人体肌骨的多柔体系统动力学研究进展

人体肌肉骨骼系统简称肌骨系统, 包括骨骼、骨骼肌与关节连接, 其力学模型是典型的多柔体系统. 从多体动力学角度研究肌骨系统, 主要关注其在运动过程中的肌肉内力、关节力矩及产生的动力学影响, 属于动力学与生物力学的交叉融合. 肌骨系统的多体动力学模型已被广泛地应用于临床医学、竞技体育、军事训练、人机工程等诸多领域, 其仿真结果可为提高人体运动能力、降低关节载荷与能耗、避免运动损伤、加快康复进程等提供重要计算参考数据. 与此同时, 上述研究亦对肌骨动力学研究提出了许多新挑战. 本文综述了人体肌骨多柔体系统动力学相关研究进展, 包括骨骼肌功能解剖与生物力学建模、神经与肌肉控制理论、肌骨系统动力学问题与求解方法, 以及近年来肌骨多体动力学在步态分析、飞行员抗荷动作、口颌手术规划等领域的典型应用. 与工程领域的机械多体系统相比, 人体肌骨多体系统具有肌肉内力主动性与肌肉控制冗余性两大特征. 现有骨骼肌模型难以同时考虑肌肉的解剖结构、三维几何与肌力产生的生物化学机制. 已有大多数肌骨模型采用静态优化假设消除肌肉冗余性, 忽略了肌肉与肌腱内力平衡及兴奋收缩耦联机制. 此外, 目前仍缺乏实现肌骨模型个性化的无创在体测试手段. 未来, 人体肌骨多体动力学研究将会向更精确、智能、个性化的方向发展, 成为动力学与生物力学交叉的热点研究领域.      

交流约瑟夫逊效应与锁频现象

微波辐照下交流约瑟夫逊效应的夏皮罗台阶是非线性耗散动力学系统的锁频现象．本文在简介锁频现象的基础上，阐明夏皮罗台阶的成因．     

用非本征吸收区的透射系数研究CdSeS量子点玻璃的非线性系数

通过CdSeS量子点玻璃在非本征吸收区的透射系数,研究CdSeS量子点玻璃的非线性光学系数与量子点半径的关系。实验测量了嵌有不同尺寸CdSeS量子点玻璃的吸收光谱,在非本征吸收区量子点玻璃的透射系数随量子点半径的增大,先增大后减小;理论分析了半导体量子点玻璃非本征吸收区透射系数与非线性光学系数之间的关系;实验与理论相结合,建立了非本征吸收区透射系数与非线性光学系数之间的联系。研究分析了CdSeS量子点玻璃的非线性光学系数随量子点半径的变化,得到CdSeS量子点玻璃在非本征吸收区的非线性光学系数随量子点半径的增大,先减小后增大的结果。     

CdSeS量子点的光吸收谱亚结构和光致发光激发谱分析

用光致发光激发(PLE)谱分析吸收谱的亚结构。实验样品是共熔法制备的CdSeS量子点玻璃,量子点的生长时间分别为2h和4h,高分辨透射电子显微镜(HRTEM)分析得到样品中量子点的平均直径分别为3．6nm和3．8nm。在室温下对样品进行了光吸收谱和光致发光激发谱研究。光吸收谱显示了量子尺寸效应,光致发光激发谱中低能端有两个明显的峰。考虑价带简并以及电子与空穴之间的相互作用,通过理论分析和数值计算,得到1S3/2-1Se和2S3/2-1Se的跃迁能量及其随量子点半径的变化,由此确认光致发光激发谱中的两个峰分别为1S3/2-1Se和2S3/2-1Se跃迁。     

Different kinds of discrete breathers in a Sine–Gordon lattice

We study a one-dimensional Sine–Gordon lattice of anharmonic oscillators with cubic and quartic nearest-neighbor interactions, in which discrete breathers can be explicitly constructed by an exact separation of their time and space dependence. DBs can stably exist in the one-dimensional Sine–Gordon lattice no matter whether the nonlinear interaction is cubic or quartic. When a parametric driving term is introduced in the factor multiplying the harmonic part of the on-site potential of the system, we can obtain the stable quasiperiodic discrete breathers and chaotic discrete breathers by changing the amplitude of the driver.     

