空间约束结合支持向量机提高毫秒激光诱导击穿光谱的铝合金中的Fe元素成分检测精度

铝合金中Fe元素的浓度会影响铝合金的软硬程度,从而影响铝合金器件的工作使用寿命,因此铝合金中Fe的含量检测精度非常重要,开展了空间约束结合支持向量机提高毫秒激光诱导击穿光谱的铝合金中的Fe元素成分检测精度研究.在平板空间约束条件下,毫秒激光诱导铝等离子体光谱出现了光谱增强,并且提高了等离子体辐射光谱稳定性,光谱辐射中的...     

Deformation and regimes of liquid column during water exit of a partially submerged sphere using the front-tracking lattice Boltzmann method

When a solid sphere exits water at a given velocity, the liquid column is pulled out of a liquid reservoir. The present study focuses on the dynamic deformation of the liquid column and on the identification of the liquid column regime on the Weber number and dimensionless time (We-t*) map. The three-dimensional model of water exit has been established on the basis of the lattice Boltzmann method. A mass tracking algorithm was incorporated to capture the free surface, where the liquid–gas​ two-phase flow is simplified to a single-phase free surface flow. The surface tension is included by adding a perturbation term and the gravity as a body force is introduced in form of calculating the equilibrium distribution with an altered velocity. The contact angle condition in numerical simulation is not considered. The accuracy of the numerical results is demonstrated through the comparisons with the prior numerical and the experimental results in the literature. A parametric study has been conducted numerically on the radius and volume and of liquid column. Besides, the relationship between the pinch-off time of a liquid column and the Froude number has been obtained. Moreover, the evolution of the liquid column shape with a wide range of the Weber number and time scales are discussed based on the number of ligaments and droplets. The liquid column exhibits different characteristics and has been divided into three regimes. The transition between different regimes has been analyzed and the critical Weber number is given.     

悬浮态上皮细胞粘附的力学-化学耦合模型及数值模拟

上皮细胞通过局部募集上皮性钙粘附蛋白 (E-cadherin) 建立胞间粘着连接, 实验证实该过程受到肌球蛋白皮层张力的调控. 为了从系统层面阐明粘着连接形成动力学过程, 本文考察皮层张力调控肌动蛋白 (F-actin) 解聚从而参与E-cadherin级联转导, 同时以马达-离合器机制模拟两细胞相互作用, 据此构建可反映悬浮态细胞粘附的力学-化学耦合数学模型; 对整体包含随机点源的非线性反应-扩散方程组与平衡微分方程耦合系统采取了自行发展的格子Boltzmann-粒子法与蒙特-卡洛法数值求解. 数值模拟表明, 由收缩性肌球蛋白 (myosin-II) 拉动胞间E-cadherin成键可提升皮层张力, 进而降低F-actin解聚速率﹑锚定更多的E-cadherin; 所构成的力学反馈回路展现出时空效应, 可帮助E-cadherin在接触区建立初始极性; E-cadherin形成顺式二聚体则将初始极性放大, 导致接触区E-cadherin展现起始、快速增长及慢速增长的积聚动力学特征. 皮层呈松散结构时刚度较小, 可通过延长胞间E-cadherin成键寿命提升张力, 而接触区弧度适中时($\approx$1.2 rad) E-cadherin峰值最高; 两者可分别作为启动力学反馈回路及调控粘着连接成熟度的有效手段.      

Moment-based boundary conditions for straight on-grid boundaries in three-dimensional lattice Boltzmann simulations

In this article, moment-based boundary conditions for the lattice Boltzmann method are extended to three dimensions. Boundary conditions for velocity and pressure are explicitly derived for straight on-grid boundaries for the D3Q19 lattice. The method is compared against the bounce-back scheme using both single and two relaxation time collision schemes. The method is verified using classical benchmark test cases. The results show very good agreement with the data found in the literature. It is confirmed from the results that the derived moment-based boundary scheme is of second-order accuracy in grid spacing and does not produce numerical slip, and therefore offers a transparent way of accurately prescribing velocity and pressure boundaries that are aligned with grid points in three-dimensional.     

Strongly anisotropic thermal conductivity in planar hexagonal borophene oxide sheet

The flat hexagonal borophene oxide (B₂O) has the highest Li storage capacity among existing two-dimensional materials. Thermal conductivity is an important parameter for the safety of Li-ion batteries. We investigate the lattice thermal conductivity of B₂O by solving phonon Boltzmann transport equation combined with the first-principles calculations. We found that the relaxation time approximation remarkably underestimate the thermal conductivity (κ) of monolayer B₂O, revealing phonon hydrodynamics characteristic. The κ of B₂O from the exact solution of Boltzmann transport equation is 53 W m⁻¹ K⁻¹ and 130 W m⁻¹ K⁻¹ along armchair-direction and zigzag-direction at 300 K, respectively. B₂O exhibits strong thermal transport anisotropy due to anisotropic phonon group velocity, obviously larger than that of other borophene allotropes. At room temperature, the phonon mean free path of B₂O is about 231 nm and 49 nm along armchair-direction and zigzag-direction, respectively. The highly anisotropic thermal conductivity of B₂O offers new possibilities for its applications in thermal management.     

