Non-local viscosity from the Green-Kubo formula.

We study through MD simulations the correlation matrix of the discrete transverse momentum density field in real space for an unconfined Lennard-Jones fluid at equilibrium. Mori theory predicts this correlation under the Markovian approximation from the knowledge of the non-local shear viscosity matrix, which is given in terms of a Green-Kubo formula. However, the running Green-Kubo integral for the non-local shear viscosity does not have a plateau. By using a recently proposed correction for the Green-Kubo formula that eliminates the plateau problem [Español et al., Phys. Rev. E 99, 022126 (2019)], we unambiguously obtain the actual non-local shear viscosity. The resulting Markovian equation, being local in time, is not valid for very short times. We observe that the Markovian equation with non-local viscosity gives excellent predictions for the correlation matrix from a time at which the correlation is around 80% of its initial value. A local in space approximation for the viscosity gives accurate results only after the correlation has decayed to 40% of its initial value.

Discrete hydrodynamics near solid planar walls.

We derive, with the projection operator technique, the equations of motion for the time-dependent average of the discrete mass and momentum densities of a fluid confined by planar walls under the assumption that the flow field is translationally invariant along the directions tangent to the walls. Shear flow and sound propagation perpendicular to the walls can be described with the discrete hydrodynamic equations. The interaction with the walls is not given through boundary conditions but rather in terms of impenetrability and friction forces appearing in the discrete hydrodynamic equations. Microscopic expressions for the transport coefficients entering the discrete equations are provided. We further show that the obtained discrete equations can be interpreted as a Petrov-Galerkin finite-element discretization of the continuum equations presented by Camargo et al. [J. Chem. Phys. 148, 064107 (2018)JCPSA60021-960610.1063/1.5010401] when restricted to planar geometries and flows.

Solution to the plateau problem in the Green-Kubo formula.

Transport coefficients appearing in Markovian dynamic equations for coarse-grained variables have microscopic expressions given by Green-Kubo formulas. These formulas may suffer from the well-known plateau problem. The problem arises because the Green-Kubo running integrals decay as the correlation of the coarse-grained variables themselves. The usual solution is to resort to an extreme timescale separation, for which the plateau problem is minor. Within the context of Mori projection operator formulation, we offer an alternative expression for the transport coefficients that is given by a corrected Green-Kubo expression that has no plateau problem by construction. The only assumption is that the Markovian approximation is valid in such a way that transport coefficients can be defined, even in the case that the separation of timescales is not extreme.

Boundary conditions derived from a microscopic theory of hydrodynamics near solids.

The theory of nonlocal isothermal hydrodynamics near a solid object derived microscopically in the study by Camargo et al. [J. Chem. Phys. 148, 064107 (2018)] is considered under the conditions that the flow fields are of macroscopic character. We show that in the limit of macroscopic flows, a simple pillbox argument implies that the reversible and irreversible forces that the solid exerts on the fluid can be represented in terms of boundary conditions. In this way, boundary conditions are derived from the underlying microscopic dynamics of the fluid-solid system. These boundary conditions are the impenetrability condition and the Navier slip boundary condition. The Green-Kubo transport coefficients associated with the irreversible forces that the solid exert on the fluid appear naturally in the slip length. The microscopic expression for the slip length thus obtained is shown to coincide with the one provided originally by Bocquet and Barrat [Phys. Rev. E 49, 3079 (1994)].

Discrete hydrodynamics near solid walls: Non-Markovian effects and the slip boundary condition.

