Molecular dynamics simulations of aqueous urea solutions: Study of dimer stability and solution structure, and calculation of the total nitrogen radial distribution function GN(r

Molecular dynamics simulations have been performed in order to study the structure of two molal urea solutions in D2O. Several initial dimer configurations were considered for an adequate sampling of phase space. Eventually all of them appeared to be unstable, when system size and periodic boundary conditions are chosen properly, even after a very careful equilibration. The total nitrogen scattering function GN(r), calculated from these simulations, is in good agreement with neutron scattering experiments when both intra- and intermolecular correlations are considered and the experimental truncation ripples are introduced by a Fourier transform of GN(r) back and forth. The simple pair potential model that we used gives results in good agreement with experiments and with a much more involved potential model, recently described in the literature [J. Chem. Phys. 95, 8419 (1991)]

From wave function to crystal morphology: application to urea and alpha-glycine

In this paper the relation between the molecular electron density distribution and the crystal growth morphology is investigated. Accurate charge densities derived from ab initio quantum chemical calculations were partitioned into multipole moments, to calculate the electrostatic contribution to the intermolecular interaction energy. For urea and alpha-glycine the F-faces or connected nets were determined according to the Hartman-Perdok PBC theory. From attachment energy and critical Ising temperature calculations, theoretical growth forms were constructed using different atom-atom potential models. These were compared to the Donnay-Harker model, equilibrium form and experimental growth forms. In the case of alpha-glycine, the theoretical growth forms are in good agreement with crystals grown from aqueous solution. Crystals obtained by sublimation seem to show some faces which are not F-faces sensu stricto

Micro-computed tomography pore-scale study of flow in porous media: Effect of voxel resolution

A fundamental understanding of flow in porous media at the pore-scale is necessary to be able to upscale average displacement processes from core to reservoir scale. The study of fluid flow in porous media at the pore-scale consists of two key procedures: Imaging - reconstruction of three-dimensional (3D) pore space images; and modelling such as with single and two-phase flow simulations with Lattice-Boltzmann (LB) or Pore-Network (PN) Modelling. Here we analyse pore-scale results to predict petrophysical properties such as porosity, single-phase permeability and multi-phase properties at different length scales. The fundamental issue is to understand the image resolution dependency of transport properties, in order to up-scale the flow physics from pore to core scale. In this work, we use a high resolution micro-computed tomography (micro-CT) scanner to image and reconstruct three dimensional pore-scale images of five sandstones (Bentheimer, Berea, Clashach, Doddington and Stainton) and five complex carbonates (Ketton, Estaillades, Middle Eastern sample 3, Middle Eastern sample 5 and Indiana Limestone 1) at four different voxel resolutions (4.4 µm, 6.2 µm, 8.3 µm and 10.2 µm), scanning the same physical field of view. Implementing three phase segmentation (macro-pore phase, intermediate phase and grain phase) on pore-scale images helps to understand the importance of connected macro-porosity in the fluid flow for the samples studied. We then compute the petrophysical properties for all the samples using PN and LB simulations in order to study the influence of voxel resolution on petrophysical properties. We then introduce a numerical coarsening scheme which is used to coarsen a high voxel resolution image (4.4 µm) to lower resolutions (6.2 µm, 8.3 µm and 10.2 µm) and study the impact of coarsening data on macroscopic and multi-phase properties. Numerical coarsening of high resolution data is found to be superior to using a lower resolution scan because it avoids the problem of partial volume effects and reduces the scaling effect by preserving the pore-space properties influencing the transport properties. This is evidently compared in this study by predicting several pore network properties such as number of pores and throats, average pore and throat radius and coordination number for both scan based analysis and numerical coarsened data

Pore-Scale Modeling of Drainage Displacement Patterns in Association With Geological Sequestration of CO2

©2020. The Authors. We investigate the immiscible displacement (drainage) of a wetting fluid in a porous medium by a nonwetting fluid using multi–graphics processing unit (GPU) lattice Boltzmann simulations with the aim of better understanding the pore-scale processes involved in the geological sequestration of CO2. Correctly resolving the dynamics involved in multiphase flow in permeable media is of paramount importance for any numerical scheme. Generally, the average fluid flow is assumed to be at low Reynolds numbers Reav. Hence, by neglecting inertial effects, this immiscible displacement should be characterized by just two dimensionless numbers, namely, the capillary number Caav and the viscosity ratio, which quantify the ratio of the relevant forces, that is, the viscous and capillary forces. Our investigation reveals that inertial effects cannot be neglected in the range of typical capillary numbers associated with multiphase flow in permeable media. Even as the average Caav and Reav decrease away from the injection point, inertial effects remain important over a transient amount of time during abrupt Haines jumps, when the nonwetting phase passes from a narrow restriction to a wider pore space. The local Rel at the jump sites is orders of magnitude higher than the average Reav, with the local dynamics being decoupled from the externally imposed flow rate. Therefore, a full Navier-Stokes solver should be used for investigating pore-scale displacement processes. Using the Ohnesorge number to restrict the parameter selection process is essential, as this dimensionless number links Caav and Reav and reflects the thermophysical properties of a given system under investigation

