Imposing accurate wall boundary conditions in corrective‐matrix‐based moving particle semi‐implicit method for free surface flow

Corrective matrix that is derived to restore consistency of discretization schemes can significantly enhance accuracy for the inside particles in the Moving Particle Semi‐implicit method. In this situation, the error due to free surface and wall boundaries becomes dominant. Based on the recent study on Neumann boundary condition (Matsunaga et al, CMAME, 2020), the corrective matrix schemes in MPS are generalized to straightforwardly and accurately impose Neumann boundary condition. However, the new schemes can still easily trigger instability at free surface because of the biased error caused by the incomplete/biased neighbor support. Therefore, the existing stable schemes based on virtual particles and conservative gradient models are applied to free surface and nearby particles to produce a stable transitional layer at free surface. The new corrective matrix schemes are only applied to the particles under the stable transitional layer for improving the wall boundary conditions. Three numerical examples of free surface flows demonstrate that the proposed method can help to reduce the pressure/velocity fluctuations and hence enhance accuracy further.     

Deciphering the Structure and Chemical Composition of Drug Nanocarriers: From Bulk Approaches to Individual Nanoparticle Characterization

Drug nanocarriers (NCs) with sizes usually below 200 nm are gaining increasing interest in the treatment of severe diseases such as cancer and infections. Characterization methods to investigate the morphology and physicochemical properties of multifunctional NCs are key in their optimization and in the study of their in vitro and in vivo fate. Whereas a variety of methods has been developed to characterize “bulk” NCs in suspension, the scope of this review is to describe the different approaches for the NC characterization on an individual basis, for which fewer techniques are available. The accent is put on methods devoid of labelling, which could lead to artefacts. For each characterization method, the principles and approaches to analyze the data are presented in an accessible manner. Aspects related to sample preparation to avoid artefacts are indicated, and emphasis is put on examples of applications. NC characterization on an individual basis allows gaining invaluable information in terms of quality control, on: i) NC localization and fate in biological samples; ii) NC morphology and crystallinity; iii) distribution of the NC components (drugs, shells), and iv) quantification of NCs’ chemical composition. The individual characterization approaches are expected to gain increasing interest in the near future.     

石英砂-聚乙烯二元混合体系的太赫兹光谱特性研究

太赫兹时域光谱技术目前逐渐应用于对岩石的研究中。在制备样品时通常需要将岩石磨碎后与粘合剂混合压片，岩石的含量、粒径等都会对测试结果造成影响。将自然界中常见的石英砂粉末（不同粒径）与聚乙烯（PE）微粒以不同的比例混合，通过压片的手段将其制成适用于太赫兹系统测试的样品，用以探究样品中石英砂的含量以及其粒径对实验结果的影响。首先研究石英砂含量对实验结果造成的影响。在保持样品中石英砂的粒径不变的情况下，发现不同石英砂含量样品的时域光谱图中，时间延迟以及峰值都呈现出非单调的变化趋势。为了探究出现这种现象的原因，对样品的折射率以及吸收系数做了进一步的分析。结果表明样品的折射率会随着样品中石英砂含量的增加而逐渐增大，通过适用于本实验的有效介质理论能够解释这一现象。样品对于太赫兹波段的吸收系数随着石英砂含量的增加呈现出先增大后减小的现象，并且在石英砂质量分数为60%时达到最大值。为了解释这一现象产生的原因，利用扫描电子显微镜对样品的微观形貌进行观察，发现随着石英砂含量的增加，在压制样品时PE颗粒破碎程度加剧，导致PE的粒径变小。根据米氏散射以及瑞利散射的原理，石英砂粒径不变而PE粒径减小，随之降低的散射强度与石英砂的吸收效应发生竞争，从而导致了吸收系数先增大后减小的现象。进一步研究了石英砂粒径对实验结果的影响，对不同粒径的石英砂样品进行测试，发现折射率不随石英砂粒径的变化而改变，但其吸收系数随着石英砂粒径的减小而逐渐减小。根据米氏散射，样品吸收系数的变化是由于散射强度随石英砂粒径的减小而逐渐减小。研究表明，样品中石英砂的含量和粒径都会对实验结果产生影响。石英砂粒径相同时，样品的吸收系数随石英砂含量的增加呈现先增大后减小的趋势，样品的折射率随石英砂含量的增加而逐渐增大；石英砂含量相同时，样品的吸收系数随石英砂粒径的减小而逐渐减小，而样品的折射率基本不变。这一结论对矿物样品的制备以及对混合物的实验结果分析有一定程度的指导意义。     

