15 resultados para Nonlinear structures

em Chinese Academy of Sciences Institutional Repositories Grid Portal


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We present a systematical numerical study of the effects of adiabatic exponent gamma on Richtmyer-Meshkov instability (RMI) driven by cylindrical shock waves, based on the gamma model for the multi-component problems and numerical simulation with high-order and high-resolution method for compressible Euler equations. The results show that the RMI of different gamma across the interface exhibits different evolution features with the case of single gamma. Moreover, the large gamma can hold back the development of nonlinear structures, such as spikes and bubbles.

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A high order difference scheme is used to simulate the spatially developing compressible axisymmetric jet. The results show that the Kelvin-Helmholtz instability appears first when the jet loses its stability, and then with development of jet the increase in nonlinear effects leads to the secondary instability and the formation of the streamwise vortices. The evolution of the three-dimensional coherent structure is presented. The computed results verify that in axisymmetric jet the secondary instability and formation of the streamwise vortices are the important physical mechanism of enhancing the flow mixing and transition occurring.

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In this paper, a logarithmic expression to describe the residual strength degradation process is developed in order to fatigue test results for normalized carbon steel. The definition and expression of fatigue damage due to symmetrical stress with a constant amplitude are also given. The expression of fatigue damage can also explain the nonlinear properties of fatigue damage. Furthermore, the fatigue damage of structures under random stress is analyzed, and an iterative formula to describe the fatigue damage process is deduced. Finally, an approximate method for evaluating the fatigue life of structures under repeated random stress blocking is presented through various calculation examples.

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The paper revisits a simple beam model used by Chater et al. (1983, Proc. IUTAM Symp. Collapse, Cambridge University Press) to examine the dynamics of propagating buckles on it. It was found that, if a buckle is initiated at a constant pressure higher than the propagation pressure of the model (P-p), the buckle accelerates and gradually reaches a constant velocity which depends upon the pressure, while if it is initiated at P-p, the buckle propagates at a velocity which depends upon the initial imperfection. The causes for the difference are also investigated.

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In this paper particular investigation is directed towards the combined effects of horizontal and vertical motions of real earthquakes to structures resting on sliding base. A simplified method is presented to treat the nonlinear effects of time dependent frictional force of the sliding base as a function of the vertical reaction produced by the foundation. As an example, the El Centro 1940 earthquake record is used on a structural model to show the structural responses due to a sliding base with different frictional and stiffness characteristics. The study shows that vertical ground motion does affect both the superstructure response and the base sliding displacement. Nevertheless, the sliding base isolator is shown to be effective for the reduction of seismic response of a superstructure.

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In laser applications, the size of the focus spot can be reduced beyond the diffraction limit with a thin film of strong nonlinear optical Kerr effect. We present a concise theoretical simulation of the device. The origin of the super-resolution is found to be mainly from the reshaping effect due to the strongly nonlinear refraction mediated multi-interference inside the thin film. In addition, both diffraction and self-focusing effects have been explored and found negligible for highly refractive and ultrathin films in comparison with the reshaping effect. Finally, the theoretic model has been verified in experiments with single Ge2Sb2Te5 film and SiN/Si/SiN/Ge2Sb2Te2 multilayer structures. (c) 2006 American Institute of Physics.

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The refractive nonlinearities of InAs/GaAs quantum dots under a dc electric field at photon energies above its band gap energy have been studied using the reflection Z-scan technique. The effect of the dc electric field on the nonlinear response of InAs/GaAs quantum dots showed similar linear and quadratic electro-optic effects as in the linear response regime at low fields. This implies that the electro-optic effect in the nonlinear regime is analogous to the response in the linear regime for semiconductor quantum dots. Our experimental results show the potential for voltage tunability in InAs quantum dot-based nonlinear electro-optic devices.

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We investigate the electronic structures of the inhomogeneous quantum dots within the framework of the effective mass theory. The results show that the energies of electron and hole states depend sensitively on the relative magnitude 77 of the core radius to the capped quantum dot radius. The spatial distribution of the electrons and holes vary significantly when the ratio eta changes. A quantum-confinement-driven type-II-type-I transition is found in GaAs/AlxGa1-xAs-capped quantum dot structures. The phase diagram is obtained for different capped quantum dot radii. The ground-state exciton binding energy shows a highly nonlinear dependence on the innner structures of inhomogeneous quantum dots, which originates from the redistribution of the electron and hole wave functions.

