Bin size determination for the measurement of mean flow velocity in molecular dynamics simulations

Extracting macroscopic properties from molecular simulation of fluids is required in most of the molecular dynamics problems. However, methods used for this purpose, their accuracy, and their dependence on the sampling and averaging schemes are still ambiguous. Macroscopic properties at any point of a molecular domain can be extracted via sampling and averaging of molecular behavior within a control region around that point, called a bin. The size of this bin has a significant impact on the accuracy of the macroscopic properties' measurement. In this research, we will focus on the measurement of mean flow velocity using ‘binning method’. On the basis of the results of this study, the most appropriate range of the bin size in which mean velocity can be measured accurately is determined. Although this range is determined based on the measurement of mean flow velocity, we believe that it can be used to estimate the proper range for the measurement of other flow quantities as well. Copyright © 2012 John Wiley & Sons, Ltd.     

Coupled Navier–Stokes/molecular dynamics simulations in nonperiodic domains based on particle forcing

This paper focuses on coupling methods for hybrid Navier–Stokes/molecular dynamics (MD) simulations. The computational domain is split in a continuum flow region, where a finite‐volume discretisation of the Navier–Stokes equations is used, and one or more particle domains, where molecular level modelling of the flow is employed. The domains are defined with a partial overlap, in which the flow states are coupled through an exchange of the velocity components. For the steady flows considered, an under‐relaxed Newton iteration method is used to drive the coupled system to convergence. The main focus of the present work is on methods to impose nonperiodic boundary conditions on the particle domain(s). A particle forcing is applied in the direction normal to the particle domain boundary to impose the boundary normal velocity component. A novel aspect of the present work is the extension of this method to more general nonplanar particle domain boundaries. The main contribution of the paper is the development of a particle forcing method in the direction tangential to the domain boundary, which is based on the equivalent continuum‐flow boundary shear stresses along with an iterative forcing strength adjustment based on the extrapolated particle boundary velocity. Furthermore, an adaptation scheme is presented, which uses the finite‐volume flux residuals of the particle bin averaged velocity field as a truncation criterion for the iterative force‐update scheme. It is demonstrated that by comparing the residual reduction for the momentum equation in the nonhomogeneous directions during the molecular dynamics simulations with that for a homogeneous direction, the forcing iteration at which the statistical noise in the velocity field dominates the uncertainty in the forcing strength can be determined. At this point the iteration can be truncated. It is shown that with adaptive schemes of this type, the total number of MD evaluations required in a coupled Navier–Stokes/MD simulation can be reduced relative to a hybrid scheme with a fixed number of forcing‐strength updates. Copyright © 2011 John Wiley & Sons, Ltd.     

Molecular dynamics studies on the local disordering of Σ3 and Σ11 grain boundaries in aluminium bicrystal

Molecular dynamics (MD) simulations using Morse interaction potential are performed in studies of [110] symmetrical tilt grain boundary (GB) structures with mis-orientation angles 50.5°(Σ11), 129.5°(Σ11), 70.5°(Σ3) and 109.5°(Σ3) at various tempratures. The GB structures are found to start local disordering at about 0.5T _m (T _m is the melting point of aluminium) for 50.5°(Σ11), 0.32T _m for 129.5° (Σ11) and 0.38T _m for 70.5°(Σ3), respectively. These results agree with conclusions deduced from the anelastic measurements. But, for twin-boundary structure 109.5°(Σ3), this disordering has not been found even when temperature increases up to 0.9T _m . The project supported by the National Natural Science Foundation of China and Laboratory for Non-linear Mechanics of Continuous Media, Institute of Mechanics, Academia Sinica.     

