8,733 research outputs found
Acoustic metafluids made from three acoustic fluids
Significant reduction in target strength and radiation signature can be
achieved by surrounding an object with multiple concentric layers comprised of
three acoustic fluids. The idea is to make a finely layered shell with the
thickness of each layer defined by a unique transformation rule. The shell has
the effect of steering incident acoustic energy around the structure, and
conversely, reducing the radiation strength. The overall effectiveness and the
precise form of the layering depends upon the densities and compressibilities
of the three fluids. Nearly optimal results are obtained if one fluid has
density equal to the background fluid, while the other two densities are much
greater and much less than the background values. Optimal choices for the
compressibilities are also found. Simulations in 2D and 3D illustrate
effectiveness of the three fluid shell. The limited range of acoustic
metafluids that are possible using only two fluid constituents is also
discussed.Comment: 12 pages, 11 figure
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
Faxen relations in solids - a generalized approach to particle motion in elasticity and viscoelasticity
A movable inclusion in an elastic material oscillates as a rigid body with
six degrees of freedom. Displacement/rotation and force/moment tensors which
express the motion of the inclusion in terms of the displacement and force at
arbitrary exterior points are introduced. Using reciprocity arguments two
general identities are derived relating these tensors. Applications of the
identities to spherical particles provide several new results, including simple
expressions for the force and moment on the particle due to plane wave
excitation.Comment: 11 pages, 4 figure
A Large Blue Shift of the Biexciton State in Tellurium Doped CdSe Colloidal Quantum Dots
The exciton-exciton interaction energy of Tellurium doped CdSe colloidal
quantum dots is experimentally investigated. The dots exhibit a strong Coulomb
repulsion between the two excitons, which results in a huge measured biexciton
blue shift of up to 300 meV. Such a strong Coulomb repulsion implies a very
narrow hole wave function localized around the defect, which is manifested by a
large Stokes shift. Moreover, we show that the biexciton blue shift increases
linearly with the Stokes shift. This result is highly relevant for the use of
colloidal QDs as optical gain media, where a large biexciton blue shift is
required to obtain gain in the single exciton regime.Comment: 9 pages, 4 figure
Cluster analysis of midlatitude oceanic cloud regimes: mean properties and temperature sensitivity
Clouds play an important role in the climate system by reducing the amount of shortwave radiation reaching the surface and the amount of longwave radiation escaping to space. Accurate simulation of clouds in computer models remains elusive, however, pointing to a lack of understanding of the connection between large-scale dynamics and cloud properties. This study uses a k-means clustering algorithm to group 21 years of satellite cloud data over midlatitude oceans into seven clusters, and demonstrates that the cloud clusters are associated with distinct large-scale dynamical conditions. Three clusters correspond to low-level cloud regimes with different cloud fraction and cumuliform or stratiform characteristics, but all occur under large-scale descent and a relatively dry free troposphere. Three clusters correspond to vertically extensive cloud regimes with tops in the middle or upper troposphere, and they differ according to the strength of large-scale ascent and enhancement of tropospheric temperature and humidity. The final cluster is associated with a lower troposphere that is dry and an upper troposphere that is moist and experiencing weak ascent and horizontal moist advection. <br><br> Since the present balance of reflection of shortwave and absorption of longwave radiation by clouds could change as the atmosphere warms from increasing anthropogenic greenhouse gases, we must also better understand how increasing temperature modifies cloud and radiative properties. We therefore undertake an observational analysis of how midlatitude oceanic clouds change with temperature when dynamical processes are held constant (i.e., partial derivative with respect to temperature). For each of the seven cloud regimes, we examine the difference in cloud and radiative properties between warm and cold subsets. To avoid misinterpreting a cloud response to large-scale dynamical forcing as a cloud response to temperature, we require horizontal and vertical temperature advection in the warm and cold subsets to have near-median values in three layers of the troposphere. Across all of the seven clusters, we find that cloud fraction is smaller and cloud optical thickness is mostly larger for the warm subset. Cloud-top pressure is higher for the three low-level cloud regimes and lower for the cirrus regime. The net upwelling radiation flux at the top of the atmosphere is larger for the warm subset in every cluster except cirrus, and larger when averaged over all clusters. This implies that the direct response of midlatitude oceanic clouds to increasing temperature acts as a negative feedback on the climate system. Note that the cloud response to atmospheric dynamical changes produced by global warming, which we do not consider in this study, may differ, and the total cloud feedback may be positive
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