Frenkel-Kontorova模型中基底势振动的影响

基于一维Frenkel-Kontorova模型, 研究了振动的基底势对系统纳米摩擦现象的影响. 分别在相邻原子间的距离与周期势场的周期比为不公度(incommensurate)、可公度(commensurate)两种情形下, 探讨了基底势振动的振幅和频率对滞回现象(hysteresis)、最大静摩擦力以及超滑现象的作用机理. 两种情形下, 固定频率, 随着振幅的增大, 滞回区域的面积以及最大静摩擦力都将减小, 对于不同的频率, 减小的趋势不同. 系统甚至产生了超滑现象. 但当频率过大时, 振幅的改变不会影响滞回区域的面积以及最大静摩擦力的大小, 此时与基底不加振动时的情形一致; 当振幅固定, 随着频率的增大, 滞回区域的面积将增大, 对于不同振幅, 增大的趋势不同. 特别地, 对于某些固定的振幅, 最大静摩擦力随着振动频率的增大先逐步减小直至出现超滑现象, 再进一步增大频率, 最大静摩擦力又转而逐步增大. 这一现象类似于共振, 表明存在最佳的振动频率促进系统内所有原子的共同运动, 使得整个系统的最大静摩擦力几乎消失. 另外, 两种情形的区别是, 对于某些固定的频率(如ω= 0.5)和不同的小振幅, 不可公度情形往往具有相同的平均终止速度, 而可公度情形则不同, 表明相同前提下后者具有更复杂的动力学行为. 关键词： Frenkel-Kontorova模型 滞回 最大静摩擦力 超润滑     

高瑞利数下水平自然热对流的幂律关系

采用数值模拟方法，研究了高度和宽度比为1∶10的狭长矩形腔内的水平自然热对流. 根据对瑞利数(Rayleigh数)Ra在10⁴ <Ra<10¹¹内情形的计算结果，将流动分为三个不同的区间：线性区、连续过渡区、1/5次幂律区. 虽然流量和努塞尔数(Nusselt数)Nu随瑞利数的变化都包括了三个参数演化区间，但从一个区间到另外一个参数区间的转变时并不是同步的，其中努塞尔数的转变总是超前流量的转变. 对比前人的研究发现，流量1/3次幂律的结果是由于瑞利数不够高所致. 此外，模拟结果也表明Siggers等的理论分析过高估计了热通量强度，实际的温度边界层内努塞尔数和瑞利数为1/5次幂律关系.     

Modeling and analysis of rigid multibody systems with driving constraints and frictional translation joints

An approach is proposed for modeling and anal- yses of rigid multibody systems with frictional translation joints and driving constraints. The geometric constraints of translational joints with small clearance are treated as bilat- eral constraints by neglecting the impact between sliders and guides. Firstly, the normal forces acting on sliders, the driv- ing constraint forces （or moments） and the constraint forces of smooth revolute joints are all described by complementary conditions. The frictional contacts are characterized by a set- valued force law of Coulomb＇s dry friction. Combined with the theory of the horizontal linear complementarity problem （HLCP）, an event-driven scheme is used to detect the transi- tions of the contact situation between sliders and guides, and the stick-slip transitions of sliders, respectively. And then, all constraint forces in the system can be computed easily. Secondly, the dynamic equations of multibody systems are written at the acceleration-force level by the Lagrange multiplier technique, and the Baumgarte stabilization method is used to reduce the constraint drift. Finally, a numerical example is given to show some non-smooth dynamical behaviors of the studied system. The obtained results validate the feasibility of algorithm and the effect of constraint stabilization.     

DEPLOYMENT DYNAMICS OF FLEXIBLE SOLAR ARRAYS CONSIDERING FRICTIONAL JOINT

研究了空间漂浮航天器太阳阵展开与锁定过程的刚柔耦合动力学问题. 基于Jourdain 速度变分原理和单项递推组集方法, 建立了太阳阵展开与锁定过程的刚柔耦合多体系统动力学模型,采用虚功率原理推导了铰摩擦对系统动力学方程的贡献. 在以上动力学模型中引入3-D 鬃毛摩擦模型来研究铰链的摩擦特性对太阳阵展开动力学的影响. 所建动力学模型的正确性通过与商业软件ADAMS 和NASTRAN 的联合仿真对比得到了验证,该模型能够有效地预测太阳阵的展开历程以及航天器姿态的动态行为.     

