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Refraction characteristics of phononic crystals
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作者 Sia Nemat-Nasser 《Acta Mechanica Sinica》 SCIE EI CAS CSCD 2015年第4期481-493,共13页
Some of the most interesting refraction prop- erties of phononic crystals are revealed by examining the anti-plane shear waves in doubly periodic elastic composites with unit cells containing rectangular and/or ellipt... Some of the most interesting refraction prop- erties of phononic crystals are revealed by examining the anti-plane shear waves in doubly periodic elastic composites with unit cells containing rectangular and/or elliptical multi- inclusions. The corresponding band structure, group velocity, and energy-flux vector are calculated using a powerful mixed variational method that accurately and efficiently yields all the field quantities over multiple frequency pass-bands. The background matrix and the inclusions can be anisotropic, each having distinct elastic moduli and mass densities. Equifrequency contours and energy-flux vectors are read- ily calculated as functions of the wave-vector components. By superimposing the energy-flux vectors on equifrequency contours in the plane of the wave-vector components, and supplementing this with a three-dimensional graph of the corresponding frequency surface, a wealth of information is extracted essentially at a glance. This way it is shown that a composite with even a simple square unit cell con- taining a central circular inclusion can display negative or positive energy and phase velocity refractions, or simply performs a harmonic vibration (standing wave), depending on the frequency and the wave-vector. Moreover, that the same composite when interfaced with a suitable homoge- neous solid can display: (1) negative refraction with negative phase velocity refraction; (2) negative refraction with pos- itive phase velocity refraction; (3) positive refraction with negative phase velocity refraction; (4) positive refraction with positive phase velocity refraction; or even (5) completereflection with no energy transmission, depending on the fre- quency, and direction and the wavelength of the plane-wave that is incident from the homogeneous solid to the interface. For elliptical and rectangular inclusion geometries, analyti- cal expressions are given for the key calculation quantities. Expressions for displacement, velocity, linear momentum, strain, and stress components, as well as the energy-flux and group velocity components are given in series form. The general results are illustrated for rectangular unit cells, one with two and the other with four inclusions, although any number of inclusions can be considered. The energy-flux and the accompanying phase velocity refractions at an inter- face with a homogeneous solid are demonstrated. Finally, by comparing the results of the present solution method with those obtained using the Rayleigh quotient and, for the lay- ered case, with the exact solutions, the remarkable accuracy and the convergence rate of the present solution method are demonstrated. 展开更多
关键词 Doubly periodic composites - Phononiccrystals Band structure Group and energy-flux vectors
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THE ACTION OF HARMONIC CONDITION ON THE STUDY OF GRAVITATIONAL WAVES
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作者 刘钧 《Science China Mathematics》 SCIE 1992年第2期186-199,共14页
To study the exact solutions of plane gravitational waves in the harmonic condition, wefirst give the definitions and the operational methods of Poisson and Lie operative symbolof the indices, respectively. With these... To study the exact solutions of plane gravitational waves in the harmonic condition, wefirst give the definitions and the operational methods of Poisson and Lie operative symbolof the indices, respectively. With these one may derive easily thc algebraic relation of theaffine connection, curvature tensor aud Ricci tensor. Thus we prove: (i) Einstein field equa-tions only have one independent equation. We obtain its excat solutions which contain Ein-stein’s Solutions for weak fields as approximations. (ii) The scalar curvature may be express-ed by the second-order partial differential equation of a component of the metric tensor.(iii) The exact solution has a relativistic action term. If the term is zero, then there are nogravitational waves in the vacuum. (iv) The exact solution contains three components of themetric tensor. If the components have singularities, so does the solution. The physical inter-pretations of the solutions are studied in detail. This paper may be regarded as a generaliza-tion of Ref. [4]. 展开更多
关键词 GRAVITATIONAL waves weak field approximation energy-flux density RELATIVISTIC action TERM indices operational SYMBOLS EXACT solutions.
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