Properties of the streamwise velocity fluctuations in the inertial layer of turbulent boundary layers and their connection to self-similar mean dynamics

Well-resolved streamwise velocity measurements are used to investigate three measures of self-similarity in the spatial inertial sublayer of turbulent boundary layers. The emergence of self-similarity in the inertial sublayer requires a high Reynolds number, and thus a relatively wide range of ${\delta }^{+}=\delta u_{\tau }/\nu$ $(1400\lesssim {\delta }^{+}\lesssim 20\text{,}000)$ is explored. The measures investigated include the Kullback–Leibler divergence (KLD) used in turbulent flow analysis by Tsuji et al. (2005), the logarithmic decrease of the even statistical moments studied by Meneveau and Marusic (2013), and the diagnostic plot of Alfredsson and Örlü (2010). These measures are compared with the analyses of Fife et al. (2005) that determine and exploit an invariant form of the mean momentum equation. A primary focus is on domain(s) where the self-similar behaviors are analytically predicted and empirically observed. The present findings indicate that the approximately constant KLD and approximately logarithmic moment profiles reside in a region that is interior to the bounds of the self-similar inertial domain associated with the mean momentum equation. Conversely, the bounds of the self-similar region on the diagnostic plot correspond closely to the theoretically estimated bounds. Results are briefly discussed relative to Townsend’s notion of outer layer similarity, and, on the inertial domain, the physical existence of uniform momentum zones segregated by narrow vortical fissures.     

A molecular dynamics study of the stress–optical behavior of a linear short-chain polyethylene melt under shear

In this study, we present details of the stress–optical behavior of a linear polyethylene melt under shear using a realistic potential model. We demonstrate the existence of the critical shear stress, above which the stress–optical rule (SOR) begins to be invalid. The critical shear stress of the SOR of this melt turns out to be 5.5 MPa, which is fairly higher than 3.2 MPa at which shear thinning starts, indicating that the SOR is valid up to a point well beyond the incipient point of shear thinning. Furthermore, contrary to conventional wisdom, the breakdown of the SOR turns out not to be correlated with the saturation of chain extension and orientation: It occurs at shear rates well before maximum chain extension is obtained. In addition to the stress and birefringence tensors, we also compare two important coarse-grained second-rank tensors, the conformation and orientation tensors. The birefringence, conformation, and orientation tensors display nonlinear relationships to each other at high values of the shear stress, and the deviation from linearity begins at approximately the critical shear stress for breakdown of the SOR.     

Computational fluid dynamics simulations of respiratory airflow in human nasal cavity and its characteristic dimension study

To study the airflow distribution in human nasal cavity during respiration and the characteristic parameters of nasal structure, three-dimensional, anatomically accurate representations of 30 adult nasal cavity models were recons- tructed based on processed tomography images collected from normal people. The airflow fields in nasal cavities were simulated by fluid dynamics with finite element software ANSYS. The results showed that the difference of human nasal cavity structure led to different airflow distribution in the nasal cavities and variation of the main airstream passing through the common nasal meatus. The nasal resistance in the regions of nasal valve and nasal vestibule accounted for more than half of the overall resistance. The characteristic model of nasal cavity was extracted on the basis of characteristic points and dimensions deduced from the original models. It showed that either the geometric structure or the airflow field of the two kinds of models was similar. The characteristic dimensions were the characteristic parameters of nasal cavity that could properly represent the original model in model studies on nasal cavity.     

Size effects in indentation response of thin films at the nanoscale: A molecular dynamics study

The indentation response of Ni thin films of thicknesses in the nanoscale was studied using molecular dynamics simulations with embedded atom method (EAM) interatomic potentials. A series of simulations were performed in films in the [1 1 1] orientation with thicknesses varying from 4 to 12.8 nm. The study included both single crystal films and films containing low angle grain boundaries perpendicular to the film surface. The simulation results for single crystal films show that as film thickness decreases larger forces are required for similar indentation depths but the contact stress necessary to emit the first dislocation under the indenter is nearly independent of film thickness. The low angle grain boundaries can act as dislocation sources under indentation. The mechanism of preferred dislocation emission from these boundaries operates at stresses that are lower as the film thickness increases and is not active for the thinnest films tested. These results are interpreted in terms of a simple model.     

