Jamming and crystallization in athermal polymer packings

Dense packings of chains of hard spheres possess characteristic features that do not have a counterpart in corresponding packings of monomeric spheres especially near the maximally random jammed (MRJ) state. From the modelling perspective the additional requirement that spheres keep their connectivity while maximizing the occupied volume fraction imposes severe constraints on generation algorithms of dense chain configurations. The extremely sluggish dynamics imposed by the uncrossability of chains precludes the use of deterministic or stochastic dynamics to generate all but dilute polymer packings. As a viable alternative, especially tailored chain-connectivity-altering Monte Carlo (MC) algorithms have been developed that bypass this kinetic hindrance and have actually been able to produce packings of hard-sphere chains in a volume fraction range spanning from infinite dilution up to the MRJ state. Such very dense athermal polymer packings share a number of structural features with packings of monomeric hard spheres, but also display unique characteristics due to the constraints imposed by connectivity. We give an overview of the most relevant results of our recent modeling work on packings of freely-jointed chains of tangent hard spheres about the MRJ state, local structure, chain dimensions and their scaling with density, topological constraints in the form of entanglements and knots, contact network at jamming, and entropically driven crystallization.     

A subgrid model for clustering of high-inertia particles in large-eddy simulations of turbulence

Clustering (or preferential concentration) of inertial particles suspended in a homogeneous, isotropic turbulent flow is strongly influenced by the smallest scales of the turbulence. In particle-laden large-eddy simulations (LES) of turbulence, these small scales are not captured by the grid and hence their effect on particle motion needs to be modelled. In this paper, we use a subgrid model based on kinematic simulations of turbulence (Kinematic Simulation based SubGrid Model or KSSGM), for the first time in the context of predicting the clustering and the relative velocity statistics of inertial particles. This initial study focuses on the special case of inertial particles in the absence of gravitational settling. We show that the KSSGM gives excellent predictions for clustering in a priori tests for inertial particles with St ≥ 2.0, where St is the Stokes number, defined as the ratio of the particle response time to the Kolmogorov time-scale. To the best of our knowledge, the KSSGM represents the first model that has been shown to capture the effect of the subgrid scales on inertial particle clustering for St ≥ 2.0. We also show that the mean inward radial relative velocity between inertial particles (?w_r?^(?), which enters into the formula for the collision kernel) is accurately predicted by the KSSGM for all St. We explain why the model captures clustering at higher St?but not for lower St?, and provide new insights into the key statistical parameters of turbulence that a subgrid model would have to describe, in order to accurately predict clustering of low-St?particles in an LES.     

Conserved scalar mixing in a confined-opposed-jets flow

The focus of this paper is on the mixing of a conserved passive scalar for Sc = 1 (Sc is the Schmidt number) in axisymmetric turbulence for which the initial injections of turbulent kinetic energy and scalar variance are similar. Two confined-opposed-jets (COJ) are experimentally studied through simultaneous PIV (particle image velocimetry) and PLIF (planar laser induced fluorescence) measurements, for different flow regimes. One-point transport equation for the scalar variance is assessed through experimental data, along the common axis of the two opposed jets, and different physical phenomena are revealed (production, diffusion, dissipation). The production of scalar variance is equilibrated by the diffusion term (～75%) and the mean dissipation of the scalar variance (～25%). To further assess the scalar behaviour at each scale in this anisotropic, but axisymmetric, flow, a scale-by-scale scalar variance budget equation is derived for axisymmetric turbulence. This equation reduces to Yaglom's 4/3 law, under additional restrictions. The equation is assessed through experimental data, in the impingement region between the two COJ. In particular, the anisotropic energy transfer along different directions is quantified. It is shown that for scales smaller than the size of the central region, Δ, the cascade of the scalar variance is completely inhibited, independently of the particular direction. For scales larger than Δ, the apparent aspect of the energy transfer is that of an inverse cascade, with positive values of the scalar variance transfer. Nonetheless, inhomogeneity of the flow and mixing at those scales is directly responsible for these positive values.     

Performance characteristics of single photon pulse ranging system using Monte Carlo simulation

Single photon pulse ranging system with extremely high sensitivity has been widely used in distance measurement and 3D imaging. To analyze the factors that affect the measurement precision and accuracy will help to improve system performance. According to system structure and principle, we mainly discussed the following factors: laser intensity, pulse width, detection efficiency and time jitter. A simulation model based on Monte Carlo stochastic method was constructed in this paper, and we get the specific influence of factors on measurement precision and accuracy by simulation. Finally, we set up laboratory experiment system and took effective experiments on ranging precision and accuracy.     

