Molecular Dynamics Simulations Of Flow Research Articles

Evaluation of different particle-actuation modes in molecular dynamics and their impact on nanoscale flow behaviors

An accurate understanding of nanoscale gas transport mechanism is a fundamental research concern in many engineering applications, which remains as a research challenge currently. Two particle-actuation modes, namely, force-driven and pressure-driven methods, are evaluated and compared by molecular dynamics simulations of flows in nano-channels focusing on the characteristics of gas adsorption and slip velocity behaviors. The force-driven method may lead to unphysical properties when fluid inhomogeneities are significant since all fluid molecules are subjected to a same external force. By contrast, fluid molecules move forwards through the central part of the flow domain as a predominate pathway in a pressure-driven method. Results show that there is a significant difference between the two methods at smooth or small rough wall conditions, while the results tend to be consistent as roughness increases. The density distribution is uniform along the flow direction in force-driven cases, while adsorbed gas density increases in pressure-driven cases, leading to a smaller slip velocity near the outlet region. The effects of fluid wettability strength on solid surfaces and system temperature on gas adsorption/flow behaviors are also investigated and analyzed. This study is helpful for better understanding nanoscale gas dynamics and has many practical implications, such as the shale gas production.

Large-scale molecular dynamics simulation of flow under complex structure of endothelial glycocalyx

In this research, large-scale molecular dynamics (MD) simulations were conducted to study the fluid dynamics inside the endothelial glycocalyx layer. A work flowchart regarding constructing the flow/glycocalyx system, undertaking production simulation using the MD method and post-processing was proposed. Following the flowchart, physiological and accelerating flow cases were simulated to reveal velocity and shear stress distributions over the dendritic (tree-like) structure of the glycocalyx, thereby contributing to understanding of the influence of biomolecular complex structures on flow profiles. Besides, the selection of thermostat algorithm was discussed. Results have shown that when the forcing is below a critical value, the velocity fluctuates around a zero mean along the height in the presence of the dendritic glycocalyx. When the forcing is larger than a critical value, the bulk flow was accelerated excessively, departing from the typical physiological flow. Furthermore, distributions of shear stress magnitude among three sub-regions in the ectodomain indicate that shear stress is enhanced near the membrane surface but is impaired in the sugar-chain-rich region due to the flow regulation by sugar chains. Finally, comparisons of velocity evolutions under two widely used thermostats (Lowe-Andersen and Berendsen thermostats) imply that the Lowe-Andersen algorithm is a suitable thermostat for flow problems.

Molecular dynamics simulation of flow through a Laval nozzle

Molecular dynamics simulation of flow through a Laval nozzle

Wavy Taylor vortices in molecular dynamics simulation of cylindrical Couette flow.

Molecular dynamics simulations of flow between concentric rotating cylinders are performed. As the relative speed between the two cylinders is increased, a spontaneous flow bifurcation occurs and vortices form in a stationary-vortex or traveling-wavy-vortex configuration. The former emerges when the axial boundary conditions constrain the flow by reflection, and the traveling-wavy-vortex flow develops when the axial boundaries are relaxed to periodic conditions. The flow bifurcation is triggered by the thermal fluctuations in the system, and the resulting flow field is in agreement with previous experimental observations. In addition, the temporal growth of the Fourier mode that characterizes the wavy-vortex motion is well described by Landau's theory for Hopf bifurcations. The spatiotemporal energy spectrum is evaluated in order to characterize the instability in terms of its azimuthal wave number and wave speed.

Nanoscale flow past a colloidal cylinder confined in a slit channel: Lubrication theory and molecular dynamics analysis

Plane Poiseuille flow past a nanoscale cylinder that is arbitrarily confined (i.e., symmetrically or asymmetrically confined) in a slit channel is studied via hydrodynamic lubrication theory and molecular dynamics simulations, considering cases where the cylinder remains static or undergoes thermal motion. Analytical expressions for the drag force and volumetric flow rate valid for high confinement and arbitrary off-center displacements are derived for the first time in this work. Lubrication theory predictions are in close agreement with molecular dynamics simulations of flows having molecularly thin lubrication gaps, despite the presence of significant structural forces induced by the crystalline structure of the modeled solid. While the maximum drag force is observed in symmetric confinement, i.e., when the cylinder is equidistant from both channel walls, the drag decays significantly as the cylinder moves away from the channel centerline and approaches a wall. Hence, significant reductions in the mean drag force on the cylinder and hydraulic resistance of the channel can be observed when thermal motion induces random off-center displacements. Analytical expressions and numerical results in this work provide useful insights into the hydrodynamics of colloidal solids and macromolecules in confinement.