修正的卵形弹丸侵彻带有预制孔混凝土靶板的理论模型与数值模拟研究

应用AUTODYN-2D程序对卵形弹丸侵彻混凝土靶板进行数值仿真,以实验为依据,确定了合理的计算参数。针对不同预制孔孔径大小,进行了一系列关于弹丸侵彻带有预制孔混凝土靶板的数值实验。对Teland侵彻带有预制孔混凝土靶板理论中靶体阻力参数进行重新定义。通过与仿真结果的对比,发现改进的Teland理论能够对侵彻深度进行很好的预测。     

对管内湍流边界层结构与流动阻力特性的数值研究

在研究紊流边界层的过程中,本文考虑了分子粘性对紊流产生的作用、雷诺数以及壁面附近脉动动能的耗散不是各向同性对紊流产生的影响,采用Jones-Launder模型对管内紊流流动边界层厚度、边界层内的脉动动能K,动能耗散ε,管壁切应力τ0以及由此可得的管内流动摩擦阻力系数λ进行了数值计算,计算结果与实验值、理论计算值得具有较好的一致性。     

基于粗粒化分子动力学的自由水与水化硅酸钙孔隙水冻结模拟

水化硅酸钙是水泥基材料的主要水化产物,其孔隙内的水分是影响水泥基材料抗冻性的主要因素。本文基于粗粒化分子动力学方法研究水化硅酸钙孔隙水的冻结机制,针对水的粗粒化P₄粒子和水化硅酸钙胶体颗粒,建立了水化硅酸钙孔隙水的冻结模型。根据此模型计算了不同孔径孔隙水冰点,分析了水泥基材料孔径孔隙在冻融破坏中的危害程度;模拟得到了水化硅酸钙孔隙内水的冻结分布特征和密度分布特征。研究工作表明,本文建立的模型有效提高了分子动力学模拟水化硅酸钙孔隙水冻结问题的规模,为后续进行水泥基材料的冻融破坏分析提供了研究基础。     

A numerical study of two- and three-dimensional detonation dynamics of pulse detonation engine by the CE／SE method

In this paper, the CE/SE method is developed to simulate the two- and three-dimensional flow-field of Pulse Detonation Engine (PDE). The conservation equations with stiff source terms for chemical reaction are solved in two steps. The detailed analysis of computational results of a PDE with a single detonation tube and a PDE with five detonation tubes are given in this paper. Complex wave systems are observed inside and outside a PDE. For a PDE with 5 detonation tubes, there is a big bow shock produced from a number of little shocks near the open ends of tubes. A lot of vortexes interact with shocks and a large expansion wave propagates forward and backward with respect to the PDE in a semi-oval shape.The project supported by the National Natural Science Foundation of China (59906005), the Teaching and Research Award Program for Outstanding Young Teachers in High Education Institutions of MOE, China     

Effects of grain boundary on irradiation-induced zero-dimensional defects in an irradiated copper

Grain boundaries(GBs) can serve as effective sinks for radiation-induced defects, thus notably influencing the service performance of materials. However, the effect of GB structures on the zero-dimensional defects induced by irradiation has not been fully elucidated. Here, the evolution of cascade collision in the single-crystal(SC),bicrystalline(BC), and twinned crystalline(TC) copper is studied by atomic simulations during irradiation. The spatial distributions of vacancies and interstitials a...     

