On the definition of cavitation intensity

Cavitation intensity has already been used to character the activity or strength of cavitation, and several methods are developed to measure the cavitation intensity. However, the previous definitions of cavitation intensity are often either vague or biased. In this paper, from the point of view of energy, the authors proposed a generalized definition of cavitation intensity, derived an approximate formula to calculate the cavitation intensity and discussed its measure method.     

Finite element computations of viscoelastic two-phase flows using local projection stabilization

A three-field local projection stabilized (LPS) finite element method is developed for computations of a three-dimensional axisymmetric buoyancy driven liquid drop rising in a liquid column where one of the liquid is viscoelastic. The two-phase flow is described by the time-dependent incompressible Navier-Stokes equations, whereas the viscoelasticity is modeled by the Giesekus constitutive equation in a time-dependent domain. The arbitrary Lagrangian-Eulerian (ALE) formulation with finite elements is used to solve the governing equations in the time-dependent domain. Interface-resolved moving meshes in ALE allows to incorporate the interfacial tension force and jumps in the material parameters accurately. A one-level LPS based on an enriched approximation space and a discontinuous projection space is used to stabilize the numerical scheme. A comprehensive numerical investigation is performed for a Newtonian drop rising in a viscoelastic fluid column and a viscoelastic drop rising in a Newtonian fluid column. The influence of the viscosity ratio, Newtonian solvent ratio, Giesekus mobility factor, and the Eötvös number on the drop dynamics are analyzed. The numerical study shows that beyond a critical Capillary number, a Newtonian drop rising in a viscoelastic fluid column experiences an extended trailing edge with a cusp-like shape and also exhibits a negative wake phenomena. However, a viscoelastic drop rising in a Newtonian fluid column develops an indentation around the rear stagnation point with a dimpled shape.     

Uncertainty quantification for dynamics of geometrically nonlinear structures coupled with internal acoustic fluids in presence of sloshing and capillarity

In this paper, we propose an uncertainty quantification analysis, which is the continuation of a recent work performed in a deterministic framework. The fluid–structure system under consideration is the one experimentally studied in the sixties by Abramson, Kana, and Lindholm from the Southwest Research Institute under NASA contract. This coupled system is constituted of a linear acoustic liquid contained in an elastic tank that undergoes finite dynamical displacements, inducing geometrical nonlinear effects in the structure. The liquid has a free surface on which sloshing and capillarity effects are taken into account. The problem is expressed in terms of the acoustic pressure field in the fluid, of the displacement field of the elastic structure, and of the normal elevation field of the free surface. The nonlinear reduced-order model constructed in the recent work evoked above is reused for implementing the nonparametric probabilistic approach of uncertainties. The objective of this paper is to present a sensitivity analysis of this coupled fluid–structure system with respect to uncertainties and to use a classical statistical inverse problem for carrying out the experimental identification of the hyperparameter of the stochastic model. The analysis show a significant sensitivity of the displacement of the structure, of the acoustic pressure in the liquid, and of the free-surface elevation to uncertainties in both linear and geometrically nonlinear simulations.     

大型偏心结构吊装欠约束问题研究

吊装施工过程中被吊模块的水平度是作业要求的重要指标,通常需要增加配重调平。传统有限元方法需要补充约束以消除单元刚体位移,且需要重复计算平衡方程来求解调平载荷,效率不高。将模块的运动分解为随动坐标系的整体运动以及相对该坐标系的弹性变形,可将欠约束问题化为多体系统的静平衡问题。基于虚功率原理推导了吊装平顺时刻的节点力平衡方程以及相应的切线刚度矩阵,并将配重表示为基础配重与载荷系数相乘的形式。通过对节点力平衡方程求导,得到一组以载荷系数为自变量的微分方程,通过求解微分方程并结合水平度判据,可快速搜寻满足水平度要求的载荷系数。数值算例表明,该方法在解决偏心模块吊装欠约束问题方面具有明显的优势,在确定配重载荷方面具有较快的速度和合理的精度。     

Freezing the Dynamic Gap for Selectivity: Motion‐Based Design of Inhibitors of the Shikimate Kinase Enzyme

Shikimate kinase (SK), the fifth enzyme of the aromatic amino acid biosynthesis, is a recognized target for antibiotic drug discovery. The potential of the distinct dynamic apolar gap, which isolates the natural substrate from the solvent environment for catalysis, and the motion of Mycobacterium tuberculosis and Helicobacter pylori SK enzymes, which was observed by molecular dynamics simulations, was explored for inhibition selectivity. The results of the biochemical and computational studies reveal that the incorporation of bulky groups at position C5 of 5‐aminoshikimic acid and the natural substrate enhances the selectivity for the H. pylori enzyme due to key motion differences in the shikimic acid binding domain (mainly helix α5). These studies show that the less‐exploited motion‐based design approach not only is an alternative strategy for the development of competitive inhibitors, but could also be a way to achieve selectivity against a particular enzyme among its homologues.     

How Does Solvation Affect the Binding of Hydrophilic Amino Saccharides to Cucurbit[7]uril with Exceptional Anomeric Selectivity?

