Two-dimensional hydrodynamic lattice-gas simulations of binary immiscible and ternary amphiphilic fluid flow through porous media

The behaviour of two dimensional binary and ternary amphiphilic fluids under flow conditions is investigated using a hydrodynamic lattice gas model. After the validation of the model in simple cases (Poiseuille flow, Darcy's law for single component fluids), attention is focussed on the properties of binary immiscible fluids in porous media. An extension of Darcy's law which explicitly admits a viscous coupling between the fluids is verified, and evidence of capillary effects are described. The influence of a third component, namely surfactant, is studied in the same context. Invasion simulations have also been performed. The effect of the applied force on the invasion process is reported. As the forcing level increases, the invasion process becomes faster and the residual oil saturation decreases. The introduction of surfactant in the invading phase during imbibition produces new phenomena, including emulsification and micellisation. At very low fluid forcing levels, this leads to the production of a low-resistance gel, which then slows down the progress of the invading fluid. At long times (beyond the water percolation threshold), the concentration of remaining oil within the porous medium is lowered by the action of surfactant, thus enhancing oil recovery. On the other hand, the introduction of surfactant in the invading phase during drainage simulations slows down the invasion process -- the invading fluid takes a more tortuous path to invade the porous medium -- and reduces the oil recovery (the residual oil saturation increases).Comment: 48 pages, 26 figures. Phys. Rev. E (in press

Medicare Reimbursement for Total Joint Arthroplasty: The Driving Forces.

BACKGROUND: Total joint arthroplasty is a large and growing part of the U.S. Medicare budget, drawing attention to how much providers are paid for their services. The purpose of this study was to examine the variables that affect total joint arthroplasty reimbursement. Along with standard economic variables, we include unique health-care variables. Given the focus on value in the Affordable Care Act, the model examines the relationship of the quality of care to total joint arthroplasty reimbursement. We hoped to find that reimbursement patterns reward quality and reflect standard economic principles. METHODS: Multivariable regression was performed to identify variables that correlate with Medicare reimbursement for total joint arthroplasty. Inpatient charge or reimbursement data on Medicare reimbursements were available for 2,750 hospitals with at least 10 discharges for uncomplicated total joint arthroplasty from the Centers for Medicare & Medicaid Services (CMS) for fiscal year 2011. Reimbursement variability was examined by using the Dartmouth Atlas to group institutions into hospital referral regions and hospital service areas. Independent variables were taken from the Dartmouth Atlas, CMS, the WWAMI (Washington, Wyoming, Alaska, Montana, Idaho) Rural Health Research Center, and the United States Census. RESULTS: There were 427,207 total joint arthroplasties identified, with a weighted mean reimbursement of $14,324.84 (range,$9,103 to $38,686). Nationally, the coefficient of variation for reimbursements was 0.19. The regression model accounted for 52.5% of reimbursement variation among providers. The total joint arthroplasty provider volume (p \u3c 0.001) and patient satisfaction (p \u3c 0.001) were negatively correlated with reimbursement. Government ownership of a hospital (p \u3c 0.001) and higher Medicare costs (p \u3c 0.001) correlated positively with reimbursement. CONCLUSIONS: Medicare reimbursements for total joint arthroplasty are highly variable. Greater reimbursement was associated with lower patient volume, lower patient satisfaction, a healthier patient population, and government ownership of a hospital. As value-based reimbursement provisions of the Affordable Care Act are implemented, there will be dramatic changes in total joint arthroplasty reimbursements. To meet these changes, providers should expect qualities such as high patient volume, willingness to care for sicker patient populations, patient satisfaction, safe outcomes, and procedural demand to correlate with their reimbursement. CLINICAL RELEVANCE: Practicing orthopaedic surgeons and hospital administrators should be aware of discrepancies in inpatient reimbursement for total joint arthroplasty from Medicare. Furthermore, these discrepancies are not associated with typical economic factors. These findings warrant further investigation and collaboration between policymakers and providers to develop value-based reimbursement

Simulating Three-Dimensional Hydrodynamics on a Cellular-Automata Machine

We demonstrate how three-dimensional fluid flow simulations can be carried out on the Cellular Automata Machine 8 (CAM-8), a special-purpose computer for cellular-automata computations. The principal algorithmic innovation is the use of a lattice-gas model with a 16-bit collision operator that is specially adapted to the machine architecture. It is shown how the collision rules can be optimized to obtain a low viscosity of the fluid. Predictions of the viscosity based on a Boltzmann approximation agree well with measurements of the viscosity made on CAM-8. Several test simulations of flows in simple geometries -- channels, pipes, and a cubic array of spheres -- are carried out. Measurements of average flux in these geometries compare well with theoretical predictions.Comment: 19 pages, REVTeX and epsf macros require

Interface Roughening in a Hydrodynamic Lattice-Gas Model with Surfactant

Using a hydrodynamic lattice-gas model, we study interface growth in a binary fluid with various concentrations of surfactant. We find that the interface is smoothed by small concentrations of surfactant, while microemulsion droplets form for large surfactant concentrations. To assist in determining the stability limits of the interface, we calculate the change in the roughness and growth exponents $\alpha$ and $\beta$ as a function of surfactant concentration along the interface.Comment: 4 pages with 4 embedded ps figures. Requires psfig.tex. Will appear in PRL 14 Oct 199

Three dimensional hysdrodynamic lattice-gas simulations of binary immiscible and ternary amphiphilic flow through porous media

We report the results of a study of multiphase flow in porous media. A Darcy's law for steady multiphase flow was investigated for both binary and ternary amphiphilic flow. Linear flux-forcing relationships satisfying Onsager reciprocity were shown to be a good approximation of the simulation data. The dependence of the relative permeability coefficients on water saturation was investigated and showed good qualitative agreement with experimental data. Non-steady state invasion flows were investigated, with particular interest in the asymptotic residual oil saturation. The addition of surfactant to the invasive fluid was shown to significantly reduce the residual oil saturation.Comment: To appear in Phys. Rev.