A simple Markovian theory for the prediction of averages and correlations of discrete hydrodynamics near parallel solid walls is presented. The discrete momentum of bins is defined through a finite element basis function. The effect of the walls on the fluid is through irreversible extended friction forces appearing in the very equations of hydrodynamics. The Markovian assumption is critically assessed from the exponential decay of the eigenvalues of the correlation matrix of the discrete transverse momentum. We observe that for bins smaller than molecular dimensions, allowing one to resolve the density layering near the wall, the dynamics near the wall is non-Markovian. Bins larger than the molecular size do behave in a Markovian way. We measure the nonlocal viscosity and frictions kernels that appear in the discrete hydrodynamic equations, which are given in terms of Green-Kubo formulas. They suffer dramatically from the plateau problem. We use a recent procedure for reliably extracting the transport kernels out of the plateau-problematic Green-Kubo formula. With the so-measured transport kernels the nonlocal theory predicts very well the decay of the average of the transverse momentum when the initial velocity profile is a plug flow. The theory allows us to derive the slip boundary condition with microscopic expressions for the slip length and the hydrodynamic position of the wall. The slip boundary condition is not satisfied at the initial stages of the discontinous plug flow, but good agreement is obtained at later stages.

Microscopic Slip Boundary Conditions in Unsteady Fluid Flows.

An algebraic tail in the Green-Kubo integral for the solid-fluid friction coefficient hampers its use in the determination of the slip length. A simple theory for discrete nonlocal hydrodynamics near parallel solid walls with extended friction forces is given. We explain the origin of the algebraic tail and give a solution of the plateau problem in the Green-Kubo expressions. We derive the slip boundary condition with a microscopic expression for the slip length and the hydrodynamic wall position, and assess it through simulations of an unsteady plug flow.

Nanoscale hydrodynamics near solids.

Density Functional Theory (DFT) is a successful and well-established theory for the study of the structure of simple and complex fluids at equilibrium. The theory has been generalized to dynamical situations when the underlying dynamics is diffusive as in, for example, colloidal systems. However, there is no such a clear foundation for Dynamic DFT (DDFT) for the case of simple fluids in contact with solid walls. In this work, we derive DDFT for simple fluids by including not only the mass density field but also the momentum density field of the fluid. The standard projection operator method based on the Kawasaki-Gunton operator is used for deriving the equations for the average value of these fields. The solid is described as featureless under the assumption that all the internal degrees of freedom of the solid relax much faster than those of the fluid (solid elasticity is irrelevant). The fluid moves according to a set of non-local hydrodynamic equations that include explicitly the forces due to the solid. These forces are of two types, reversible forces emerging from the free energy density functional, and accounting for impenetrability of the solid, and irreversible forces that involve the velocity of both the fluid and the solid. These forces are localized in the vicinity of the solid surface. The resulting hydrodynamic equations should allow one to study dynamical regimes of simple fluids in contact with solid objects in isothermal situations.

[Postoperative blood pressure alterations after carotid endarterectomy : Implications of different reconstruction methods]. / Postoperative Blutdruckschwankungen nach Karotisendarteriektomie : Implikationen der verschiedenen Rekonstruktionstechniken.

BACKGROUND: Postoperative blood pressure alterations after carotid endarterectomy (CEA) are associated with an increased risk of morbidity and mortality. OBJECTIVE: To outline the influence of the two commonly used surgical reconstruction techniques, conventional CEA with patch plasty (C-CEA) and eversion CEA (E-CEA), as well as the innovative carotid sinus-preserving eversion CEA (SP-E-CEA) technique on postoperative hemodynamics, taking the current scientific knowledge into consideration. METHODS: Assessment of the current clinical and scientific evidence on each operative technique found in the PubMed (NLM) database ranging from 1974 to 2017, excluding case reports. RESULTS: A total of 34 relevant papers as well as 1 meta-analysis, which scientifically dealt with the described topic were identified. The results of the studies and the meta-analysis showed that E­CEA correlates with an impairment of local baroreceptor functions as well as with an elevated need for vasodilators in the early postoperative phase, whereas C­CEA and SP-E-CEA seem to have a more favorable effect on the postoperative blood pressure. CONCLUSION: The CEA technique influences the postoperative blood pressure regulation, irrespective of the operative technique used. Accordingly, close blood pressure monitoring is recommended at least during the postoperative hospital stay. Further studies are mandatory to evaluate the importance of SP-E-CEA as an alternative to the classical E­CEA.