Two-dimensional discrete gap breathers in a two-dimensional discrete diatomic Klein-Gordon lattice

We study the existence and stability of two-dimensional discrete breathers in a two-dimensionai discrete diatomic Klein-Gordon lattice consisting of alternating light and heavy atoms, with nearest-neighbor harmonic coupling. Localized solutions to the corresponding nonlinear differential equations with frequencies inside the gap of the linear wave spectrum, i.e. two-dimensional gap breathers, are investigated numerically. The numerical results of the corresponding algebraic equations demonstrate the possibility of the existence of two-dimensional gap breathers with three types of symmetries, i.e., symmetric, twin-antisymmetric and single-antisymmetric. Their stability depends on the nonlinear on-site potential （soft or hard）, the interaction potential （attractive or repulsive） and the center of the two-dimensional gap breathers （on a light or a heavy atom）.     

A theoretical description for solitons in polyacetylene

The bond-alternation domain walls or the solitons in the dimerized polyacetylene are analyzed theoretically. The width of the soliton is many times the period of the chain, so that the soliton can be reasonably well described by a continuum model. Because of the existence of the bond-alternation domain walls, the electron density is different definitely. Thus the electron density can be used to describe the formation of the domain walls, and a self-trapped potential is discussed and introduced in the Hamiltonian. It is shown that the envelope of the wave functions of the chain is governed by the nonlinear Schr?dinger equation which has soliton solutions. Then the shape of the soliton is determined analytically which is in accordance with the numerical calculations by Su, Schrieffer and Heeger. This implies that the bond-alternation domain wall or the soliton is observed as the envelope of the wave function.     

基于单向测量超声背散射系数的晶粒尺寸评价高效方法

镍基高温合金GH4742具有优异的机械性能,而晶粒尺寸是影响其性能的关键因素.基于物理模型的超声背散射法可以实现晶粒尺寸高效和准确的评价,但受限于复杂模型或多角度声束测量.因此,本文提出了一种只需单向测量的背散射系数法,且无需考虑测量系统等无关因素的影响.基于独立散射模型,推导了只与材料相关的背散射系数;利用空间相关函数描述了晶粒尺寸与背散射系数的关系;采用参考信号剔除干扰因素的影响,实现实验背散射系数的快速提取.制备三组不同晶粒尺寸的GH4742试块进行相控阵超声实验和平均晶粒尺寸评价,并与金相法结果进行对比.结果表明本文方法得到的晶粒度与金相法结果最大相对误差为–22.7%,最小相对误差为–3.7%.     

多柔体系统动力学建模与优化研究进展

多柔体系统是由柔性部件和运动副组成的力学系统,在航空、航天、车辆、机械与兵器等众多工程领域具有广泛的应用前景, 其典型的代表包括柔性机械臂、直升机旋翼、卫星的可展开天线、太阳帆航天器等. 近年来,随着工程技术的发展,多柔体系统动力学问题日益突出,尤其是含变长度柔性部件的多柔体系统,不仅涉及其动力学 建模与计算,还涉及其动力学优化设计. 事实上,部件柔性对多柔体系统的动力学行为影响很大,直接影响到优化结果,因此需要发展基于多柔体系统动力学的优化设计方法. 本文首先阐述了多柔体系统动力学优化的研究背景及意义,简要回顾了多柔体系统动力学建模的3类方法:浮动坐标方法、几何 精确方法和绝对节点坐标方法,并介绍了含变长度柔性部件的多柔体系统动力学建模方法. 系统概述了多柔体系统动力学响应优化、动力学特性优化和动力学灵敏度分析3个方面的研究进展,并从尺寸优化、形状优化和 拓扑优化 3 个方面综述了多柔体系统部件优化的研究进展. 本文最后提出了在多柔体系统动力学优化研究中值得关注的若干问题.      

Periodic,quasiperiodic, and chaotic breathers in two-dimensional discrete β-Fermi-Pasta-Ulam lattice

Using numerical method, we investigate whether periodic, quasiperiodic, and chaotic breathers are supported by the two-dimensional discrete Fermi-Pasta-Ulam (FPU) lattice with linear dispersion term. The spatial profile and time evolution of the two-dimensional discrete β -FPU lattice are segregated by the method of separation of variables, and the numerical simulations suggest that the discrete breathers (DBs) are supported by the system. By introducing a periodic interaction into the linear interaction between the atoms, we achieve the coupling of two incommensurate frequencies for a single DB, and the numerical simulations suggest that the quasiperiodic and chaotic breathers are supported by the system, too.