托卡马克中局域磁扰动引起的高能量离子损失研究

基于单粒子导心运动代码ORBIT，采用测试粒子模拟方法，研究了托卡马克等离子体内部不同径向位置处局域磁场扰动对高能量离子的损失的影响。研究表明，在局域磁扰动主要分布在某磁面附近、其环向具有类似纹波场形式下，可造成一些靠近等离子体中心区域的高能量离子损失，但对靠近等离子边界的离子损失影响相对不大。这些损失的高能量离子均为捕获离子，离子的投掷角越大就越容易损失。此外，造成高能量离子最大损失率的局域场径向位置与这些损失离子的初始径向位置通常存在一定的偏移，而且这个偏移与这些离子的能量密切相关。当局域场出现在某些位置时，能量较低的离子会有一定的损失，能量较高的离子反而不会损失。     

基于临界状态的砂土本构模型研究

砂土孔隙比及所受压力是其力学特性的重要影响因素. 本文基于砂土临界状态线特性分析,采用以e-(p/p_a)ξ平面内的线性关系描述其等向压缩线. 通过对比分析两种不同压缩线函数 与临界状态线函数之间的关系提出更适合描述砂土在等向压缩下的参考压缩线,并给出了基于参考压缩线的等向硬化规律. 建议了适用于 描述砂土剪切特性的屈服面函数,并给出利用等向压缩和等p路径确定屈服面形状参数μ的方法. 将不同应力比对应的压缩线作为砂土状态参量参考线,以获取潜在强度M_f与特征状态应力比M_c,进而描述砂土压缩与剪切特性;基于等向压缩与等p路径建立了当前应力比与状态参量参考线之间的相关关系,从而实现了砂土状 态参量参考线由参考压缩线向临界状态线平稳过渡. 建立的砂土本构模型共11个参数,均能够通过常规土工试验或经验获取. 基于模型预测与Toyoura砂的等向压缩、三轴不排水剪切试验及排水剪切试验的对比结果,本文建立的砂土本构模型很好地描述了Toyoura 砂在不同孔隙比和不同压力下的压缩与剪切特性.      

A high‐order PIC method for advection‐dominated flow with application to shallow water waves

A hybrid Eulerian‐Lagrangian particle‐in‐cell–type numerical method is developed for the solution of advection‐dominated flow problems. Particular attention is given over to the high‐order transfer of flow properties from the particles to the grid. For smooth flows, the method presented is of formal high‐order accuracy in space. The method is applied to solve the nonlinear shallow water equations resulting in a new, and novel, shock capturing shallow water solver. The approach is able to simulate complex shallow water flows, which can contain an arbitrary number of discontinuities. Both trivial and nontrivial bottom topography is considered, and it is shown that the new scheme is inherently well balanced, exactly satisfying the ‐property. The scheme is verified against several one‐dimensional benchmark shallow water problems. These include cases that involve transcritical flow regimes, shock waves, and nontrivial bathymetry. In all the test cases presented, very good results are obtained.     

Space potential particles to enhance the stability of projection-based particle methods

Particle methods have been seldom verified by a Karman vortex simulation, which is commonly performed as a typical benchmark in computational fluid dynamics. This is mainly due to a difficulty in suppression of occurrence of unphysical voids manifested usually in a strong vortex on account of definition of free surface by the Lagrangian tracking framework with inconsistency in volume conservation. This paper presents a simple and effective scheme as a free-surface boundary condition of projection-based particle methods, namely the MPS (moving particle semi-implicit) and Incompressible SPH (ISPH) methods to handle the free surface with consistency in volume conservation. The new scheme is introduced into the Poisson pressure equation (PPE) with consideration of a potential in void space as space potential particle (SPP), to reproduce physical motions of particles around free surface through a particle–void interaction. The enhancing effect of the newly proposed SPP scheme is shown by simulating a few numerical tests, including a whirling water flow, a two-phase surfacing flow, and a set of Karman vortex simulations.     

