We examine the effects of hydrodynamics on the late-stage kinetics in spinodal decomposition. From computer simulations of a lattice Boltzmann scheme we observe, for critical quenches, that single-phase domains grow asymptotically like tα, with α0.66 in two dimensions and α1.0 in three dimensions, both in excellent agreement with theoretical predictions. © 1993 The American Physical Society.

Alexander F. J.;Chen S.;Grunau D. W.

Physical Review B

1993

Using a Langevin description of spinodal decomposition in fluids, we examine domain growth in the diffusive, viscous, and inertial regimes. In the framework of this model, numerical results corroborate earlier theoretical predictions based on scaling arguments and dimensional analysis. © 1996 The American Physical Society.

Lookman Turab;Wu Yanan;Alexander Francis J.;Chen Shiyi

Physical Review E Statistical Physics Plasmas Fluids and Related Interdisciplinary Topics

1996

Pore networks derived from synchrotron X-ray microtomographic images of reservoir rocks were used to provide realistic geometries for simulation of oil displacement by water. The Lattice Boltzmann Method was used to compute two-phase flow dynamics with constant pore geometry - but different pressure driving forces and imposed wettability distributions at the pore wall boundaries. Such examinations can be used to formulate more physically meaningful functional forms for multiphase flow relations to replace empirical constructs of relative permeability with only implicit dependencies on wettability and pore structure. Each simulation started with the same low initial water saturation distribution achieved by primary drainage network simulation in an initially strongly water-wet, reasonably homogeneous pore system. Mixed-wet scenarios were produced by altering the boundary conditions of those surfaces contacted by the non-wetting phase in the initial water distribution image. In one set of simulations comparing water-wet and mixed-wet boundary conditions, capillary numbers in the vicinity of 10^-4were not low enough for differences in wettability to give markedly different displacement dynamics. In a second set of simulations, as the driving force for flow was reduced further, we saw differences in the fluid distributions, but only minor changes in the relative permeabilities extracted from simulation output. The biggest difference between waterflood simulations with different wettabilities appears to be the additional recovery possible after breakthrough in the mixed-wet scenario. Capillary pressure, and not relative permeability, controls whether or not a particular saturation state can be observed. It is therefore concluded that capturing wettability effects in capillary pressure relations is probably more important than trying to roll such effects into relative permeability functions. Wettability is expected to play a more important role in heterogeneous systems. © 1998 Elsevier Science B.V. All rights reserved.

Hazlett R. D.;Chen S. Y.;Soll W. E.

Journal of Petroleum Science and Engineering

1998

Sub-grid nonlinear interaction and energy transfer were analyzed using direct numerical simulations of isotropic turbulence. Influences of cutoff wave number at different ranges of scale on the energetics and dynamics have been investigated. It is observed that sub-grid-sub-grid interaction dominates the turbulent dynamics when cut-off wave number locates in the energy-containing range while resolved-sub-grid interaction dominates if it is in the dissipation range. By decomposing the sub-grid energy transfer and nonlinear interaction into `forward' and `backward' groups according to the sign of triadic interaction, it is found that individually each group has very large contribution, but the net of them is much smaller, implying that tremendous cancellation happens between these two groups.

Gong H.;Chen S.;He G. W.

Acta Mechanica Sinica Lixue Xuebao

1999

Flow in porous media is studied at the pore-scale with lattice Boltzmann simulations on pore geometries reconstructed from computed microtomographic images. Pore scale results are analyzed to give quantities such as permeability, porosity and specific surface area at various scales and at various locations. With this, some fundamental issues such as scale dependency and medium variability can be assessed quantitatively. More specifically, the existence and size of the well known concept, representative elementary volume (REV), can be quantified. It is found that the size of an REV varies spatially and depends on the quantity being represented. For heterogeneous media, a better measure may be the so called 'statistical REV', which has weaker requirements than does the deterministic REV.

