Surface reconstruction on stishovite Si02, HfO2 and rutile TiO2 （001）

This paper systematically investigates the surface reconstruction processes and patterns on stishovite SiO2, HfO2 and rutile TiO2 （001） by using classical molecular dynamics. It is found that these three surfaces relax instead of reconstruction at 0 K, and have little possibility to reconstruct below 40 K. Above 40 K, surface reconstructions take place as collective atomic motion which can be speeded by higher temperature or compressed strain. Several reconstruction patterns with approximate surface energies are found, and electrostatic potentials on them are also provided in comparison with possible microscopic results.     

A 2D Staggered Multi-Material ALE Code Using MOF Interface Reconstruction

Hydrocodes are necessary numerical tools in the fields of implosion and high-velocity impact, which often involve large deformations with changing-topology interfaces. It is very difficult for Lagrangian or Simplified Arbitrary Lagrangian-Eulerian (SALE) codes to tackle these kinds of large-deformation problems, so a staggered Multi-Material ALE (MMALE) code is developed in this paper, which is the explicit time-marching Lagrange plus remap type. We use the Moment Of Fluid (MOF) method to reconstruct the interfaces of multi-material cells and present an adaptive bisection method to search for the global minimum value of the nonlinear objective function. To keep the Lagrangian computations as long as possible, we develop a robust rezoning method named as Combined Rezoning Method (CRM) to generate the convex, smooth grids for the large-deformation domain. Regarding the staggered remap phase, we use two methods to remap the variables of Lagrangian mesh to the rezoned one. One is the first-order intersection-based remapping method that doesn't limit the distances between the rezoned and Lagrangian meshes, so it can be used in the applications of wide scope. The other one is the conservative second-order flux-based remapping method developed by Kucharika and Shashkov [22] that requires the rezoned element to locate in its adjacent old elements. Numerical results of triple point problem show that the result of first-order remapping method using ALE computations is gradually convergent to that of second-order remapping method using Eulerian computations with the decrease of rezoning, thereby telling us that MMALE computations should be performed as few as possible to reduce the errors of the interface reconstruction and the remapping. Numerical results provide a clear evidence of the robustness and the accuracy of this MMALE scheme, and that our MMALE code is powerful for the large-deformation problems.     

光谱重构油菜叶绿素含量快速检测方法及设备研制

为了实现油菜叶片中叶绿素含量的快速无损检测,开发了手持式多光谱成像系统用于采集油菜叶片在460,520,660,740,840和940 nm 六个波段的光谱图像。将一台能够采集可见光/近红外（380～1 023 nm）512个波段光谱图像但是价格高昂且体积大的室内高光谱成像系统作为参考仪器,将手持式多光谱成像系统作为目标仪器后,采用伪逆法（pseudo-inverse method）求得高光谱成像系统和多光谱成像系统两台仪器之间的转换矩阵F,从而实现6个波段的多光谱图像向512个波段的高光谱图像的重构,提高了手持式设备的光谱分辨率。运用偏最小二乘回归算法（PLSR）建立了重构的光谱与油菜叶片的叶绿素含量之间的关系模型。结果表明,重构的可见光范围内的光谱反射率与叶绿素浓度之间具有很强的相关性,PLSR回归模型建模集的决定系数R2_c为0.82,建模集均方根误差RMESC为1.98,预测集的决定系数R2_p为0.78,预测集均方根误差RMESP为1.50,RPD为2.14。虽然应用本文开发的手持式成像系统结合PLSR模型实现油菜叶绿素含量快速无损预测的精度低于基于室内高光谱成像系统获得的高光谱图像建立的PLSR模型（R2_c,RMESC,R2_p,RMESP和RPD分别为0.90,1.41,0.82,1.36和2.37）,但是明显优于基于原始多光谱成像系统4个波段（460,520,660和740 nm）反射率建立的PLSR模型得到的结果（R2_c,RMESC,R2_p,RMESP和RPD分别为0.78,2.06,0.72,1.85和1.88）。表明光谱重构技术可提高多光谱成像预测油菜叶绿素含量的精度,并且与室内高光谱成像系统相比,开发的手持式设备具有体积小、成本低廉和操作简便等优点,可为田间油菜叶片的生理状态和养分检测及可视化表达提供技术支持。     

基于PICCS的非局部TGV能谱CT重建算法

能谱CT将宽谱划分为窄谱,导致通道内光子数目明显减少,加大了噪声影响,故从噪声投影中重建出高质量图像是能谱CT的一个研究热点.传统全变分(total variational,TV)容易造成重建图像中出现块状伪影等问题,总广义全变分(total generalized variation,TGV)算法可以逼近任意阶函数,再结合非局部均值算法的思想,同时考虑到不同能谱通道下重建图像的相关性,将高质量全能谱重建图像作为先验图像指导能谱CT重建,提出了基于先验图像约束压缩感知(prior image constrained compressed sensing,PICCS)的非局部TGV重建算法.实验结果表明,所提算法在抑制噪声的同时能够有效复原图像细节及边缘信息,且收敛速度快.     