Velocity slip on curved surfaces.

The Navier boundary condition for velocity slip on flat surfaces, when expressed in tensor form, is readily extended to surfaces of any shape. We test this assertion using molecular dynamics simulations of flow in channels with flat and curved walls and for rotating cylinders and spheres, all for a wide range of solid-liquid interaction strengths. We find that the slip length as conventionally measured at a flat wall in Couette flow is the same as that for all other cases with curved and rotating boundaries, provided the atomic interactions are the same and boundary shape is properly taken into account. These results support the idea that the slip length is a material property, transferable between different flow configurations.

Anomalous flow behavior in nanochannels: A molecular dynamics study

We report molecular dynamics simulations of flow of water in nanochannels with a range of surface wettability characteristics (hydrophobic to strongly hydrophilic) and driving forces (pressures). Our results show apparently anomalous behavior. At low pressures, the rate is higher in nanochannels with hydrophilic surfaces than that with hydrophobic surfaces; however, with high pressure driven flow we observe opposite trends. This apparently anomalous behavior can be explained on the basis of molecular thermodynamics and fluid mechanics considerations. Understanding such behavior is important in many nanofluidic devices such as nanoreactors, nanosensors, and nanochips that are increasingly being designed and used.

Nanoscale Fluid Flow Over Two Side-by-Side Cylinders With Atomically Rough Surface

A molecular dynamics simulation of flow over two side-by-side cylinders with atomically rough surfaces is presented. The model is two-dimensional with 3×105 liquid argon atoms. The surface roughness is constructed by external protrusion of atoms on the surface of the cylinders with specified amplitude and width. Two cylinders, with diameters of d=79.44 (molecular units), are placed at a distance of D in a vertical line. The solids atoms are allowed to vibrate around their equilibrium coordinates to mimic the real solid structure. The influence of various parameters, such as roughness amplitude, topology, periodicity, and the gap between cylinders on the hydrodynamics of flow, especially drag and lift forces, is studied. It was noted that even very little surface roughness, with amplitude on the order of a few nanometers, can influence the drag forces. Both roughness texture and the number of roughening elements affects the drag and lift coefficients. The gap between the cylinders showed to be an effective parameter, especially on the lift force for flow over the nanoscale cylinders.

Supercritical fluids in porous composite materials: Direction-dependent flow properties

The results of extensive nonequilibrium molecular dynamics simulations of flow and transport of a pure fluid, as well as a binary fluid mixture, through a porous material composed of a macropore, a mesopore, and a nanopore, in the presence of an external pressure gradient, are reported. We find that under supercritical conditions, unusual phenomena occur that give rise to direction-dependent and pressure-dependent permeabilities for the fluids' components. The results, which are also in agreement with a continuum formulation of the problem, indicate that the composite nature of the material, coupled with condensation, give rise to the direction-dependent permeabilities. Therefore, modeling flow and transport of fluids, in the supercritical regime, in porous materials with the type of morphology considered in this paper (such as supported porous membranes) would require using effective permeabilities that depend on both the external pressure drop and the direction along which it is applied to the materials.

Effect of patterned slip on micro- and nanofluidic flows

We consider the flow of a Newtonian fluid in a nano- or microchannel with walls that have patterned variations in slip length. We formulate a set of equations to describe the effects on an incompressible Newtonian flow of small variations in slip and solve these equations for slow flows. We test these equations using molecular dynamics simulations of flow between two walls which have patterned variations in wettability. Good qualitative agreement and a reasonable degree of quantitative agreement is found between the theory and molecular dynamics simulations. The results of both analyses show that patterned wettability can be used to induce complex variations in flow. Finally we discuss the implications of our results for the design of microfluidic mixers using slip.

