Assembling hydrodynamic cloaks to conceal complex objects from drag

Transformation hydrodynamics and the corresponding metamaterials have been proposed as a means to exclude the drag force acting on an object. Here, we report a strategy to deploy the hydrodynamic cloaks in a more practical manner by assembling different-shaped cloaking parts. Our strategy is to first model a square-shaped cloak and a carpet cloak and then combine them to conceal a more complex-shaped space in the three-dimensional hydrodynamic flow. With the derivation of transformation hydrodynamics, the coordinate transformations for each hydrodynamic cloaking are demonstrated with the calculated viscosity tensors. The pressure and velocity fields of the square, triangular (carpet), and exemplary three-dimensional house-shaped cloaks are numerically simulated, thus showing a cloaking effect and reduced drag. This study suggests an efficient way of cloaking complex architectures from fluid-dynamic forces.     

水下对转螺旋桨流致辐射噪声机理与预报方法*

水下对转螺旋桨流致辐射噪声的预报对于水下目标的特征提取和分类识别具有重要意义。由桨叶的旋转引起的湍流场是水下对转螺旋桨流致辐射噪声的源场。分述了水下对转螺旋桨湍流边界层脉动、旋转干涉效应和空化效应引发的水动力噪声机制和研究进展,比较了目前工程应用中的3种对转螺旋桨流致辐射噪声预报方法的特点。在分析对转螺旋桨流致辐射噪声数值预报难点的基础上,综述了对转螺旋桨流致辐射噪声计算方法的研究进展,指出间接数值模拟方法是工程中进行对转螺旋桨流致辐射噪声预报的有效方法。     

离子浓度及表面结构对岩石孔隙内水流动特性的影响

酸性环境引发的岩石孔隙表面溶解增加了孔隙内水溶液的盐离子浓度,破坏了孔隙的表面结构.本文采用分子动力学模拟的方法研究了纳米级岩石孔隙内水溶液的流动特性,分析了盐离子浓度和孔隙表面结构对水流速度分布的影响及原因.研究结果表明:纳米级岩石孔隙内的水溶液流动符合泊肃叶流动特性,流速呈"抛物线"分布;随盐离子浓度增加,水溶液内部氢键网络变得更为致密,水黏度随其呈线性增长;水溶液中离子浓度越大,孔隙表面对水流动的阻力越大,最大流速越小,速度分布的"抛物线"曲率半径越大;岩石孔隙表面结构的破坏改变了流动表面的粗糙程度,增加了孔隙表面对H2O分子的吸引力.随表面结构破坏程度的增大,水溶液在近壁区域的密度增大,流速降低;当表面破坏程度达到50%时,水溶液在近壁区域出现了明显的负边界滑移现象.     

交错跃迁Hofstadter梯子的量子流相

为玻色Hofstadter梯子模型引入交错跃迁,来扩展模型支持的量子流相.基于精确对角化和密度矩阵重整化群计算发现,无相互作用时,系统中包含横流相、涡旋相和纵流相;横流相来自均匀跃迁时Hofstadter梯子模型的Meissner相,纵流相是交错跃迁时才可见的流相.强相互作用极限下系统的超流区也包含横流相、纵流相和涡旋相,但存在更多的相变级数;超流区的横流相、纵流相之间存在相变但Mott区的不存在,把Mott区的"横、纵流相"称为Mott-均匀相,在Mott区只存在均匀相和涡旋相.跃迁的交错会压缩涡旋相存在的区域,使Mott区最终只剩下均匀相;跃迁的交错不仅能驱动Mott-超流相变,还使磁通的改变也能够驱动系统的Mott-超流相变.对这一系统的研究丰富了磁通系统中的量子流相,同时为研究拓扑流特性提供了模型支持.     

Monte Carlo simulations of magnetic particle suspensions with a simple assessment method for the particle overlap between magnetic spheroids

We have developed a simple assessment method for the overlap between spheroidal particles, which neither requires the complex manipulation of vectors and matrices that is indispensable in the ordinary methods, nor is based on a model potential. Moreover, we have developed an evaluation method for the interaction energy arising from the overlap of the steric layer coating spheroidal particles. This is based on a sphere-connected particle model, but some modifications are introduced in order to express an appropriate repulsive interaction energy at the deepest overlapping position. We have investigated the phase change in a magnetic spheroidal particle suspension for a two-dimensional system by means of Monte Carlo simulations. In the case of no external magnetic field, if the magnetic particle-particle interaction is sufficiently strong to favour cluster formation, long raft-like clusters tend to be formed in a dilute situation. With decreasing values of area fraction, a chain-like structure in a dense situation transforms into a raft-like structure within a narrow range of the particle area fraction. Similarly, the raft-like clusters are preferred in a weak applied magnetic field, but an increase in the field strength induces a phase change from a raft-like into a chain-like structure.

Highlights of the present paper:

1. A simple assessment method has been proposed for the overlap between two spheroidal particles.

2. The particle overlap assessment is free from a complex mathematical manipulation regarding vectors and matrices.

3. A modified sphere-connected model has been proposed in order to more accurately evaluate a repulsive interaction due to the overlap of the steric layers coating spheroidal particles.

