect and report each plant health issue, which will vary depending on how distinctive the symptoms are and the experience of the observer. It is also vital to assess how representative the reports are and whether they occur more frequently in certain locations. Methods are increasingly available to predict species distributions from large datasets, but more work is needed to understand how these apply to rare events such as new introductions. One solution for general surveillance is to develop and maintain a network of tree health volunteers, but this requires a large investment in training, feedback and engagement to maintain motivation. There are already many working examples of passive surveillance programmes and the suite of options to interpret the resulting datasets is growing rapidly
Information Storage and Retrieval for Probe Storage using Optical Diffraction Patterns
A novel method for fast information retrieval from a probe storage device is
considered. It is shown that information can be stored and retrieved using the
optical diffraction patterns obtained by the illumination of a large array of
cantilevers by a monochromatic light source. In thermo-mechanical probe
storage, the information is stored as a sequence of indentations on the polymer
medium. To retrieve the information, the array of probes is actuated by
applying a bending force to the cantilevers. Probes positioned over
indentations experience deflection by the depth of the indentation, probes over
the flat media remain un-deflected. Thus the array of actuated probes can be
viewed as an irregular optical grating, which creates a data-dependent
diffraction pattern when illuminated by laser light. We develop a low
complexity modulation scheme, which allows the extraction of information stored
in the pattern of indentations on the media from Fourier coefficients of the
intensity of the diffraction pattern. We then derive a low-complexity maximum
likelihood sequence detection algorithm for retrieving the user information
from the Fourier coefficients. The derivation of both the modulation and the
detection schemes is based on the Fraunhofer formula for data-dependent
diffraction patterns. We show that for as long as the Fresnel number F<0.1, the
optimal channel detector derived from Fraunhofer diffraction theory does not
suffer any significant performance degradation.Comment: 14 pages, 11 figures. Version 2: minor misprints corrected,
experimental section expande
Consequences of spontaneous reconnection at a two-dimensional non-force-free current layer
Magnetic neutral points, where the magnitude of the magnetic field vanishes
locally, are potential locations for energy conversion in the solar corona. The
fact that the magnetic field is identically zero at these points suggests that
for the study of current sheet formation and of any subsequent resistive
dissipation phase, a finite beta plasma should be considered, rather than
neglecting the plasma pressure as has often been the case in the past. The
rapid dissipation of a finite current layer in non-force-free equilibrium is
investigated numerically, after the sudden onset of an anomalous resistivity.
The aim of this study is to determine how the energy is redistributed during
the initial diffusion phase, and what is the nature of the outward transmission
of information and energy. The resistivity rapidly diffuses the current at the
null point. The presence of a plasma pressure allows the vast majority of the
free energy to be transferred into internal energy. Most of the converted
energy is used in direct heating of the surrounding plasma, and only about 3%
is converted into kinetic energy, causing a perturbation in the magnetic field
and the plasma which propagates away from the null at the local fast
magnetoacoustic speed. The propagating pulses show a complex structure due to
the highly non-uniform initial state. It is shown that this perturbation
carries no net current as it propagates away from the null. The fact that,
under the assumptions taken in this paper, most of the magnetic energy released
in the reconnection converts internal energy of the plasma, may be highly
important for the chromospheric and coronal heating problem
Source amplitudes for active exterior cloaking
The active cloak comprises a discrete set of multipole sources that
destructively interfere with an incident time harmonic scalar wave to produce
zero total field over a finite spatial region. For a given number of sources
and their positions in two dimensions it is shown that the multipole amplitudes
can be expressed as infinite sums of the coefficients of the incident wave
decomposed into regular Bessel functions. The field generated by the active
sources vanishes in the infinite region exterior to a set of circles defined by
the relative positions of the sources. The results provide a direct solution to
the inverse problem of determining the source amplitudes. They also define a
broad class of non-radiating discrete sources.Comment: 21 pages, 17 figure
Quantifying the contribution of sediment compaction to late Holocene salt-marsh sea-level reconstructions, North Carolina, USA
Salt-marsh sediments provide accurate and precise reconstructions of late Holocene relative sea-level changes. However, compaction of salt-marsh stratigraphies can cause post-depositional lowering (PDL) of the samples used to reconstruct sea level, creating an estimation of former sea level that is too low and a rate of rise that is too great. We estimated the contribution of compaction to late Holocene sea-level trends reconstructed at Tump Point, North Carolina, USA. We used a geotechnical model that was empirically calibrated by performing tests on surface sediments from modern depositional environments analogous to those encountered in the sediment core. The model generated depth-specific estimates of PDL, allowing samples to be returned to their depositional altitudes. After removing an estimate of land-level change, error-in-variables changepoint analysis of the decompacted and original sea-level reconstructions identified three trends. Compaction did not generate artificial sea-level trends and cannot be invoked as a causal mechanism for the features in the Tump Point record. The maximum relative contribution of compaction to reconstructed sea-level change was 12%. The decompacted sea-level record shows 1.71 mm yr−1 of rise since AD 1845
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