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The linear electro-optic (Pockels) effect of wurtzite gallium nitride (GaN) films and six-period GaN/AlxGa1-xN superlattices with different quantum structures were demonstrated by a polarization-maintaining fiber-optical Mach-Zehnder interferometer system with an incident light wavelength of 1.55 mu m. The samples were prepared on (0001) sapphire substrate by low-temperature metalorganic chemical vapor deposition (MOCVD). The measured coefficients of the GaN/AlxGa1-xN superlattices are much larger than those of bulk material. Taking advantage of the strong field localization due to resonances, GaN/AlxGa1-xN SL can be proposed to engineer the nonlinear responses.

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K(4)Ln(2)(CO3)(3)F-4 (Ln=Pr, Nd, Sm, Eu, Gd) is a special type of frequency doubling compound, whose crystal structure exhibits a scarcity of fluorine ions. This leads to two different coordination polyhedrons in the general position of K(2) atoms: [K(2)O6F(1)(2)F(2)] and [K(21)O6F(1)(2)] in a 2/1 ratio. The chemical bonding structures of all constituent atoms of the compound K4Gd2(CO3)(3)F-4 (KGCOF) are comprehensively studied; moreover, the relationship between the chemical bonding structure and the nonlinear optical (NLO) properties is investigated from the chemical bond viewpoint. The theoretical prediction of the NLO tensor coefficient d(11) of KGCOF is in agreement with experimental observation. Theoretical analyses show that the nonlinearity of this crystal type mainly originates from K-O bonds. In addition, the correlation between the NLO tensor d(11) and the refractive index n(0) of KGCOF is discussed. (C) 2000 American Institute of Physics. [S0021-8979(00)07506-X].

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Second order nonlinear optical (NLO) properties of single crystals with complex structures are studied, from the chemical bond viewpoint. Contributions of each type of constituent chemical bond to the total linearity and nonlinearity are calculated from the actual crystal structure, using the chemical bond theory of complex crystals and the modified bond charge model. We have quantitatively proposed certain relationships between the crystal structure and its NLO properties. Several relations have been established from the calculation. Our method makes it possible for us to identify, predict and modify new NLO materials according to our needs. (C) 1999 Elsevier Science B.V. All rights reserved.

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The second-order nonlinear optical tensor coefficients of both KTiOPO4 (KTP) and KTiOAsO4 (KTA) are calculated from the chemical bond viewpoint. All constituent chemical bonds of both crystals are considered, and contributions of each type of bond to the total linearity and nonlinearity are determined. Calculated results agree satisfactorily with experimental data in both signs and numerical values. The calculation shows that though TiO6 groups and P(1)O-4 or As(1)O-4 groups have relatively larger linear contributions, they can only produce an advantageous environment for KOx (x = 8, 9) groups and P(2)O-4 or As(2)O-4 groups in nonlinear optical contributions. The origin of nonlinearity of KTP family crystals comes from the KOx (x = 8, 9) and P(2)O-4 groups in their crystal structures. Furthermore, the difference in optical nonlinearities of KTP type crystals is analyzed, based on the detailed calculation of nonlinearities of both KTP and KTA. (C) 1999 Academic Press.

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Linear and nonlinear optical (NLO) properties of KNbO3 in various phases are calculated from their crystal structures. Nonlinear optical tensor coefficients are found to be very sensitive to the atomic geometry, changing as much as 85% between the ferroelectric tetragonal and rhombohedral phases. The predicted principal refractive indices are also found to be sensitive to their structural changes. In the tetragonal phase KNbO3 has the largest NLO responses, in the orthorhombic phase KNbO3 has the relative larger NLO coefficients, and in the rhombohedral structure KNbO3 has the large and homogeneous NLO properties. (C) 1998 Elsevier Science B.V. All rights reserved.

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This work considers the isomorphous optically active crystals NaClO3 and NaBrO3. The connection between their second-order nonlinear optical (NLO) responses and chemical bond structures is established, starting from the experimental optical activities. The calculation reproduces the well-known experimental fact that crystals of NaClO3 and NaBrO3 with similar structures have different signs of optical rotation and of second harmonic generation (SHG). Unlike previous bond charge models, the method may include more than one type of bond in the calculation, and therefore may be used to study the optical activity and nonlinear optical properties of more general crystals. (C) 1998 Elsevier Science B.V. All rights reserved.