Crystal Structure and Improved Synthesis of 1‐(2H‐Tetrazol‐5‐yl)guanidium Nitrate

Energetic derivatives of tetrazoles are one of the key areas of research focus in pursuit of novel high energy materials, useful as propellants and explosives. Herein, the crystal structure and an improved synthetic procedure of 1‐(2H‐tetrazol‐5‐yl)guanidine ( 1 ) and its nitrate salt ( 2 ) are reported. The compounds were structurally characterized by spectroscopic (FT‐IR, ¹H NMR, ¹³C NMR) and elemental analysis. The molecular structure of tetrazolyl guanidium nitrate ( 2 ) was solved using low temperature single‐crystal X‐ray diffraction. 2 crystallized as its hemihydrate in the orthorhombic space group Fdd2, with a crystal density of 1.69 g cm⁻³. Thermal behavior and decomposition of the molecules were studied with differential scanning calorimetry (DSC). Molar enthalpy of formation (Δ_fH^m) of compound 2 was back calculated from heat of combustion (Δ_cH⁰) value obtained experimentally using bomb calorimetric measurements. Lattice enthalpy of 1‐(2H‐tetrazol‐5‐yl)guanidium nitrate was directly calculated from measured crystal density using Jenkins equation. Preliminary ballistic parameters of the compound were predicted and compared with reported high nitrogen tetrazole derivatives.     

Bidirectional Evolutionary Structural Optimization (BESO) based design method for lattice structure to be fabricated by additive manufacturing

Unlike traditional manufacturing methods, additive manufacturing can produce parts with complex geometric structures without significant increases in fabrication time and cost. One application of additive manufacturing technologies is the fabrication of customized lattice-skin structures which can enhance performance of products while minimizing material or weight. In this paper, a novel design method for the creation of periodic lattice structures is proposed. In this method, Functional Volumes (FVs) and Functional Surfaces (FSs) are first determined based on an analysis of the functional requirements. FVs can be further decomposed into several sub-FVs. These sub-FVs can be divided into two types: FV with solid and FV with lattice. The initial design parameters of the lattice are selected based on the proposed guidelines. Based on these parameters, a kernel based lattice frame generation algorithm is used to generate lattice wireframes within the given FVs. At last, traditional bidirectional evolutionary structural optimization is modified to optimize distribution of lattice struts’ thickness. The design method proposed in this paper is validated through a case study, and provides an important foundation for the wide adoption of additive manufacturing technologies in the industry.     

Dynamic behavior of droplet transport on realistic gas diffusion layer with inertial effect via a unified lattice Boltzmann method

The dynamic behavior of liquid droplets on a reconstructed real gas diffusion layer (GDL) surface with the inertial effect produced by the three dimensional (3D) flow channel is investigated using an improved pseudopotential multiphase model within the unified lattice Boltzmann model (ULBM) framework, which can realize thermodynamic consistency and tunable surface tension. The microstructure of the GDL (Toray-090) including carbon fibers and polytetrafluoroethylene (PTFE) is reconstructed by a stochastic and mixed-wettability model. The critical force formulation for the Cassie-Wenzel transition of a droplet on GDL surface is derived. The effects of inertia and contact angles on the liquid droplet transport process on a reconstructed real GDL surface with a 3D flow channel are investigated. The results show the normalized center-of-mass coordinate X may enter the channel wall area or fluctuate around the initial position. With increased inertia applied on the droplet, the normalized center-of-mass coordinate Y grows faster and the normalized center-of-mass coordinate Z decreases. It is found by the ULBM for the first time that the liquid droplet is pushed back into the GDL by inertial effect. With the increase of inertia and the decrease of contact angle of GDL, both the droplet penetration depth in GDL and the droplet invasion fraction increase. The droplet invasion fraction in GDL is up to 30%.     

Investigation of coalesced droplet vertical jumping and horizontal moving on textured surface using the lattice Boltzmann method

The coalesced droplet vertical jumping and horizontal moving on conical posts textured surface are numerically studied using the three-dimensional (3D) multi-relaxation-time (MRT) pseudopotential lattice Boltzmann model. The influences of wettability gradient and roughness gradient are investigated systematically. It is found that the coalesced droplet on the flat and conical posts textured surfaces can move horizontally from superhydrophobic bend to hydrophobic bend without the roughness gradient. Moreover, the coalesced droplet is able to spontaneously move from lower conical post density region to higher conical post density region without the wettability gradient. Specifically, the in-line array textured surface is more beneficial to the coalesced droplet horizontal moving than the staggered array at the same wettability parameter. However, the staggered array textured surface is more beneficial to the coalesced droplet vertical jumping than the in-line array. The hybrid effect of wettability gradient and roughness gradient plays critical roles in coalesced droplet vertical jumping and horizontal moving. The present work demonstrates that the dropwise condensation heat transfer can be enhanced in a self-sustained manner if the wettability and roughness of the textured surface are properly designed. It is also confirmed that the 3D MRT pseudopotential lattice Boltzmann model is of potential to simulate coalesced droplet behaviors on textured surface.     

