Dissipative particle dynamics simulation on the rheological properties of heavy crude oil

The rheological properties of heavy crude oil have a significant impact on the production, refining and transportation. In this paper, dissipative particle dynamics (DPD) simulations were performed to study the effects of the addition of light crude oil and emulsification on the rheological properties of heavy crude oil. The simulation results reflected that the addition of light crude oil reduced the viscosity effectively. The shear thinning behaviour of crude oil mixtures were becoming less distinct as the increase of the mass fraction of light crude oil. According to the statistics, the shear had an influence on the aggregation and spatial orientation of asphaltene molecules. In addition, the relationship between the viscosity and the oil mass fraction was investigated in the simulations of emulsion systems. The viscosity increased with the oil mass fraction slowly in oil-in-water emulsions. When the oil mass fraction was higher than 50%, the increase became much faster since systems had been converted into water-in-oil emulsions. The equilibrated morphologies of emulsion systems were shown to illustrate the phase inversion. The surfactant-like feature of asphaltenes was also studied in the simulations.     

Effect of shear equilibrium flow in Tokamak plasma on resistive wall modes

A code named LARWM with non-ideal magnetohydrodynamic equations in cylindrical model is used to describe the instability in Tokamak plasma surrounded by a conducting wall with finite resistivity. We mainly take three factors related to the shear equilibrium plasma flow into consideration to study the stabilizing effect of the shear flow on the resistive wall modes (RWMs). The three factors are the velocity amplitude of flow, the shear rate of flow on plasma surface, and the inertial energy of equilibrium plasma flow. In addition, a local shear plasma flow is also calculated by the LARWM code. Consequently, it is found that the inertial energy of the shear equilibrium plasma flow has an important role in the stabilization of the RWMs.     

Calculation of the thermodynamic properties of copper by molecular dynamics simulation

It is shown that the thermodynamic characteristics of a system can be accurately described using many-body potentials of the interatomic interaction in molecular dynamics calculations. Original Russian Text ? O.V. Stepanyuk, D.B. Alekseev, A.M. Saletskii, 2009, published in Vestnik Moskovskogo Universiteta. Fizika, 2009, No. 2, pp. 115–116.     

Analytic solution to the problem of the couette flow in a plane channel with infinitely large parallel walls

An analytic (in the form of a Neuman series) solution to the problem of the Couette flow in a plane channel with infinitely large parallel walls is constructed using the kinetic approach in the isothermal approximation. For the basic equation, the Bhatnagar-Gross-Krook (BGK) model of the kinetic Boltzmann equation is used, while the boundary condition is determined by the diffuse reflection model. The mass flux through half the channel thickness in the direction parallel to the channel walls as well as the nonzero component of the viscous stress tensor are calculated taking into account the constructed distribution function. The results are compared with analogous data obtained by numerical methods.     

Shear zones and wall slip in the capillary flow of concentrated colloidal suspensions

We image the flow of a nearly random close packed, hard-sphere colloidal suspension (a "paste") in a square capillary using confocal microscopy. The flow consists of a "plug" in the center while shear occurs localized adjacent to the channel walls, reminiscent of yield-stress fluid behavior. However, the observed scaling of the velocity profiles with the flow rate strongly contrasts yield-stress fluid predictions. Instead, the velocity profiles can be captured by a theory of stress fluctuations originally developed for chute flow of dry granular media. We verified this both for smooth and rough walls.     

Effect of slow dynamics on elastic properties of materials with residual and shear strains

The effects of fast and slow dynamics in acoustic resonators from rock and metal samples from the D16 microcrystalline aluminum alloy are studied using resonance ultrasonic spectroscopy. Before the experiment, residual shear strains were artificially created in the metal samples. A decrease in the elasticity modulus in the fast dynamics process has been revealed in resonators from rock and the D16 alloy with residual strains. Based on an analysis of the experimental results, the following conclusion was drawn: the experimentally observed slow dynamics effect cannot be explained by thermoelastic effects alone. The slow dynamics effect is to a great extent related to metastable states of the defect structure the latter passes through to due to the force action (dynamic or static) applied to the sample. After its removal, the defect structure slowly relaxes to its equilibrium state.     

On the effect of an anisotropy-resolving subgrid-scale model on large eddy simulation predictions of turbulent open channel flow with wall roughness

A large eddy simulation (LES) was conducted of turbulent flow in a channel with a rough wall on one side and a free surface on the other by adopting an anisotropy-resolving subgrid-scale (SGS) model. A shear Reynolds number of Re_τ = 395 was used based on the mean friction velocity and channel height. To investigate the grid dependency of the LES results caused by the SGS model, three grid resolutions were tested under the same definition of a roughness shape by using the immersed boundary method. The results obtained were compared with direct numerical simulation data with and without the wall roughness and those without the extra anisotropic term. The primary focus was on how the present anisotropic SGS model with coarser grid resolutions can properly provide the effects of roughness on the mean velocity and turbulent stresses, leading to a considerable reduction of the computational cost of LES.     

