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Radiation Damping (radiation + damping)

Distribution by Scientific Domains
Engineering80%

Selected Abstracts

Visualizing feedback-enhanced contrast in magnetic resonance imaging

CONCEPTS IN MAGNETIC RESONANCE, Issue 6 2007
Susie Y. Huang
Abstract A new approach to magnetic resonance imaging (MRI) contrast enhancement has recently been developed that exploits nonlinear feedback interactions to amplify contrast arising from small variations in the underlying MRI parameters. A unified framework for understanding feedback-enhanced contrast is presented here based on the concepts of instability and positive feedback. The specific mechanisms governing contrast enhancement under the feedback interactions of radiation damping, the distant dipolar field, and their joint effect are elucidated through numerical simulations illustrating the involved spin dynamics. Experimental demonstrations of feedback-enhanced contrast are shown on samples of in vitro human brain tissue, and applications to improving lesion detection in disease states such as epilepsy and cancer are discussed. © 2007 Wiley Periodicals, Inc. Concepts Magn Reson Part A 30A: 378,393, 2007. [source]

Influence of dynamic soil,structure interaction on the nonlinear response and seismic reliability of multistorey systems

EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 3 2007
Armando Bárcena
Abstract A set of reinforced concrete structures with gravitational loads and mechanical properties (strength and stiffness) representative of systems designed for earthquake resistance in accordance with current criteria and methods is selected to study the influence of dynamic soil,structure interaction on seismic response, ductility demands and reliability levels. The buildings are considered located at soft soil sites in the Valley of Mexico and subjected to ground motion time histories simulated in accordance with characteristic parameters of the maximum probable earthquake likely to occur during the system's expected life. For the near-resonance condition the effects of soil,structure interaction on the ductility demands depend mainly on radiation damping. According to the geometry of the structures studied this damping is strongly correlated with the aspect ratio, obtained by dividing the building height by its width. In this way, for structures with aspect ratio greater than 1.4 the storey and global ductility demands increase with respect to those obtained with the same structures but on rigid base, while for structures with aspect ratio less than 1.4 the ductility demands decrease with respect to those for the structures on rigid base. For the cases when the fundamental period of the structure has values very different from the dominant ground period, soil,structure interaction leads in all cases to a reduction of the ductility demands, independently of the aspect ratio. The reliability index , is obtained as a function of the base shear ratio and of the seismic intensity acting on the nonlinear systems subjected to the simulated motions. The resulting reliability functions are very similar for systems on rigid or on flexible foundation, provided that in the latter case the base rotation and the lateral displacement are removed from the total response of the system. Copyright © 2006 John Wiley & Sons, Ltd. [source]

Dynamic dam,reservoir,interaction , treatment of radiation damping by the "Mixed,Variables,Technique"

PROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2003
C. Trinks Dipl.-Ing.
In this paper, a method for the consistent description of a fluid channel of infinite extent in a fully coupled time,domain dam,reservoir interaction analysis is proposed. The method is based on an analytical solution with respect to the unbounded dimensions of the reservoir. Thus, an accurate representation of radiation damping is achieved. [source]

Fractional calculus applied to radiation damping

PROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2003
C. Trinks Dipl.-Ing.
Separate treatment of the low, and high,frequency part of the dynamic stiffness is essential when approximating the latter in the frequency,domain. In this paper, a doubly,asymptotic rational approximation of the low,frequency part is combined with an analytical interpretation of the asymptotic part leading to a system of fractional differential equations to represent the force,displacement relationship. Here, an analogy between fading memory of viscoelastic materials and radiation damping of unbounded domains is visible. [source]