Coming of age in L3 initial stages transfer models: deriving developmental predictions and looking towards the future

Aims: Over the past decade in particular, formal linguistic work within L3 acquisition has concentrated on hypothesizing and empirically determining the source of transfer from previous languages—L1, L2 or both—in L3 grammatical representations. In view of the progressive concern with more advanced stages, we aim to show that focusing on L3 initial stages should be one continued priority of the field, even—or especially—if the field is ready to shift towards modeling L3 development and ultimate attainment. Approach: We argue that L3 learnability is significantly impacted by initial stages transfer, as such forms the basis of the initial L3 interlanguage. To illustrate our point, the insights from studies using initial and intermediary stages L3 data are discussed in light of developmental predictions that derive from the initial stages models. Conclusions: Despite a shared desire to understand the process of L3 acquisition in whole, inclusive of offering developmental L3 theories, we argue that the field does not yet have—although is ever closer to—the data basis needed to effectively do so. Originality: This article seeks to convince the readership for the need of conservatism in L3 acquisition theory building, whereby offering a framework on how and why we can most effectively build on the accumulated knowledge of the L3 initial stages in order to make significant, steady progress. Significance: The arguments exposed here are meant to provide an epistemological base for a tenable framework of formal approaches to L3 interlanguage development and, eventually, ultimate attainment

Computer simulations of domain growth and phase separation in two-dimensional binary immiscible fluids using dissipative particle dynamics

We investigate the dynamical behavior of binary fluid systems in two dimensions using dissipative particle dynamics. We find that following a symmetric quench the domain size R(t) grows with time t according to two distinct algebraic laws R(t) = t^n: at early times n = 1/2, while for later times n = 2/3. Following an asymmetric quench we observe only n = 1/2, and if momentum conservation is violated we see n = 1/3 at early times. Bubble simulations confirm the existence of a finite surface tension and the validity of Laplace's law. Our results are compared with similar simulations which have been performed previously using molecular dynamics, lattice-gas and lattice-Boltzmann automata, and Langevin dynamics. We conclude that dissipative particle dynamics is a promising method for simulating fluid properties in such systems.Comment: RevTeX; 22 pages, 5 low-resolution figures. For full-resolution figures, connect to http://www.tcm.phy.cam.ac.uk/~ken21/tension/tension.htm

Bounds on the mass-to-radius ratio for non-compact field configurations

It is well known that a spherically symmetric compact star whose energy density decreases monotonically possesses an upper bound on its mass-to-radius ratio, $2M/R\leq 8/9$. However, field configurations typically will not be compact. Here we investigate non-compact static configurations whose matter fields have a slow global spatial decay, bounded by a power law behavior. These matter distributions have no sharp boundaries. We derive an upper bound on the fundamental ratio max_r{2m(r)/r} which is valid throughout the bulk. In its simplest form, the bound implies that in any region of spacetime in which the radial pressure increases, or alternatively decreases not faster than some power law $r^{-(c+4)}$, one has $2m(r)/r \leq (2+2c)/(3+2c)$. [For $c \leq 0$ the bound degenerates to $2m(r)/r \leq 2/3$.] In its general version, the bound is expressed in terms of two physical parameters: the spatial decaying rate of the matter fields, and the highest occurring ratio of the trace of the pressure tensor to the local energy density.Comment: 4 page

Shear-Induced Isotropic-to-Lamellar Transition in a Lattice-Gas Model of Ternary Amphiphilic Fluids

Although shear-induced isotropic-to-lamellar transitions in ternary systems of oil, water and surfactant have been observed experimentally and predicted theoretically by simple models for some time now, their numerical simulation has not been achieved so far. In this work we demonstrate that a recently introduced hydrodynamic lattice-gas model of amphiphilic fluids is well suited for this purpose: the two-dimensional version of this model does indeed exhibit a shear-induced isotropic-to-lamellar phase transition.Comment: 17 pages, LaTeX with epsf and REVTeX, PostScript and EPS illustrations included. To appear in J. Phys. Cond. Ma

Channel Network Scaling Laws in Submarine Basins

Fluvial drainage basin area is often related to channel length and local slope through power law relationships and the relatively small range of exponents observed in these relationships is thought to result from physical mechanisms. Proposed mechanisms assume that the observed correlation between drainage area and fluid discharge is caused by precipitation. Using high resolution DEMs of channelized continental slope settings offshore Monterey, CA and Brunei Darussalam we extracted submarine channel profiles and drainage area statistics from five basins. In-situ and remote observations suggest discharge in these oceanic settings is determined by boundary conditions at the shelf-edge. In spite of substantial differences in environment and physical process, the data yield submarine scaling exponents within the range of terrestrial (fluvial) observations. The convergence in scaling relationships from two very different settings supports theoretical arguments that channel network structure results from the aggregation of random walks