4. 2D Monte Carlo simulations have been performed to elucidate the phenomenon of a phase change by magnetic spheroidal particles on a material plane surface.

5. A phase change between a raft-like and a chain-like aggregate structure is able to be controlled by the area fraction of particles and an external magnetic field.

     

氯化钠水溶液结构的研究

利用上海光源(SSRF)的第三代同步辐射光源测定室温下摩尔浓度分别为0.172 mol/L、0.343 mol/L、0.699 mol/L、1.064 mol/L、2.832 mol/L、3.910 mol/L、5.289 mol/L的NaCl水溶液的X射线散射数据。由X射线散射数据可知，随着NaCl水溶液浓度的增大，X射线散射曲线的特征峰由12.6°到13.4°发生偏移。运用Pair Distribution Function(PDF)理论对X射线散射数据进行处理，得到了不同浓度NaCl水溶液及纯水的差值对分布函数，其中的O-O峰随着浓度的增大逐渐分裂为两峰，O-O峰位在0.282 nm处。利用分子动力学模拟研究不同浓度的NaCl水溶液，表明Na+、Cl-的引入对水分子的氢键结构有一定的破坏，当浓度大于15%时，这种效果尤其明显。Na+、Cl-均存在两层水化层，各离子间配位数随浓度的增大而减少。H2O分子的自扩散系数远大于Na+和Cl-的自扩散系数，后两者的值随浓度的增大逐渐减少，Na+和Cl-的水化半径均随浓度增大而降低。     

单颗金刚石切削无氧铜的声发射关联维特征*

声发射技术可以实现无氧铜切削加工特征的监测与评价。采用声发射技术监测单颗金刚石磨粒旋转切削无氧铜，利用G-P算法重构出声发射时域信号相空间，采用自相关函数法计算出相空间时间延迟参数，通过相空间双对数曲线的计算，得到不同切削工况下的关联维数。研究结果表明，进给速度和切削速度对声发射信号影响较不显著，切深与声发射信号振幅呈正效应关系；声发射信号双对数曲线呈现阶段性增加趋势，并逐渐收敛于饱和状态，关联维数随着嵌入维数的增加先快速下降后趋于平稳；金刚石切削无氧铜的声发射信号具有混沌运动变化特性，在较小嵌入维数时，关联维数与切深和切削速度呈现线性负效应关系，与进给速度呈现线性正效应关系。该研究为无氧铜的切削加工提供理论参考。     

Simultispin: A versatile graphical user interface for the simulation of solid-state continuous wave EPR spectra

Solid-state continuous wave (cw) electronic paramagnetic resonance (EPR) spectroscopy is particularly suitable for metal complex analysis. Extracting magnetic parameters by simulation is often necessary to describe the electronic structure of the studied molecular compounds that can have various electronic spin states and characterized by different parameters like g-values, hyperfine coupling or zero field splitting values. Easyspin toolbox on MATLAB is a powerful tool, but for the user, it requires spending time with coding and could discourage nonexperts. Facing this context, we have developed a graphical user interface called Simultispin, dedicated to solid-state cw-EPR spectra simulation. Some examples of experimental spectra of metal complexes (mixture of low spin and high spin Fe^III complexes, dynamic disorder of a Cu^II complex, photogeneration of a Mn^III complex), highlighting specific solid-state functions, are described and analyzed based on simulations performed with Simultispin. We hope that its ergonomy and the ease to set up a complete set of parameters to get reliable simulations could help a large EPR community to improve the efficiency of their interpretations.     

Prediction of branch on branch and topological characteristics of low‐density polyethylene polymerization by a novel stochastic approach

A novel approach using Monte Carlo method applied to simulation of low‐density polyethylene (LDPE) polymerization in tubular reactor showing topological characteristics, and the comprehensive kinetic mechanism has been taken into consideration. The results show the precise details of the structure of a chain in the three levels of the backbone, the main branches, and branches on branch. The chain types include dead polymer, dead polymer with unsaturated end, and live polymer with primary radical, secondary radical, and tertiary radical. In this work, the branches on branch were identified in terms of number, length, and position of the branch. Sixty percent of branches on branch are 1 to 5 carbons long, and the longest branch on branch is about 50 carbons. Thus, this study provides a tool for more accurately mapping the polymer chains architecture, superior to determine the number, and position of long‐ and short‐chain branches in past researches. Finally, this approach will advance the prediction of microstructure‐related properties of polymer one step further.     

单层n型MoS2/p型c-Si异质结太阳电池数值模拟

单层二硫化钼(MoS₂)是一种具有优异光电性能的半导体材料,在太阳能能量转换中表现出很大的应用潜力。本文基于AMPS模拟软件,对单层n型MoS₂/p型c-Si异质结太阳电池进行了数值模拟与分析。通过模拟优化,n型MoS₂的电子亲和能为3.75 eV、掺杂浓度为10¹⁸ cm^-3,p型c-Si的掺杂浓度为10¹⁷ cm^-3时,太阳电池能够取得最高22.1%的转换效率。最后模拟了n型MoS₂/p型c-Si异质结界面处的界面态对太阳电池性能的影响,发现界面态密度超过10¹¹ cm^-2·eV^-1时会严重影响太阳电池的光伏性能。