Investigation of viscous coupling effects in three-phase flow by lattice Boltzmann direct simulation and machine learning technique

The momentum transfer across fluid interfaces in multi-phase flow leads to a non-negligible viscous coupling effect. In this study, we use the lattice Boltzmann method (LBM) as a direct simulator to solve the three-phase flow at pore scale. The viscous coupling effects are investigated for various fluid configurations in simple pore geometries with different conditions in terms of saturation, wettability and viscosity ratio. It is found that the viscous coupling effect can be significant for certain configurations. A parametric modification factor for conventional three-phase conductance model is then proposed to estimate the viscous coupling effect. The modification factor as a function of viscosity ratios can be easily incorporated into existing pore network model (PNM) to eliminate errors from viscous coupling effect. Moreover, an elegant approach using machine learning technique is proposed to predict the multi-phase permeability by a trained Artificial Neural Network (ANN) from the direct simulation database. Such data-driven approach can be extended to develop a more sophisticated PNM for a better prediction of transport properties taking account of the viscous coupling effects

Molecular-dynamics simulations of interfaces between water and crystalline urea

Molecular-dynamics simulations of several water-crystalline urea interfaces have been performed. The structure and dynamics of water close to the urea crystal surface are discussed in terms of density profiles, positional and orientational distribution functions, and diffusion coefficients. The water structure close to the interface is strongly determined by the structure of the crystal surface: the (001) and (111) interfaces reveal strong adsorption of water while the (110) and () interfaces do so to a lesser extent. Assuming that the growth rate of a specific crystal face decreases with increasing solvent adsorption, the appearance of only (111) on the urea growth form is predicted. We argue that on the other hand the dominance of (110) over (001) cannot be explained using a simple layer growth model

Dissipative Particle Dynamics with energy conservation

Dissipative particle dynamics (DPD) does not conserve energy and this precludes its use in the study of thermal processes in complex fluids. We present here a generalization of DPD that incorporates an internal energy and a temperature variable for each particle. The dissipation induced by the dissipative forces between particles is invested in raising the internal energy of the particles. Thermal conduction occurs by means of (inverse) temperature differences. The model can be viewed as a simplified solver of the fluctuating hydrodynamic equations and opens up the possibility of studying thermal processes in complex fluids with a mesoscopic simulation technique.Comment: 5 page

The Control System for a new Pixel Detector at the sLHC

For the upgrade of the LHC, the sLHC (super Large Hadron Collider), a new ATLAS Pixel Detector is planned, which will require a completely new control system. To reduce the material budget new power distribution schemes are under investigation, where the active power conversion is located inside the detector volume. Such a new power supply system will need new control strategies. Parts of the control must be located closer to the loads. The minimization of mass, the demand for less cables and the re-use of the outer existing services are the main restrictions to the design of the control system. The requirements of the DCS (Detector Control System) and a first concept will be presented. We will focus on a control chip which necessarily has to be implemented in the new system. A setup of discrete components has been built up to investigate and verify the chip’s requirements. We report on the status of the work

Static and Dynamic Properties of Dissipative Particle Dynamics

The algorithm for the DPD fluid, the dynamics of which is conceptually a combination of molecular dynamics, Brownian dynamics and lattice gas automata, is designed for simulating rheological properties of complex fluids on hydrodynamic time scales. This paper calculates the equilibrium and transport properties (viscosity, self-diffusion) of the thermostated DPD fluid explicitly in terms of the system parameters. It is demonstrated that temperature gradients cannot exist, and that there is therefore no heat conductivity. Starting from the N-particle Fokker-Planck, or Kramers' equation, we prove an H-theorem for the free energy, obtain hydrodynamic equations, and derive a non-linear kinetic equation (the Fokker-Planck-Boltzmann equation) for the single particle distribution function. This kinetic equation is solved by the Chapman-Enskog method. The analytic results are compared with numerical simulations.Comment: 22 pages, LaTeX, 3 Postscript figure