2,557 research outputs found
Wave Chaos in Elastodynamic Cavity Scattering
The exact elastodynamic scattering theory is constructed to describe the
spectral properties of two- and more-cylindrical cavity systems, and compared
to an elastodynamic generalization of the semi-classical Gutzwiller unstable
periodic orbits formulas. In contrast to quantum mechanics, complex periodic
orbits associated with the surface Rayleigh waves dominate the low-frequency
spectrum, and already the two-cavity system displays chaotic features.Comment: 7 pages, 5 eps figures, latex (with epl.cls
Nearly cloaking the elastic wave fields
In this work, we develop a general mathematical framework on regularized
approximate cloaking of elastic waves governed by the Lam\'e system via the
approach of transformation elastodynamics. Our study is rather comprehensive.
We first provide a rigorous justification of the transformation elastodynamics.
Based on the blow-up-a-point construction, elastic material tensors for a
perfect cloak are derived and shown to possess singularities. In order to avoid
the singular structure, we propose to regularize the blow-up-a-point
construction to be the blow-up-a-small-region construction. However, it is
shown that without incorporating a suitable lossy layer, the regularized
construction would fail due to resonant inclusions. In order to defeat the
failure of the lossless construction, a properly designed lossy layer is
introduced into the regularized cloaking construction . We derive sharp
asymptotic estimates in assessing the cloaking performance. The proposed
cloaking scheme is capable of nearly cloaking an arbitrary content with a high
accuracy
Lubricant rheology applied to elastohydrodynamic lubrication
Viscosity measurements in a high pressure rheometer, elastohydrodynamic simulator studies (including the development of a temperature measuring technique), and analytical fluid modeling for elastohydrodynamic contacts are described. The more recent research which is described concerns infrared temperature measurements in elastohydrodynamic contacts and the exploration of the glassy state of lubricants. A correlation, of engineering significance, was made between transient surface temperature measurements and surface roughness profiles. Measurements of glass transitions of lubricants and the study of the effect of rate processes on materials lead to the conclusion that typical lubricants go into the glassy state as they pass through the contact region of typical elastohydrodynamic contacts
A hybrid T matrix/boundary element method for elastic wave scattering from a defect near a non-planar surface
The in-plane P-SV scattering of elastic waves bya defect and a close non-planar surface is considered. A hybrid T matrix/boundary element approach is used, where a boundary integral equation is used for the non-planar surface and the Greenâs tensor in this integral equation is chosen as the one for the defect and thus incorporates the transition
(T ) matrix of the defect. The integral equation is iscretized by the boundary element method in a standard way.
Also models of ultrasonic probes in transmission and reception are included. In the numerical examples the defect is for simplicity chosen as a circular cavity. This cavity is located close to a non-planar surface, which is planar except for a smooth transition between two planar parts. It is illustrated that the scattering by the cavity and the non-planar surface becomes quite complicated, and that shielding and masking may appear
Near-Surface Scattering From High Velocity Carbonates In West Texas
Seismic data acquired directly over near-surface limestone formations are commonly
observed to be of inferior quality. A possible cause for this degradation is scattering
in the near-subsurface by, e.g., the weathering layer, rough free-surface topography,
or heterogeneities such as cavities or clusters of vugs. We applied different numerical
scattering schemes to study the effects of each of these three scattering mechanisms.
For a particular dataset acquired in West Texas, we find that a weathering layer is
the dominant cause of noise on records acquired in valleys. However on mesas, nearsubsurface heterogeneity is the primary cause of scattered wave-energy. Topography
turned out to be of only secondary importance.
As additional attributes, we use energy-density and energy-flux vectors to study
the frequency dependence of the different scattering models. These attributes allow
us to study where energy concentrates and in which direction it flows. For example,
we observed that near sub-surface heterogeneities build up waveguides which efficiently
trap seismic energy near the surface.Massachusetts Institute of Technology. Borehole Acoustics and Logging Consortiu
A time-domain finite element boundary integral approach for elastic wave scattering
The response of complex scatterers, such as rough or branched cracks, to incident elastic waves is required in many areas of industrial importance such as those in non-destructive evaluation and related fields; we develop an approach to generate accurate and rapid simulations. To achieve this we develop, in the time domain, an implementation to efficiently couple the finite element (FE) method within a small local region, and the boundary integral (BI) globally. The FE explicit scheme is run in a local box to compute the surface displacement of the scatterer, by giving forcing signals to excitation nodes, which can lie on the scatterer itself. The required input forces on the excitation nodes are obtained with a reformulated FE equation, according to the incident displacement field. The surface displacements computed by the local FE are then projected, through time-domain BI formulae, to calculate the scattering signals with different modes. This new method yields huge improvements in the efficiency of FE simulations for scattering from complex scatterers. We present results using different shapes and boundary conditions, all simulated using this approach in both 2D and 3D, and then compare with full FE models and theoretical solutions to demonstrate the efficiency and accuracy of this numerical approach
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