Bammann-Chiesa-Johnson粘塑性本构模型材料参数的一种识别方法

粘塑性本构模型能否成功模拟金属高应变率大应变变形过程依赖于材料参数识别结果的好坏。由于BCJ模型考虑了应变率、温度与材料硬化之间的耦合效应以及应变率、温度历史效应,同时模型中包含了多个材料参数,因此很难通过试验直接识别模型的材料参数。本文针对BCJ模型中的耦合效应和历史效应,基于对模型中材料参数物理涵义的界定,给出了一种对材料参数解耦、分离并进行估计的方法,获得了模型材料参数估计公式,估计了材料参数的取值范围。在此基础上,编制了BCJ模型应力积分径向返回算法和粒子群优化算法的计算程序,应用重新设计了BCJ模型耦合效应和历史效应的反分析方法,在参数取值范围内对材料参数进行了优化识别。以OFHC Cu为例,应用提出的识别方法对BCJ模型的材料参数进行了识别,计算结果和试验结果符合较好。     

Modified Lagrangian vortex method with improved boundary conditions for water waves past a thin bottom‐standing barrier

Herein, the modified Lagrangian vortex method (LVM), a hybrid analytical‐numerical algorithm per se, is devised to simulate the process of vortex formation and shedding from the sharp edge of a zero‐thickness vertical plate under linear water‐wave attack. Application of the Helmholtz decomposition facilitates a convenient switch between the inviscid‐ and viscous‐flow models, thereby enabling easy incorporation of vorticity effects into the potential‐flow calculations for the viscous‐dominated region. In evaluating the potential‐flow component, making good use of the quickly convergent technique with singular basis functions, correctly capturing the singular behavior in velocity fields near the tip of the plate, leads to a considerable reduction of computational burdens and to 12‐decimal‐place accuracy. The viscous correction is carried out via the meshless LVM with improved boundary conditions. Comparisons with previously published results show good agreement. Simulations of vortex generation and evolution illuminate the ability of the present method, and provide a supplement to pertinent experimental works. The hybrid scheme proposed herein allows flexibility for the former LVM and convenience in the code development. Such a compromise fits particularly well for the high‐resolution modeling of sharp‐edged vortex shedding without heavy numerical developments. Copyright © 2014 John Wiley & Sons, Ltd.     

颗粒毛细效应影响因素的离散元分析

颗粒毛细效应是指将一根细管插入填充有颗粒物质的容器中并对管施加竖直振动时颗粒在管内上升并最终达到一个稳定的高度的现象, 该现象为颗粒物料的逆重力输运提供了一种潜在的技术途径. 为探究颗粒毛细效应的影响因素, 采用离散元方法, 模拟再现了颗粒毛细效应过程,展示了不同管径下颗粒竖直方向速度演变特性, 考察了不同容器宽度和振动条件下颗粒最终毛细上升高度随管径的演变规律. 结果表明, 在容器宽度与粒径比为40、管振幅与粒径比为14.33、管振动频率为12 Hz情况下, 管径与粒径比$D/d = 3.33$时, 管内颗粒堵塞严重, 使得颗粒上升缓慢,并造成颗粒柱中断; $D/d = 8.33$时, 起初毛细上升高度增加迅速, 随后毛细上升高度的增大逐渐减缓, 管内颗粒在管径方向几乎不存在速度梯度; $D/d =15$时, 随着颗粒毛细上升高度的增大, 管内颗粒柱分离为速度截然不同的两层, 上层颗粒在管径方向几乎不存在速度梯度, 而下层颗粒存在明显的速度梯度.研究还发现, 在毛细效应能够发生的管径范围内, 存在一个对应于颗粒最终毛细上升高度最大值的临界管径, 当管径小于临界管径时, 颗粒最终毛细上升高度随管径的增大而增大, 当管径大于临界管径时, 颗粒最终毛细上升高度随管径的增大而趋于减小; 增大容器宽度,临界管径有所增大; 增大振幅、适当提高频率能够有效促进临界管径的增大.