Selection of Embedding Dimension and Delay Time in Phase Space Reconstruction via Symbolic Dynamics

The modeling and prediction of chaotic time series require proper reconstruction of the state space from the available data in order to successfully estimate invariant properties of the embedded attractor. Thus, one must choose appropriate time delay τ∗ and embedding dimension p for phase space reconstruction. The value of τ∗ can be estimated from the Mutual Information, but this method is rather cumbersome computationally. Additionally, some researchers have recommended that τ∗ should be chosen to be dependent on the embedding dimension p by means of an appropriate value for the time delay τw=(p−1)τ∗, which is the optimal time delay for independence of the time series. The C-C method, based on Correlation Integral, is a method simpler than Mutual Information and has been proposed to select optimally τw and τ∗. In this paper, we suggest a simple method for estimating τ∗ and τw based on symbolic analysis and symbolic entropy. As in the C-C method, τ∗ is estimated as the first local optimal time delay and τw as the time delay for independence of the time series. The method is applied to several chaotic time series that are the base of comparison for several techniques. The numerical simulations for these systems verify that the proposed symbolic-based method is useful for practitioners and, according to the studied models, has a better performance than the C-C method for the choice of the time delay and embedding dimension. In addition, the method is applied to EEG data in order to study and compare some dynamic characteristics of brain activity under epileptic episodes     

A numerical method for the viscous incompressible Oseen flow in shape reconstruction

This paper is concerned with the shape reconstruction of a bounded domain with a viscous incompressible fluid driven by the Oseen equations. For the approximate solution of the ill-posed and nonlinear problem we propose a regularized Gauss-Newton method. A theoretical foundation for the method is given by establishing the differentiability of the boundary value problem with respect to the boundary in the sense of the domain derivative. The results of several numerical experiments show that our theory is useful for practical purpose, and the proposed algorithm is feasible.     

An implicit high-order preconditioned flux reconstruction method for low-Mach-number flow simulation with dynamic meshes

A fully implicit high-order preconditioned flux reconstruction/correction procedure via reconstruction (FR/CPR) method is developed to solve the compressible Navier-Stokes equations at low Mach numbers. A dual-time stepping approach with the second-order backward differentiation formula (BDF2) is employed to ensure temporal accuracy for unsteady flow simulation. When dynamic meshes are used to handle moving/deforming domains, the geometric conservation law is implicitly enforced to eliminate errors due to the resolution discrepancy between BDF2 and the spatial FR/CPR discretization. The large linear system resulted from the spatial and temporal discretizations is tackled with the restarted generalized minimal residual solver in the PETSc (portable, extensible toolkit for scientific computation) library. Through several benchmark steady and unsteady numerical tests, the preconditioned FR/CPR methods have demonstrated good convergence and accuracy for simulating flows at low Mach numbers. The new flow solver is then used to study the effects of Mach number on unsteady force generation over a plunging airfoil when operating in low-Mach-number flows. It is observed that weak compressibility has a significant impact on thrust generation but has a negligible effect on lift generation of an oscillating airfoil.     

应用隧道探索算法提取莫尔条纹偏移量

为了精确提取莫尔条纹偏移量,提出了隧道探索算法,并将其应用于实际莫尔条纹分析.使用双光栅产生莫尔条纹,抓拍多方向投影.应用数字图像处理技术,追踪分析实验莫尔条纹.首先,追踪各条纹的极大值分布,根据条纹极大值分布挖掘条纹隧道,调整挖掘宽度,即像素宽度,既实现隧道贯通,又不干预相邻条纹.然后,二值化莫尔图,噪声滤波,滤除条纹隧道壁上的噪声毛刺,使隧道壁趋于光滑.最后,根据两侧隧道壁的布局,探索条纹隧道走向,提取隧道走向数据,获得条纹偏移量.并进一步转换为偏折角,即光学计算机层析投影.应用非线性自适应迭代重建算法对偏折角进行迭代重建.结果发现,最高重建截面温度是492℃,额定电热器表面温度大约500℃,重建截面恰恰在电热器上表面上方,所以重建结果是有效的.     

Connections between the discontinuous Galerkin method and high‐order flux reconstruction schemes

With high‐order methods becoming more widely adopted throughout the field of computational fluid dynamics, the development of new computationally efficient algorithms has increased tremendously in recent years. One of the most recent methods to be developed is the flux reconstruction approach, which allows various well‐known high‐order schemes to be cast within a single unifying framework. Whilst a connection between flux reconstruction and the more widely adopted discontinuous Galerkin method has been established elsewhere, it still remains to fully investigate the explicit connections between the many popular variants of the discontinuous Galerkin method and the flux reconstruction approach. In this work, we closely examine the connections between three nodal versions of tensor‐product discontinuous Galerkin spectral element approximations and two types of flux reconstruction schemes for solving systems of conservation laws on quadrilateral meshes. The different types of discontinuous Galerkin approximations arise from the choice of the solution nodes of the Lagrange basis representing the solution and from the quadrature approximation used to integrate the mass matrix and the other terms of the discretization. By considering both linear and nonlinear advection equations on a regular grid, we examine the mathematical properties that connect these discretizations. These arguments are further confirmed by the results of an empirical numerical study. Copyright © 2014 John Wiley & Sons, Ltd.