Effect of thermal annealing on nanoimprinted Cu-Ni alloys using molecular dynamics simulation

The mechanical behaviors of nanoimprinted Cu-Ni alloys before and after annealing were studied using molecular dynamics simulation with a tight-binding potential. The results showed that when the punch is advancing, the punching force obtained from the simulation with a tight-binding potential is lower than with the Morse potential. During and after withdrawing the punch from the specimen, the adhesive phenomena are observed and the large residual stress in the Cu-Ni alloys is induced. During the annealing process, the internal energy of Cu-Ni alloys decreased with increasing the temperature and the component of Cu. In addition, comparing the maximum residual stress in the Cu-Ni alloys with and without annealing treatment, the stress is significantly released after annealing, especially in the higher component of Ni.     

Nonequilibrium molecular dynamics simulation of coupling between nanoparticles and base-fluid in a nanofluid

The intent of this study is to examine nonequilibrium heat transfer in a copper-argon nanofluid by molecular dynamics simulation. Two different methods, the physical definition method and the curve fitting method, are introduced to calculate the coupling factor between nanoparticles and base fluid. The results show that the coupling factors obtained by these two methods are consistent. The coupling factor is proportional to the volume fraction of the nanoparticle and inversely proportional to nanoparticle diameter. In the temperature range of 90-200 K, the coupling factor is not affected by temperature. The nanoparticle aggregation results in a decrease of the coupling factor.     

Non-equilibrium momentum exchange algorithm for molecular dynamics simulation of heat flow in multicomponent systems

A new non-equilibrium molecular dynamics algorithm is presented based on the original work of Müller-Plathe, (1997, J. chem. Phys., 106, 6082), for the non-equilibrium simulation of heat transport maintaining fixed the total momentum as well as the total energy of the system. The presented scheme preserves these properties but, unlike the original algorithm, is able to deal with multicomponent systems, that is with particles of different mass independently of their relative concentration. The main idea behind the new procedure is to consider an exchange of momentum and energy between the particles in the hot and cold regions, to maintain the non-equilibrium conditions, as if they undergo a hypothetical elastic collision. The new algorithm can also be employed in multicomponent systems for molecular fluids and in a wide range of thermodynamic conditions.     

The influence of viscous heating and wall thermal conditions on the thermal ignition of a poiseuille/couette reactive flow

This paper investigates the stationary thermal stability of an exothermic reactive viscous non-Newtonian fluid between two parallel walls in the plane Poiseuille and generalized Couette flow configurations for two different thermal conditions. Assuming the system is adiabatic with negligible reactant consumption, the closed-form solution obtained from the momentum equation was inserted into the energy equation due to the dissipative effect of viscosity. The resulting energy equation was analyzed for criticality using a variational technique. The influence of the viscous heating parameter (Γ), wall dynamical (Λ) and the thermal conditions of the walls on the thermal ignition parameters were examined.     

Thermophysical properties of stable and metastable liquid copper and nickel by molecular dynamics simulation

Molecular dynamics simulation combined with an embedded atom method (EAM) potential was applied to the calculation of the specific heat and the diffusion coefficient for superheated and undercooled liquid copper and nickel as functions of temperature. The system contains 108,000 atoms. The calculated results show that the enthalpy increases linearly with the rise of temperature. There are no breaks at their melting temperatures of 1356 and 1726 K. It is found that the calculated specific heats of Cu and Ni are 32.75 and 36.11 J/mol/K respectively. The calculated mean square displacements increase linearly with calculated time. The diffusion coefficients are exponentially dependent on temperature. Moreover, the calculated results are in good agreement with the reported experimental results for the specific heat and diffusion coefficient.     

A molecular dynamics simulation on the convective heat transfer in nanochannels

The understanding of the flow and heat transfer processes for fluid through micro- and nanochannels becomes imperative due to its wide application in micro- and nano-fluidic devices. In this paper, the method to simulate the convective heat transfer process in molecular dynamics is improved based on a previous study. With this method, we simulate a warm dense fluid flowing through a cold parallel-plate nanochannel with constant wall temperature. The characteristics of the velocity and temperature fields are analysed. The temperature difference between the bulk average temperature of fluid and the wall temperature decreases in an exponential form along the flow direction. The Nusselt number for the laminar flow in parallel-plate nanochannel is smaller than its corresponding value at macroscale. It could be attributed to the temperature jump at the fluid–wall interface, which decreases the temperature gradient near the wall. The results also reveal that the heat transfer coefficient is related to the surface wettabilities, which differs from that in the macroscopic condition.     

Study on the relations of sensitivity with energy properties for HMX and HMX-based PBXs by molecular dynamics simulation

Molecular dynamics simulation was applied to study the structure and energy properties of β-HMX (β-cyclotetramethylene tetranitramine) crystal and its composite PBXs (polymer-bonded explosives) with F₂₃₁₁ as a polymer binder under different temperatures and F₂₃₁₁ concentrations. The interface interaction energy of HMX and F₂₃₁₁, the interaction energy E_N–N between N atoms in N–NO₂ trigger bond in HMX molecules, and the cohesive energy density (CED) are presented and analyzed. A meaningful finding is that there exists correlation between E_N–N and the sensitivity of β-HMX and its composites, i.e. the less the E_N–N is, the larger the sensitivity is. Additionally, molecular interactions are inherently disclosed by using pair correlation function (PCF) to analyze the interfacial structure between (1 0 0)HMX crystal surface and F₂₃₁₁ molecular chain.