甲烷及其氟氯代物在TATB晶体表面吸附的模

采用Discover/MaterialStudio中的分子动力学方法(COMPASS力场、NVT)及结构优化方法,模拟了甲烷及其氟氯代物(共15种化合物)在TATB超胞的(001)、(100)及(010)晶面的吸附作用。结果表明:(1)TATB晶面与大多数分子相互作用的强弱次序为:(010)(100)(001),这可能与晶面结构差异及氢键的形成有关;(2)三种晶面上,吸附能最大(为负值)的都是CH4、CF4,表明如果氟聚合物中氢或氟的含量过高均可能导致聚合物与TATB相互作用减弱。     

A study on the behaviour of high-order flux reconstruction method with different low-dissipation numerical fluxes for large eddy simulation

This work focuses on the numerical dissipation features of high-order flux reconstruction (FR) method combined with different numerical fluxes in turbulence flows. The famous Roe and AUSM+ numerical fluxes together with their corresponding low-dissipation enhanced versions (LMRoe, SLAU2) and higher resolution variants (HR-LMRoe, HR-SLAU2) are incorporated into FR framework, and the dissipation interplay of these combinations is investigated in implicit large eddy simulation. The numerical dissipation stemming from these convective numerical fluxes is quantified by simulating the inviscid Gresho vortex, the transitional Taylor–Green vortex and the homogenous decaying isotropic turbulence. The results suggest that low-dissipation enhanced versions are preferential both in high-order and low-order cases to their original forms, while the use of HR-SLAU2 has marginal improvements and the HR-LMRoe leads to degenerated solution with high-order. In high-order the effects of numerical fluxes are reduced, and their viscosity may not be dissipative enough to provide physically consistent turbulence when under-resolved.     

Simulation of oscillatory flow in an aortic bifurcation using FVM and FEM: A comparative study of implementation strategies

Cardiovascular diseases are one of the major causes of long‐term morbidity and mortality in human beings. The nearly epidemic increase in prevalence of such diseases poses a serious threat to public health and calls for efficient methods of diagnosis and treatment. Non‐invasive diagnostic procedures such as MRI are often used in this context; however, these are limited in terms of spatial and temporal resolution and do not provide information on time‐dependent pressures and wall shear stresses—key quantities considered to be partially responsible for the formation and development of related pathologies. The present study is concerned with the numerical simulation of oscillatory flow through the abdominal aortic bifurcation. Computational fluid dynamics simulation of oscillatory flow in a branched geometry at high Reynolds numbers poses considerable challenges. The present study reports a detailed comparison of simulations performed with a finite volume and a finite element method, two approaches with significant differences in their discretization strategy, treatment of boundary conditions and other numerical aspects. Both solvers were parallelized, using loop parallelization of the BiCGStab linear solver for the finite volume and domain decomposition based on the Schur complement method for the finite element technique. The experience gained with these two approaches for the solution of flow in a bifurcation forms the focus of this study. Although similar results were obtained for both methods, the computation time required for convergence was found to be significantly smaller for the finite element approach. Copyright © 2010 John Wiley & Sons, Ltd.     

Landmarks in the application of electrical tomography in particle science and technology

Selected milestones in the development and use of electrical tomography in powder conveying, slurry processing and multi-phase flow are highlighted. The ability to map concentration in opaque mixtures under process-realistic conditions was a major innovation for the method and has had far reaching implications. Subsequent developments have enabled velocity information to be abstracted resulting in the ability to measure component flux and motion.     

A numerical study on the unsteady evolution of weak axisymmetric fountain flows in a homogeneous ambient

Weak axisymmetrical fountains resulting from the injection of a dense fluid upwards into a large container of homogeneous fluid of lower density has been studied numerically in this paper using a time-accurate finite volume scheme. The behaviour of fountains for both the uniform and parabolic profiles of the discharge velocity at the fountain source has been investigated. The evolution of transient fountain flow has been analysed and two distinct stages of evolution have been identified. The time trace of the position of the fountain front has been presented and the initial, temporary and final fountain height and fountain width have been determined. The project is jointly supported by the National Natural Science Foundation of China, the Special Fund for the Young and Middle-aged Academic and Technical Leaders Training Scheme of Yunnan Province, and the Australian Development Cooperation Scholarship to Prof. Lin Wenxian. Author for correspondence: Li Yuncang.     