Periodic-shell boundary condition for soft molecular systems: molecular dynamics simulations of flow past a circular cylinder

The periodic-shell boundary condition, developed as an outer boundary condition for molecular dynamics simulations, is applied to soft molecular systems, using a molecular dynamics simulation of the flow of a Lennard-Jones liquid past a two-dimensional circular cylinder. The usual periodic boundary condition is found to give rise to a significantly distorted velocity field. For example, the fluid in the side areas of the cylinder near the boundary surfaces flows along the boundary surfaces, i.e., in the uniform flow direction. The velocities in the rear area near the boundary surface are much larger than the results of the periodic-shell boundary condition and the Navier–Stokes equations. Also the pair of vortices behind the cylinder is significantly compressed in the direction of the cylindrical centre. In contrast to these findings, the results of the periodic-shell boundary condition agree well with the Navier–Stokes result qualitatively; good agreement is obtained concerning the formation of a pair of vortices behind the cylinder and the flow fields in the side region of the cylinder. These results show clearly that the periodic-shell boundary condition is very useful for soft molecular systems as well as for rigid molecular systems.

Density waves in the flows of granular media.

We study density waves in the flows of granular particles through vertical tubes and hoppers using both analytic methods and molecular dynamics (MD) simulations. We construct equations of motion for quasi one-dimensional systems. The equations, combined with the Bagnold's law for friction, are used to describe the time evolutions of the density and the velocity fields for narrow tubes and hoppers. The solutions of the equations can have two types of density waves, kinetic and dynamic. For tubes, we can show the existence of kinetic waves, and obtain the condition for dynamic waves for tubes from the equations. For hoppers, we obtain the solutions of the equations up to the first order of the opening angle, which also show the existence of kinetic waves. We reproduce density waves in the MD simulations for tubes. The waves are believed to be kinetic based on a few evidences, including a well defined flux-density curve. In MD simulations of flows in hoppers, we find density waves, which are also believed to be kinetic.

Molecular dynamics study of flow at a fluid-wall interface.

A novel wall model, including inelastic fluid-wall collisions, molecular structure of the wall, and a long-ranged fluid-wall interaction, is used in molecular dynamics simulations of flow in two dimensions. For strong fluid-wall attraction the fluid velocity goes smoothly to zero at the wall; there is no velocity slip. In less dense fluids the velocity drops to zero in an ever-narrower region. Our results differ from standard kinetic theory but agree with Eu's theory [Phys. Rev. A 36, 400 (1987)]. The nature of the fluid-wall interaction is important; repulsive walls give velocity slip.

Molecular-dynamics simulations of flow with binary diffusion in a two-dimensional channel with atomically rough walls.

Molecular-dynamics simulations of flow with binary diffusion in a two-dimensional channel with atomically rough walls.

Capillary condensation and molecular-dynamics simulations of flow in narrow pores.

The Hagen-Poiseuille flow of liquidlike drops formed during capillary condensation of Lennard-Jones molecules through narrow cylindrical pores is studied by molecular-dynamics simulations. The observed time evolution of the system depends upon the strength of the wall-fluid potential and the strength of the gravitational-type driving force. In the case of weak adsorbing walls, the whole drop slips along the pore, and during the flow changes in its local structure are observed. For stronger fluid-wall interaction and for larger driving forces, the phenomenon of vanishing of the drop has been found.

Molecular-dynamics simulation of flow in a two-dimensional channel with atomically rough walls.

Fluid flows in a two-dimensional channel with atomically rough walls were studied by molecular-dynamics simulations. Both sinusoidally and randomly roughened walls with various roughness amplitudes A were used. It was found that the no-slip condition arises when the molecular mean free path l is comparable with A. Therefore the ratio l/A should be employed to validate the no-slip condition, rather than the traditionally used ratio of the mean free path to the channel or tube diameter (Knudsen number).

Molecular dynamics simulation of flow past a plate

Molecular dynamics simulations with a soft-sphere potential have been carried out to model two dimensional fluid flow obstructed by a plate. At fluid velocities large enough to obtain adequate signal to noise resolution, two counter-circulating vortices are observed behind the obstruction. The stationary state length scale of these vortices is found to be roughly proportional to the average velocity in the system, as predicted by the hydrodynamic theory.