Zhang Dongxiao;Soll Wendy E.;Zhang Raoyang;Chen Shiyi

Geophysical Research Letters

2000

Chen Shiyi;Sheng Ping;Tao Ruibao

International Journal of Modern Physics B

2003

In this paper, we use a lattice Boltzmann (LB) multiphase/multicomponent model to study the flow of two immiscible fluids with different viscosities. The approach is first validated for a two-dimensional layered flow. The velocity profiles and the relative permeability coefficients are compared with the analytic results. We then apply this method to studying fingering in a two-dimensional channel where one fluid is displaced by another. The effects of viscosity ratio, capillary number, and wettability are investigated. The simulation results show that with the increase of the viscosity ratio or capillary number, both the finger width and the slip distance of the contact lines decrease, while the finger length increases. With the decrease of the wettability of the displacing fluid, the finger length and its change rate with time increase while the slip distance of the contact lines and its change rate with time decrease, and the minimum capillary number to form a stable finger decreases. Hence the finger growth is enhanced when the displacing fluid is nonwetting to the wall and otherwise suppressed. An indented part near the beginning of the fingers is clearly observed when a wetting fluid is displacing a nonwetting one. The finger width, however remains nearly unchanged when the wettability of the fluids changes. © 2003 Elsevier Ltd. All rights reserved.

Kang Qinjun;Zhang Dongxiao;Chen Shiyi

Advances in Water Resources

2004

Flow driven by moving a wall that bounds a fluid-filled cavity is a classic example of a multiscale problem. Continuum equations predict that every scale contributes roughly equally to the total force on the moving wall, leading to a logarithmic divergence, and that there is an infinite hierarchy of vortices at the stationary corners. A multiscale approach is developed that retains an atomistic description in key regions. Following the stress over more than six decades in length in systems with characteristic scales of up to millimeters and milliseconds allows us to resolve the singularities and determine the force for the first time. We find a universal dependence on the macroscopic Reynolds number, and large atomistic effects that depend on wall velocity and interactions. © 2006 The American Physical Society.

Nie Xiaobo;Robbins Mark O.;Chen Shiyi

Physical Review Letters

2006

Electric potential distribution in nanoscale electroosmosis has been investigated using the nonequilibrium molecular dynamics (NEMD), whose results are compared with the continuum based Poisson-Boltzmann (PB) theory. If the bin size of the MD simulation is no smaller than a molecular diameter and the focusing region is limited to the diffusion layer, the ionic density profiles on the bins of the MD results agree well with the predictions based on the PB theory for low and moderate bulk ionic concentrations. The PB equation breaks down at high bulk ionic concentrations, which is also consistent with the macroscopic description.

Wang M.;Liu J.;Chen S.

Molecular Simulation

2007

We report an investigation of inverse energy cascade in steady-state two-dimensional turbulence by direct numerical simulation (DNS) of the two-dimensional Navier-Stokes equation, with small-scale forcing and large-scale damping. We employed several types of damping and dissipation mechanisms in simulations up to 2048² resolution. For all these simulations we obtained a wavenumber range for which the mean spectral energy flux is a negative constant and the energy spectrum scales as k^-5/3, consistent with the predictions of Kraichnan (Phys. Fluids, vol. 439, 1967, p. 1417). To gain further insight, we investigated the energy cascade in physical space, employing a local energy flux defined by smooth filtering. We found that the inverse energy cascade is scale local, but that the strongly local contribution vanishes identically, as argued by Kraichnan (J. Fluid Mech., vol. 47, 1971, p. 525). The mean flux across a length scale ℓ was shown to be due mainly to interactions with modes two to eight times smaller. A major part of our investigation was devoted to identifying the physical mechanism of the two-dimensional inverse energy cascade. One popular idea is that inverse energy cascade proceeds via merger of like-sign vortices. We made a quantitative study employing a precise topological criterion of merger events. Our statistical analysis showed that vortex mergers play a negligible direct role in producing mean inverse energy flux in our simulations. Instead, we obtained with the help of other works considerable evidence in favour of a 'vortex thinning' mechanism, according to which the large-scale strains do negative work against turbulent stress as they stretch out the isolines of small-scale vorticity. In particular, we studied a multi-scale gradient (MSG) expansion developed by Eyink (J. Fluid Mech., vol. 549, 2006 a, p. 159) for the turbulent stress, whose contributions to inverse cascade can all be explained by 'thinning'. The MSG expansion up to second order in space gradients was found to predict well the magnitude, spatial structure and scale distribution of the local energy flux. The majority of mean flux was found to be due to the relative rotation of strain matrices at different length scales, a first-order effect of 'thinning'. The remainder arose from two second-order effects, differential strain rotation and vorticity gradient stretching. Our findings give strong support to vortex thinning as the fundamental mechanism of two-dimensional inverse energy cascade. © 2008 Cambridge University Press.

Xiao Z.;Chen S.;Wan M.;Eyink G. L.

Journal of Fluid Mechanics

2009