On the influence of material non-linearities in geometric modeling of kink band instabilities in unidirectional fiber composites

The axial compressive failure of aligned fiber composites triggered by kink band instabilities is the topic of investigation herein. Particular emphasis is put on the accurate prediction of the post-failure regime, where fiber composites are known to exhibit substantial post-failure strength. In this regard, a previous analytical model, based on geometric relationships and energy principles, is enhanced by consistently taking into account material non-linearities. Therefore, a non-linear constitutive law is introduced herein based on a newly developed exponential formulation. This non-linear constitutive law is subsequently implemented into the stress–strain response in interlaminar shearing as well as the compression response. The model enhancements are validated against published experimental data yielding excellent comparisons. Furthermore, the relevance of modeling non-linear material behavior in interlaminar dilation and bending is assessed using a bilinear constitutive law. However, implementing non-linear material behavior does not yield any improvements and can therefore be neglected.     

具有攻击角约束的非奇异终端滑模导引律设计

为了满足导弹拦截高速大机动目标时高精度制导的需求,首先对二维平面内的弹目相对运动方程进行状态扩张,对于影响制导性能的目标总扰动采用了扩张状态观测器的方法进行动态补偿。然后在非奇异终端滑模面的基础上选取了两种滑模趋近律,设计了两种具有攻击角约束的非奇异终端滑模导引律。最后数值仿真结果表明,在观测器对扩张系统状态进行实时有效估计的前提下,针对不同的期望视线角和目标机动方式,所设计的两种导引律在满足期望的性能要求的同时,可实现导弹对目标的高精度快速打击。     

Analysis of plasma sheath propagation attenuation based on ray tracing

Based on the complex Snell's law of lossy media, a ray tracing method is proposed to study the propagation attenuation characteristics of electromagnetic (EM) waves in plasma sheaths. The plasma sheath is modelled as layered media. This method considers the complex ray characteristics of inhomogeneous plane EM waves, tracks the propagation rays of EM waves in each layer of media, and calculates the propagation attenuation of EM waves in each layer of media according to the propagation direction of the complex rays. The attenuation during numerical cumulative propagation is the total attenuation of EM waves through the plasma sheath. By comparing the results with that obtained from the WKB method, the accuracy of the ray tracing algorithm is proved. The results of the propagation attenuation of a blunt cone model are calculated by the proposed method, and the effects of different parameters on the EM wave propagation attenuation in the plasma sheath are analysed at different heights, velocities, incident angles, and incident positions. Studying the propagation characteristics of EM waves in the plasma sheath is of importance in application for radar target tracking, blackout communication, and other issues.     

冲击载荷下圆环纵向压缩力学行为研究

选用外径、内径和高度三者比值为6∶3∶2的铝合金圆环试件作为研究对象,在粗磨、细磨和润滑三种端面粗糙条件下,利用SHPB实验方法研究了其纵向动态压缩行为。实验结果表明:随着端面摩擦系数的增大,对应的屈服强度和硬化强度增大明显。同时,圆环纵向压缩应力应变曲线有一个明显的应力下降段。其原因是圆环在塑性压缩过程从内而外发生了动态塑性失稳。由于端面应力不均匀和压缩过程端面摩擦系数不均匀变化,试件前后端面的压缩变形明显不一致。数值模拟分析了不同表面粗糙情况下的圆环纵向压缩过程,得到了和实验相类似的现象。     

激光网格化淬火表面渗硫/纳米微粒复合层的摩擦学性能研究

利用MFT-R4000摩擦磨损试验机考察了激光网格化淬火表面渗硫复合层(简称SLD层)和激光网格化淬火表面渗硫/纳米微粒复合层(简称NLD层)在不同试验载荷时的减摩抗磨性能,利用SEM和XPS分析了不同试验条件下磨损表面形貌和所含元素及其化学价态.结果表明:NLD层不仅在重载荷下的减摩性能比SLD层的优异,其在干摩擦条件下的摩擦系数仅为0.075左右,而且在较低试验载荷下具有很长的使用寿命.磨损表面元素分析发现S元素以硫化物和硫酸盐形式存在,但没有发现Si元素,这表明磨损表面形成了主要由硫化物、硫酸盐和氧化物组成的摩擦化学反应膜,而n-SiO2颗粒在摩擦表面可能起到了"微纳滚珠"的作用.     

酚醛纤维增强纸基摩擦材料的性能研究

采用湿式成型工艺制备出5种不同酚醛纤维含量的纸基摩擦材料,研究了酚醛纤维含量对纸基摩擦材料各项性能的影响.通过力学试验机测试了材料剪切强度和压缩-回弹性能,利用同步热分析仪和湿式摩擦试验机测试了材料热性能及摩擦磨损性能,通过扫描电镜观察材料摩擦表面和剪切断裂面的微观形貌.结果表明:随着酚醛纤维含量的增加,材料层间剪切强度逐渐提高,而压缩率变化较小,回弹率先增大后减小;材料耐热性随酚醛纤维含量的增加而降低;随着酚醛纤维含量的增加,磨损率逐渐降低,同时摩擦系数保持稳定.