振动主动控制中的时滞动力学问题

反馈环节的时滞会导致受控系统失稳，制约着振动主动控制技术的发展和应用。本文概述了作者近期对含时滞振动控制系统动力学的研究，包括：线性单自由度时滞系统的稳定性判据，线性多自由度小时滞系统的稳定性分析方法，Ｄｕｆｉｎｇ时滞系统的主共振与亚谐共振分析，时滞系统周期运动及稳定性的数值分析等。最后，指出了值得研究的一些问题。     

On the Lamb Impulse Dynamics in Superfluid Turbulent HeII

Macroscopic dynamics of the hydrodynamic (Lamb) impulse J _V induced by the chaotic vortex filaments in the counterflowing superfluid turbulent HeII is studied. Both the Lamb impulse and its dynamics are considered on the basis of the Gaussian model of chaotic vortex tangle elaborated earlier by one of the authors (S.N.). The rate of change of the quantity J _V is shown to be equal to the force F _sn exerted by the normal component on the vortex tangle. That contradicts the obvious fact that in steady case the quantity J _V should have a constant value. Resolving that apparent paradox we draw some conclusions concerning the stochastic dynamics of the quantized vortex lines.     

Strain-softening behavior of an Fe-6.5 wt%Si alloy during warm deformation and its applications

An Fe-6.5 wt%Si alloy with columnar grains was compressed at a temperature below its recrystallization temperature. The Vickers hardness and structure of the alloy before and after deformation were investigated. The results showed that with an increase in the degree of deformation, Vickers hardness of the alloy initially increased rapidly and then decreased slowly, indicating that the alloy had a strain-softening behavior after a large deformation. Meanwhile, the work-hardening exponent of the alloy decreased significantly. Transmission electron microscopy confirmed that the decrease of the order degree was responsible for the strain-softening behavior of the deformed alloy. Applying its softening behavior, the Fe-6.5 wt%Si alloy with columnar grains was rolled at 400 °C and then at room temperature. An Fe-6.5 wt%Si thin strip with thickness of 0.20 mm was fabricated. The surface of the strip was bright and had no obvious edge cracks.     

基于变量变换级数展开法的光波导矢量本征模分析

基于变量变换级数展开法,获得了掩埋矩形光波导及脊形光波导的矢量本征模及其传播常数。变量变换使无限平面映射成单位平面,从而使单位平面边界上的电磁场自然为零,消除了非物理反射,提高了计算精度;另外,由于这种方法所导致的矩阵阶数小,因此计算效率较高。分析的结果与已发表的结果吻合较好,可以为优化波导光电子器件的结构提供参考。     

Droplet Dynamics On a Body Surface In a Gas Flow

High Temperature - The results of previous studies of the water droplet dynamics on a horizontal surface blown by air are briefly reviewed. The experiments were carried out on rectangular wing...     

Epistemic Communities and Cluster Dynamics: On the Role of Knowledge in Industrial Districts

This paper questions the prevailing notions that firms within industrial clusters have privileged access to “tacit knowledge” that is unavailable—or available only at high cost—to firms located elsewhere, and that such access provides competitive advantages that cause the growth and development of both firms and regions. It outlines a model of cluster dynamics emphasizing two mutually interdependent processes: the concentration of specialized and complementary epistemic communities, on the one hand, and entrepreneurship and a high rate of new firm formation on the other.     

Spin Dynamics in Disordered Solids

Modern state of studies in spin dynamics of statically disordered media is presented. Next four fundamental problems are attended mainly: 1) delocalization of nuclear polarization in subsystem of impurity nuclei (it is exemplified in model nuclear spin system ⁸Li-⁶ Li in the LiF single crystal) 2) nuclear relaxation via paramagnetic impurities in crystals of arbitrary space dimension d 3) free induction decay and EPR line form function at d ≤ 3; and 4) form function of the hole, burned on the wing of the dipolar EPR line.     