Cucurbit[7]uril (CB[7]) is known to bind strongly to hydrophilic amino saccharide guests with exceptional α‐anomer selectivities under aqueous conditions. Single‐crystal X‐ray crystallography and computational methods were used to elucidate the reason behind this interesting phenomenon. The crystal structures of protonated galactosamine (GalN) and glucosamine (GluN) complexes confirm the inclusion of α anomers inside CB[7] and disclose the details of the host–guest binding. Whereas computed gas‐phase structures agree with these crystal structures, gas‐phase binding free energies show preferences for the β‐anomer complexes over their α counterparts, in striking contrast to the experimental results under aqueous conditions. However, when the solvation effect is considered, the binding structures drastically change and the preference for the α anomers is recovered. The α anomers also tend to bind more tightly and leave less space in the CB[7] cavity toward inclusion of only one water molecule, whereas loosely bound β anomers leave more space toward accommodating two water molecules, with markedly different hydrogen‐bonding natures. Surprisingly, entropy seems to contribute significantly to both anomeric discrimination and binding. This suggests that of all the driving factors for the strong complexation of the hydrophilic amino saccharide guests, water mediation plays a crucial role in the anomer discrimination.     

Molecular dynamic study of dielectric polarization and ferroelectricity in a model polar polymer

Molecular dynamics simulations are used to explore the polarization response of a lamellar crystal consisting of folded chains of a highly simplified model polar polymer. The system is based on a united atom model of polyethylene with constrained bond lengths and bond angles, and it is endowed with artificial partial charges placed on the united atoms to give it a simple polar character. Simulations performed with various temperatures, electric field directions, and electric field application histories reveal a complicated sequence of reorientation processes, including pronounced ferroelectric behavior. The sequence includes a weak, temperature‐independent prompt response, and a slow‐rising delay regime with stretched exponential behavior and thermally‐activated reorientation parameters consistent with trans‐gauche (TG) barrier crossings in the amorphous phase. When the delay regime has progressed sufficiently, a primary large‐amplitude response due to organized rotation of large subsegments in the crystalline phase occurs in a rapid manner that requires relatively few TG barrier crossings. A final, extremely slow rise in residual polarization completes the sequence. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 740–759     

Molecular dynamics simulation of DNA‐directed assembly of nanoparticle superlattices using patterned templates

DNA‐directed assembly is a well developed approach in constructing desired nano‐architectures. On the other hand, E‐beam lithography is widely utilized for high resolution nano‐scale patterning. Recently, a new technique combining these two methods was developed to epitaxially grow DNA‐mediated nanoparticle superlattices on patterned substrates. However, defects are observed in epitaxial layers which restricts this technique from building large‐scale superlattices for real applications. Here we use molecular dynamics simulations to study and predict defect formation on adsorbed superlattice monolayers. We demonstrate that this epitaxial growth is energetically driven by maximizing DNA hybridization between the epitaxial layer and the substrate and that the shape anisotropy of the DNA‐mediated template posts leads to structural defects. We also develop design rules to dramatically reduce defects on epitaxial layers. Ultimately, with the assist of the computational study, this technique will open the door to constructing well‐ordered, three‐dimensional novel nanomaterials. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1687–1692     

Highly Selective Zeolite Topologies for Flue Gas Separation

The separation of carbon dioxide from flue gas is essential for the reduction of greenhouse gas emissions. In adsorptive methods, the challenge lies in the choice of suitable porous materials. Among all zeolite topologies, a number of adsorbents with pore dimensions in the range of the guest molecules were identified to allow an excellent separation by diffusion, and MRE and AFO zeolite topologies appear to be the best candidates based on equilibrium adsorption. Also, it was found that the behavior of this gas mixture in DFT and APD zeolites differed from the normal behavior.     

CL-20/TNT共晶炸药的分子动力学研究

运用分子动力学(MD)方法,选择凝聚态分子势能优化力场(COMPASS),对六硝基六氮杂异伍兹烷(ε-CL-20)、2,4,6-三硝基甲苯(TNT)晶体及其等摩尔比的CL-20/TNT混合炸药和共晶炸药进行不同温度下恒定粒子数等压等温(NPT)系综模拟研究.结果表明,CL-20/TNT共晶的内聚能密度(CED)和结合能随温度的升高逐渐减小;共晶的CED比混合炸药的大,结合能是混合炸药的2倍多,预示其稳定性明显增强.对相关函数和局部放大结构显示共晶中组分分子间作用主要来自TNT中H和CL-20中O以及CL-20中H和TNT中O之间形成的氢键.通过波动法求得的弹性力学性能结果表明,CL-20/TNT共晶的拉伸模量(E)、体积模量(K)和剪切模量(G)介于ε-CL-20和TNT晶体之间,且随温度的升高而下降,符合一般预期;但共晶炸药的柯西压(C12-C44,Cij弹性系数)、K/G和泊松比(ν)均比其组分炸药ε-CL-20和TNT高得多,预示该共晶具有异常高的延展性和弹性伸长,主要是二组分呈层状交替排列且之间存在较强相互作用所致.