二维湍流外力尺度影响的格子Boltzmann方法研究

该文基于格子Boltzmann方法,研究系统外力尺度kmax/kf对二维湍流的能谱和统计行为的影响。考虑了两种系统外力(高斯力和Kolmogorov力)和三种外力尺度max/fk k。在不存在线性摩擦力情况下,验证了描述二阶涡量结构函数与能谱关系的Benzi理论。研究表明:当外力尺度kmax/kf逐渐增大时,Kolmogorov外力情况下的二阶涡量结构函数与能谱之间的关系逐渐符合Benzi理论。在存在线性摩擦力情况下,出现能谱双能级。随着外力尺度kmax/kf逐渐增大,Kolmogorov外力情况下的直接惯性区能谱逐渐接近Kraichnann理论。在两种系统外力情况下涡量场和速度场的概率分布函数(PDF)具有指数尾迹,与Falkovich和Lebedev理论预测的结果一致。PDF随着kmax/kf增加有更大的峰度(Kurtosis),这意味着在直接级联惯性区具有更强的间歇性。     

Adaptive burnup stepsize selection using control theory for 2-D lattice depletion simulations

A control theory approach is adopted to determine the temporal discretization during two-dimensional lattice physics depletion simulations. Two primary applications of automated and adaptive stepsize control are identified: (i) the presence of strong absorbers such as gadolinium, where the accurate burnout of the isotopes requires a depletion stepsize smaller than typically required, and (ii) high fidelity multiphysics simulations, e.g. loosely coupled physics, where the coupled physics are nonlinear in time and stepsize changes may be necessary to obtain an accurate coupled solution. A conventional predictor–corrector method is used to address the nonlinearity of the nuclide transmutation and neutron flux. An adaptive stepsize method is developed based on monitoring the one-group scalar neutron flux at both the predictor and corrector steps to approximate the convergence residual of the nonlinear solution. A user-specified tolerance on the L² relative error norm of the scalar neutron flux is utilized by the stepsize controller. Controllers that include integral, proportional, and/or derivative components are investigated and parameterized using Latin hypercube sampling of the controller input parameters. Three distinct fuel loadings of pressurized water reactor 17 × 17 fuel pin assemblies are considered, including no burnable absorbers, Integral Fuel Burnable Absorber, and gadolinium fuel pins. The required depletion stepsizes, as predicted throughout the cycle by the controller, are compared with a very small stepsize (0.01 MW d/kgHM) reference solution and a solution obtained by a typical rule of thumb depletion stepsize sequence.     

Fusion of multifocus images by lattice structures

Image fusion methods based on multiscale transform (MST) suffer from high computational load due to the use of fast Fourier transforms (ffts) in the lowpass and highpass filtering steps. Lifting wavelet scheme which is based on second generation wavelets has been proposed as a solution to this issue. Lifting Wavelet Transform (LWT) is composed of split, prediction and update operations all implemented in the spatial domain using multiplications and additions, thus computation time is highly reduced. Since image fusion performance benefits from undecimated transform, it has later been extended to Stationary Lifting Wavelet Transform (SLWT). In this paper, we propose to use the lattice filter for the MST analysis step. Lattice filter is composed of analysis and synthesis parts where simultaneous lowpass and highpass operations are performed in spatial domain with the help of additions/multiplications and delay operations, in a recursive structure which increases robustness to noise. Since the original filter is designed for the undecimated case, we have developed undecimated lattice structures, and applied them to the fusion of multifocus images. Fusion results and evaluation metrics show that the proposed method has better performance especially with noisy images while having similar computational load with LSWT based fusion method.     

Developing community codes for materials modeling

For this article, we call scientific software a community code if it is freely available, written by a team of developers who welcome user input, and has attracted users beyond the developers. There are obviously many such materials modeling codes. The authors have been part of such efforts for many years in the field of atomistic simulation, specifically for two community codes, the LAMMPS and GULP packages for molecular dynamics and lattice dynamics respectively. Here we highlight lessons we have learned about how to create such codes and the pros and cons of being part of a community effort. Many of our experiences are similar, but we also have some differences of opinion (like modeling vs modelling). Our hope is that readers will find these lessons useful as they design, implement, and distribute their own materials modelling software for others to use.     

C60 Dimers Revisited

Abstract

Several dimers: C₁₂₀, C₁₄₀, C₁₃₀ and their oxygen‐containing derivatives as well as some oligomers or polymers of the well known spherical fullerenes have been synthesized so far. Some of the formulas attributed to such compounds represent the first steps in a more complex fate leading to trivalent capped tubulenes. Such a way could even be followed by functionalized fullerenes. A detailed network transformation is given for C₆₀ and its mono‐ and di‐epoxy derivatives. Semiempirical calculations support the presented findings.     

Large-scale parallel lattice Boltzmann–cellular automaton model of two-dimensional dendritic growth

An extremely scalable lattice Boltzmann (LB)–cellular automaton (CA) model for simulations of two-dimensional (2D) dendritic solidification under forced convection is presented. The model incorporates effects of phase change, solute diffusion, melt convection, and heat transport. The LB model represents the diffusion, convection, and heat transfer phenomena. The dendrite growth is driven by a difference between actual and equilibrium liquid composition at the solid–liquid interface. The CA technique is deployed to track the new interface cells. The computer program was parallelized using the Message Passing Interface (MPI) technique. Parallel scaling of the algorithm was studied and major scalability bottlenecks were identified. Efficiency loss attributable to the high memory bandwidth requirement of the algorithm was observed when using multiple cores per processor. Parallel writing of the output variables of interest was implemented in the binary Hierarchical Data Format 5 (HDF5) to improve the output performance, and to simplify visualization. Calculations were carried out in single precision arithmetic without significant loss in accuracy, resulting in 50% reduction of memory and computational time requirements. The presented solidification model shows a very good scalability up to centimeter size domains, including more than ten million of dendrites.