Two-Hole Dynamics in Spin Ladders

We present an analytic theory for the energy spectrum of a two-leg spin ladder doped with two holes. Starting from a pseudo-fermion-bond-boson representation of the corresponding t _1,2 –J _1,2 Hamiltonian we apply a diagrammtic approach adapted to the limit of strong rung coupling, which includes both, the coupling of holes to the spin background as well as the two-hole interactions. The two-hole spectrum is calculated and the formation of bound states is discussed. Additionally the evolution of the spin gap of the ladder upon doping is analyzed. A comparison with existing exact diagonalization data is presented and good agreement is found.     

包装动力学中的若干问题

对现代包装动力学的含义、研究目的与涉及的基本内容作了比较深入的阐述。对包装防震缓冲、物品易损度、缓冲包装材料、包装防震缓冲设计等方面的进展和存在的问题作了综述和分析。提出了今后的主要研究方向。     

Exciton Dynamics in Semiconductor Nanocrystals

This review article provides an overview of recent advances in the study and understanding of dynamics of excitons in semiconductor nanocrystals (NCs) or quantum dots (QDs). Emphasis is placed on the relationship between exciton dynamics and optical properties, both linear and nonlinear. We also focus on the unique aspects of exciton dynamics in semiconductor NCs as compared to those in bulk crystals. Various experimental techniques for probing exciton dynamics, particularly time‐resolved laser methods, are reviewed. Relevant models and computational studies are also briefly presented. By comparing different materials systems, a unifying picture is proposed to account for the major dynamic features of excitons in semiconductor QDs. While the specific dynamic processes involved are material‐dependent, key processes can be identified for all the materials that include electronic dephasing, intraband relaxation, trapping, and interband recombination of free and trapped charge carriers (electron and hole). Exciton dynamics play a critical role in the fundamental properties and functionalities of nanomaterials of interest for a variety of applications including optical detectors, solar energy conversion, lasers, and sensors. A better understanding of exciton dynamics in nanomaterials is thus important both fundamentally and technologically.     

Spin Dynamics in ZnO-Based Materials

In this work, we address the issue of spin relaxation and its relevance to spin detection in ZnO-based materials, by spin-polarized, time-resolved magneto-optical spectroscopy. We have found that spin relaxation is very fast, i.e. about 100 ps for donor bound excitons in wurtzite ZnO, despite of a weak spin–orbit interaction. We also reveal that alloying of ZnO with Cd enhances spin relaxation, prohibiting ZnCdO/ZnO structures for efficient optical spin detection. On the other hand, a variation in strain field induced by lattice mismatch with substrates does not seem to lead to a noticeable change in spin relaxation. The observed fast spin relaxation, together with the limitation imposed by the band structure, are thus identified as the two most important factors that limit the efficiency of optical spin detection in the studied ZnO-based materials.     

Trapped Bubble Dynamics in Cryogenic Fluids

Initial studies of the properties of helium bubbles acoustically trapped in liquid nitrogen at 66 K have been carried out. Laser scattering is used to monitor the dynamics of the bubble radius as a function of the acoustic driving field. In this apparatus, no sonoluminescence has yet been observed from helium bubbles in liquid nitrogen, although sonoluminescence was previously observed from xenon bubbles in ethanol cooled to 165 K. It is found that bubbles can be stably trapped at quite low acoustic drive levels, enabling the observation of new dynamical bubble behavior.     

Dynamics of macromolecules in convergent-channel flows

The deformation of flexible and rigid macromolecules is analyzed under conditions of convergent-channel flows.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 55, No. 5, pp. 743–750, November, 1988.     

Vortex Dynamics in Trapped Bose-Einstein Condensate

We perform numerical simulations of vortex motion in a trapped Bose-Einstein condensate by solving the two-dimensional Gross-Pitaevskii Equation in the presence of a simple phenomenological model of interaction between the condensate and the finite temperature thermal cloud. At zero temperature, the trajectories of a single, off-centred vortex precessing in the condensate, and of a vortex-antivortex pair orbiting within the trap, excite acoustic emission. At finite temperatures the vortices move to the edge of the condensate and vanish. By fitting the finite-temperature trajectories, we relate the phenomenological damping parameter to the friction coefficients α and α′, which are used to describe the interaction between quantised vortices and the normal fluid in superfluid helium.