A study of the effect of step excrescences and free‐stream disturbances on boundary layer stability

In this work, a study of the mechanism by which free‐stream acoustic and vorticity disturbances interact with a boundary layer flow developing over a flat plate featuring a step excrescence located at a certain distance from a blunt leading edge is included. The numerical tool is a high‐fidelity implicit numerical algorithm solving for the unsteady, compressible form of the Navier–Stokes equations in a body‐fitted curvilinear coordinates and employing high‐accurate compact differencing schemes with Pade‐type filters. Acoustic and vorticity waves are generated using a source term in the momentum and energy equations, as opposed to using inflow boundary conditions, to avoid spurious waves that may propagate from boundaries. The results show that the receptivity to surface step excrescences is largely the result of an overall adverse pressure gradient posed by the step, and that the free‐stream disturbances accelerate the generation of instabilities in the downstream. As expected, it is found that the acoustic disturbance interacting with the surface imperfection is more efficient in exciting the Tollmien–Schlichting waves than the vorticity disturbance. The latter generates Tollmien–Schlichting waves that are grouped in wave packets consistent with the wavelength of the free‐stream disturbance. Copyright © 2015 John Wiley & Sons, Ltd.     

Numerical study on the stability and mixing of vertical round buoyant jet in shallow water

IntroductionAsimplewaytodisposeofthelargequantitiesofwasteheatresultingfromsteamelectricpowergeneration ,istodischargetheheatedcondenserwaterthroughasubmergedroundoutfalllocatedatthebottomofthereceivingwater.Inrecentyears,suchkindsofhighvelocityinjectionsystemshaveincreasinglybeenusedaseffectivemixingdevices,frequently ,thesedevicesarelocatedinwatersofonlyafewportdiametersdeep ,anddischargingareaisrelativelysmall.Thusanunderstandingoftheinducedexcesstemperaturedistribution ,inrelationshiptothe…     

A simulation-based analysis of the effect of a reflecting surface on aeroacoustic time-reversal source characterization and comparison with beamforming

This paper presents a simulation-based analysis of the effect of a reflecting surface on aeroacoustic Time-Reversal (TR) source localization/characterization and compares the results of TR with those obtained using cross-spectral Conventional Beamforming (CB). The TR technique is shown to require the use of at least two line arrays of microphones to accurately characterize the nature of aeroacoustic sources. This work, however, shows that in the presence of a rigid surface, only one line array of microphones is sufficient to accurately localize and characterize idealized aeroacoustic sources. Forward simulations were carried out using the 2-D Linearized Euler Equations on a rectangular domain with a rigid bottom boundary (modeling a 2-D semi-infinite space) for the test-cases of stationary idealized tonal aeroacoustic (monopole, dipole and lateral quadrupole) sources located in a fully-developed mean shear flow field wherein the acoustic pressure time–history was stored at the computational boundaries. A set of TR simulations are implemented that show for each test-case that only the top line array is required to accurately characterize the idealized aeroacoustic sources in the presence of a reflecting bottom boundary, thereby suggesting the redundancy of acoustic pressure measurement at the rigid surface. The test-case of convecting (moving) idealized aeroacoustic source was also considered and the TR simulation using only the top line array in the presence of reflecting bottom boundary was able to accurately retrieve the source trajectory and simultaneously characterize its nature. This numerical experiment demonstrates in principle that when a rigid surface is mounted on the floor of an Anechoic Wind Tunnel, the use of only one (top) line array of microphones should be sufficient to characterize the nature and location of experimental flow-induced noise source. Acoustic source maps were also obtained using the CB method based on the Method of Images (to model the reflecting surface) and incorporation of the Ray-Tracing algorithm necessary to account for the effect of mean flow. The CB results were found to be highly comparable to those obtained using TR for the test-cases of non-convecting sources; thereby demonstrating the conceptual equivalence of the Method of Images and directly implementing the rigid-wall condition during TR for source localization/characterization.