方腔内相变材料固液相变传热研究

基于相变材料(PCM,phase change material)的相变储能设备具有储能密度高的特点。本文建立了基于相变储能元件伪焓模型的固液相变格子Boltzmann模型,研究了内部管道位置、方腔倾斜角度对PCM融化过程的影响规律。结果表明,在内管道靠近方腔上部时,由于上部界面(固液相变界面或上壁面)对自然对流阻碍作用,使PCM的融化速率减慢。但是,在此时使方腔发生倾斜,会改变管道热流体到上部界面的距离,强化PCM的热质传递过程,使融化加快。     

A rotating reference frame‐based lattice Boltzmann flux solver for simulation of turbomachinery flows

In this paper, the newly developed lattice Boltzmann flux solver (LBFS) is developed into a version in the rotating frame of reference for simulation of turbomachinery flows. LBFS is a finite volume solver for the solution of macroscopic governing differential equations. Unlike conventional upwind or Godunov‐type flux solvers which are constructed by considering the mathematical properties of Euler equations, it evaluates numerical fluxes at the cell interface by reconstructing local solution of lattice Boltzmann equation (LBE). In other words, the numerical fluxes are physically determined rather than by some mathematical approximation. The LBE is herein expressed in a relative frame of reference in order to correctly recover the macroscopic equations, which is also the basis of LBFS. To solve the LBE, an appropriate lattice Boltzmann model needs to be established in advance. This includes both the determinations of the discrete velocity model and its associated equilibrium distribution functions. Particularly, a simple and effective D1Q4 model is adopted, and the equilibrium distribution functions could be efficiently obtained by using the direct method. The present LBFS is validated by several inviscid and viscous test cases. The numerical results demonstrate that it could be well applied to typical and complex turbomachinery flows with favorable accuracy. It is also shown that LBFS has a delicate dissipation mechanism and is thus free of some artificial fixes, which are often needed in conventional schemes. Copyright © 2016 John Wiley & Sons, Ltd.     

Quadratic Majorization for Nonconvex Loss with Applications to the Boosting Algorithm

Classical robust statistical methods dealing with noisy data are often based on modifications of convex loss functions. In recent years, nonconvex loss-based robust methods have been increasingly popular. A nonconvex loss can provide robust estimation for data contaminated with outliers. The significant challenge is that a nonconvex loss can be numerically difficult to optimize. This article proposes quadratic majorization algorithm for nonconvex (QManc) loss. The QManc can decompose a nonconvex loss into a sequence of simpler optimization problems. Subsequently, the QManc is applied to a powerful machine learning algorithm: quadratic majorization boosting algorithm (QMBA). We develop QMBA for robust classification (binary and multi-category) and regression. In high-dimensional cancer genetics data and simulations, the QMBA is comparable with convex loss-based boosting algorithms for clean data, and outperforms the latter for data contaminated with outliers. The QMBA is also superior to boosting when directly implemented to optimize nonconvex loss functions. Supplementary material for this article is available online.     

A partial entropic lattice Boltzmann MHD simulation of the Orszag–Tang vortex

Karlin has introduced an analytically determined entropic lattice Boltzmann (LB) algorithm for Navier-Stokes turbulence. Here, this is partially extended to an LB model of magnetohydrodynamics, on using the vector distribution function approach of Dellar for the magnetic field (which is permitted to have field reversal). The partial entropic algorithm is benchmarked successfully against standard simulations of the Orszag–Tang vortex [Orszag, S.A.; Tang, C.M. J. Fluid Mech. 1979, 90 (1), 129–143].     

β型锑烯热电输运性质的第一性原理研究

利用第一性原理与半经典玻尔兹曼方程，计算并分析β型锑烯的声子色散、声子群速度、声子弛豫时间、晶格热导率及不同温度下的塞贝克系数、电导率和电子热导率随化学势的变化；结果表明：β型锑烯由于非平面六角结构，三支声学声子在Γ点附近均呈线性变化；声学声子对整个晶格热导率的贡献高达96.68%，而光学声子仅仅占到3.32%；由于较大的声光带隙（a-o gap）导致LA支在声子群速度和弛豫时间中占据主导地位，从而增大了LA支声子对整个热导的贡献；热电优值随温度的升高而增大，在费米面附近其绝对值最大可达0.275.