Stochastic rearrangement inequalities

We develop a unified theory for obtaining stochastic rearrangement inequalities and show how this theory may be applied in statistical contexts such as ranking problems, hypothesis testing, contamination models, and optimal assembly of systems.     

Flow-induced oscillation of two circular cylinders in tandem arrangement at low Re

Results are presented for flow-induced vibrations of a pair of equal-sized circular cylinders of low nondimensional mass (m^*=10) in a tandem arrangement. The cylinders are free to oscillate both in streamwise and transverse directions. The Reynolds number, based on the free-stream speed and the diameter of the cylinders, D is 100 and the centre-to-centre distance between the cylinders is 5.5D. The computations are carried out for reduced velocities in the range 2≤U^*≤15. The structural damping is set to zero for enabling maximum amplitudes of oscillation. A stabilized finite element method is utilized to carry out the computations in two dimensions. Even though the response of the upstream cylinder is found to be qualitatively similar to that of an isolated cylinder, the presence of a downstream cylinder is found to have significant effect on the behaviour of the upstream cylinder. The downstream cylinder undergoes very large amplitude of oscillations in both transverse and streamwise directions. The maximum amplitude of transverse response of the downstream cylinder is quite similar to that of a single cylinder at higher Re beyond the laminar regime. Lock-in and hysteresis are observed for both upstream and downstream cylinders. The downstream cylinder undergoes large amplitude oscillations even beyond the lock-in state. The phase between transverse oscillations and lift force suffers a 180 jump for both the cylinders almost in the middle of the synchronization regime. The phase between the transverse response of the two cylinders is also studied. Complex flow patterns are observed in the wake of the freely vibrating cylinders. Based on the phase difference and the flow patterns, the entire flow range is divided into five sub-regions.     

串列双圆柱绕流问题的数值模拟

本文运用有限体积方法,对绕串列放置的双圆柱的二维不可压缩流动进行了数值计算。为研究两圆柱不同间距对圆柱相互作用和尾流特征的影响,选取间距比Ｌ／Ｄ（Ｌ为两圆柱中心间的距离,Ｄ为圆柱直径）在１．５～５．０之间每隔０．５共八个有代表性的间距进行了计算模拟。计算均在Ｒｅ＝２００条件下进行。计算结果表明：对该绕流问题,流动特征在很大程度上取决于间距的大小。且间距存在一临界值,间距比从小于临界值变化到大于临界     

New relationships for multi-neighborhood search for the minimum linear arrangement problem

We develop a series of theorems about the graph structure of the classical Minimum Linear Arrangement (MinLA) problem which disclose properties that can be exploited by Multi-Neighborhood Search (MNS) algorithms. As a foundation, we differentiate between swaps of labels attached to adjacent and non-adjacent nodes to create two new neighborhood classes, and show how our theorems yield efficient algorithms for updating key arrays used by local search procedures. In addition, we introduce a class of neighborhoods called set-based neighborhoods supported by a theorem that identifies solutions (labelings) for the MinLA problem in polynomial time that dominate exponential numbers of alternative solutions. The component neighborhoods within this new neighborhood class can be applied in various sequences in conjunction with the first two new neighborhoods introduced. Our results also apply to problems with objectives different than those of MinLA. Finally, our results make it possible to exploit the new neighborhoods according to the user's choice of MNS protocols and alternative local search algorithms.     

双圆柱斜置排列时下游圆柱压力分布和振荡频率的实验研究

当两个圆柱斜置排列并靠得较近时,绕流下游圆柱的流动明显地不同于单一圆柱.本文研究了下游圆柱压力分布受到的影响,并着重于用FFT来分析圆柱表面某些特征点上的压力频谱,研究频谱与圆柱之间的相对距离和位置的关系.当圆柱间距较远时,压力振荡的频率与单圆柱卡门涡节的频率相接近,随着横向间距的接近,频率逐渐降低,然而当两圆柱十分接近时,外侧压力振荡的频率继续降低,但圆柱内侧压力振荡的频率却突然提高,出现了一个圆柱体两侧作用有不同频率压力的情况.     

Strain-rate history effects and microstructural aspects in lithium fluoride and aluminium single crystals after dynamic loading

Single crystals of LiF and Al are deformed in shear at a number of constant strain-rates in the range 10^–4 to 1600 s^–1. These constant rate tests are supplemented by a series of jump tests in which a sharp increment in strain rate is imposed during the quasi-static straining. Dislocation arrangements are observed by etch-pits technique for LiF crystals and by TEM for Al crystals. It is shown that cell sizes vary inversely with flow stress and strain-rate sensitivity.     

大湍流度高雷诺数时并列双圆柱的平均和脉动压力分布

本文通过风洞实验研究了来流湍流度,Iu=10％雷诺数分别为Re=1.95×10~9和Re=6.5×10~5时单个圆柱和不同间距比下并列双圆柱的平均和脉动压力分布。结果表明:在Re=1.95×10~5时单个圆柱的平均压力分布类似于低湍流度高超临界雷诺数时的压力分布;当雷诺数增大至6.5×10~5时,绕圆柱表面流动的分离点前移和背压绝对值提高,总的阻力系数随之增加。并列双圆柱的间距比变化对圆柱表面压力分布影响很大,在极小间距比(N/d=1.05)时,双圆柱间的缝隙流使附近柱面产生高达-5的压力系数峰值(Re=6.5×10~5),同时脉动压力也大为增加;在较小间距比时(1.5     

纤维排列方式对复合材料总体粘弹性常数的影响

对于金属基或高分子聚合物基复合材料，在特定情况下会表现出明显的粘弹性特性。本文采用Riemann—Liouville形式的分数阶导数模型描述基体的粘性特性，通过渐进均匀化方法给出了预测纤维加强复合材料整体本构关系的解析表达式，给出应用于基体具有Makris粘弹性关系的具体形式。最后，考察了圆截面纤维正方形排列和对角排列时的总体粘弹性弹性常数随纤维比的变化曲线。结果表明，这类复合材料仍具有粘弹性特性，其整体粘弹性本构关系的弹性部分综合了纤维弹性和基体弹性的贡献，粘性部分来自基体粘性的贡献，复合材料具有和基体相同的粘性系数和分数阶。为分析微结构特征对整体特性的贡献，须求解两类局部问题。在相同纤维体积比情况下，正方形排列的总体弹性系数大于正方形对角排列，而粘性常数相反。     

Optimization of multilayer radial beam array in weak turbulent atmosphere

The on-axis scintillation index of multilayer radial beam array with incoherent combination propagating through weak turbulence is studied. The results indicate that for large beam number, the scintillation index can be reduced by adjusting beams on different concentric circles. Further studies manifest that optimum radii arrangement of the three-layer array can be adopted to get the smallest scintillation index.