Response properties at the dynamic water/dichloroethane liquid–liquid interface

ABSTRACT

We present a novel approach for calculating the static dielectric permittivity profile of a liquid–liquid interface (LLI) from molecular dynamics simulations. To obtain well-defined features, comparable to those observed at solid–liquid interfaces, we find it essential to reference to the instantaneous liquid–liquid interface rather than the more commonly used average Gibbs interface. We provide a coarse-grained approach for the practical definition of the instantaneous interface and present numerical results for the prototypical water/1,2-dichloroethane system. These results show that the parallel components of the dielectric permittivity tensor can be accurately extracted. In contrast, the perpendicular component does not converge to the correct bulk value at large distances from the LLI, highlighting a flaw in the regularly applied coarse-graining procedure.     

宽光谱棱镜型太阳光谱仪设计

为实现大气层外太阳光谱辐照度(SSI)变化的长期例行监测,设计了一种星载宽光谱太阳光谱仪结构。全系统仅使用单片折反式曲面棱镜实现太阳光谱250～2500 nm的分光,并通过棱镜转动实现谱平面上多探测器的同步扫描探测;同时基于Huygens子波点扩展函数（PSF）仿真了光谱仪的光谱响应函数（SRF）和光谱分辨率。分光棱镜在±2.5°扫描转角内的全谱段子午像差小于8μm;光谱分辨率在紫外谱段(250～400 nm)为0.7～3.5 nm,可见/近红外谱段(400～1000 nm)为3.5～35.0 nm,短波红外谱段(1000～2500 nm)内为28.5～41.2 nm。整个系统结构简单紧凑,性能稳定可靠,分光和像差校正能力满足大气层外太阳光谱辐照度长期监测需求。     

Lagrangian and Eulerian velocity intermittency

Usual turbulence experiments, based on the Taylor hypothesis, differ from true Eulerian measurements. This is the origin of the apparent discrepancy between a recent two point correlation analysis and the multiplicative cascade picture. Indeed, both Eulerian and Lagrangian observations perfectly agree with this picture. Received 19 June 2002 / Received in final form 29 July 2002 Published online 14 October 2002 RID="a" ID="a"e-mail: bcastain@ens-lyon.fr     

Numerical simulation of the flow field in a reactor for titanium dioxide production

A physicochemical and fluid dynamic model is formulated for the numerical simulation of the flow field in a reactor for titanium dioxide production, the turbulence motion is described by theKε equation, the governing equations are solved by the SIMPLER algorithm devised by Partankar and Spalding. The velocity, tmperature and concentration fields are obtained for three cases: A) with chemical reaction and thermal insulation on the walls; B) with chemical reaction and wall temperature is 450K; C) without chemical reaction and thermal insulation on the walls, and the physicochemical numerical simulation for the titanium dioxide production has been done. The results of the paper can be used as a theoretical guide for the engineers in the design of such reactors.     

Hybrid Particle–Continuum Simulations of Polymer Brushes in Shear Flow

A hybrid particle–continuum method is used to study the shear flow confined between two opposing walls, one of which is coated with polymer chains. Molecular dynamics (MD) is used in the particle region near the brush and Navier–Stokes (NS) equations are applied in the remaining region where the continuum assumption holds. The information exchange from the continuum region to the particle region is implemented using the constrained particle dynamics. Both Couette shear flow and oscillatory flow are considered in the present work. The effect of the shear flow on the conformational characteristics of polymer brushes is analyzed. In the overlap region, the velocities obtained from MD simulations are smoothly connected with those from NS equations. Our investigations demonstrate that the hybrid particle–continuum model is valid in exploring the shear behavior of polymer brushes.     

Prediction of Structure and Energy of Trans-1,4-Polybutadiene Glassy Surface by Atomistic Simulations of Free-Standing Ultrathin Films

Atomistic modeling of amorphous trans-1,4-polybutadiene (TPBD), using molecular mechanics and molecular dynamics (MD) simulations, is performed to generate three-dimensionally periodic bulk and two-dimensionally periodic thin film condensed phases. The condensed structures are constructed using multiple polymer chains. Structural and energetic relaxations and sampling of properties are performed using MD in the canonical ensemble (NVT) by a procedure that relieves local high-energy spots and brings the system to realistic thermodynamic states. The calculated surface energy for TPBD, 30.72 erg/cm², is in excellent agreement with the reported experimental value of 31 erg/cm². The structure of the surface layers is probed in terms of the atomic mass density variations, bond-bond orientation function profiles, and the distribution of the dihedral angles about the rotatable backbone bonds. The thickness of the surface layer over which the density varies smoothly but rapidly is found to be approximately 15 Å. The level of agreement of the calculated surface energy with the experimental value is superior in comparison to previous investigations in the literature using the atomistic approach for flexible polymers.