Finite element discretization of non-linear diffusion equations with thermal fluctuations.

We present a finite element discretization of a non-linear diffusion equation used in the field of critical phenomena and, more recently, in the context of dynamic density functional theory. The discretized equation preserves the structure of the continuum equation. Specifically, it conserves the total number of particles and fulfills an H-theorem as the original partial differential equation. The discretization proposed suggests a particular definition of the discrete hydrodynamic variables in microscopic terms. These variables are then used to obtain, with the theory of coarse-graining, their dynamic equations for both averages and fluctuations. The hydrodynamic variables defined in this way lead to microscopically derived hydrodynamic equations that have a natural interpretation in terms of discretization of continuum equations. Also, the theory of coarse-graining allows to discuss the introduction of thermal fluctuations in a physically sensible way. The methodology proposed for the introduction of thermal fluctuations in finite element methods is general and valid for both regular and irregular grids in arbitrary dimensions. We focus here on simulations of the Ginzburg-Landau free energy functional using both regular and irregular 1D grids. Convergence of the numerical results is obtained for the static and dynamic structure factors as the resolution of the grid is increased.

Coarse-graining Brownian motion: from particles to a discrete diffusion equation.

We study the process of coarse-graining in a simple model of diffusion of Brownian particles. At a detailed level of description, the system is governed by a Brownian dynamics of non-interacting particles. The coarse-level is described by discrete concentration variables defined in terms of Delaunay cells. These coarse variables obey a stochastic differential equation that can be understood as a discrete version of a diffusion equation. We study different models for the two basic building blocks of this equation which are the free energy function and the diffusion matrix. The free energy function is shown to be non-additive due to the overlapping of cells in the Delaunay construction. The diffusion matrix is state dependent in principle, but for near-equilibrium situations it is shown that it may be safely evaluated at the equilibrium value of the concentration field.

Effects of periodic perturbations on the oscillatory behavior in the NO+H2 reaction on Pt(100).

The mean field model proposed by Makeev and Nieuwenhuys [J. Chem. Phys. 108, 3740 (1998)] simulates the oscillatory behavior experimentally observed in the NO+H2 reaction on the surface Pt(100). This model reproduces quite well the kinetic oscillations and the transition to chaos via the Feigenbaum route, that is to say, through bifurcations involving period doubling. From this model, we analyze the response of the natural oscillations of period-1 (P1, one maximum) to periodic perturbations superposed to the partial pressure of one of the reactants. The perturbed model reproduces the periodic states found in the autonomous model, the route to chaos through bifurcations with period doubling, and the appearance of chaos via the route of intermittency, which shows alternation of periodic oscillations with intervals of disordered oscillations in the same time evolution. Experimentally it has been observed that the reaction shows a great sensitivity to reactant partial pressures and temperature. In experimental conditions slightly different to those considered in Makeev and Nieuwenhuys (MN) model, oscillations with period-3 (P3, three maxima) have been observed. At T=457 K and certain pressures, these P3 oscillations do not appear in MN model, although they appear at T=456 K . The same effect (P3 oscillations) is obtained at T=457 K in our perturbed model, due to the modulation of pH2. In a second step we show how the modulation of the perturbing frequency influences on the oscillations P1 of the perturbed system. The results show that the periodic behavior loses its regularity at low values of the normalized amplitude and of the modulated frequency of the perturbation. Other aspect observed in the perturbed model is that the amount of products varies in relation to nonperturbed model. When the oscillations are periodic or they follow the Feigenbaum route to chaos, the production average decreases or slightly increases, whereas it always increases if there are intermittencies, the most significant percentage increase being for NH3 (nearly 10%).

El láser CO2 en cirugía Oncológica de cabeza y cuello perspectivas de su asociación con radioquimioterapia y carbógeno inhalado en las neoplasias avanzadas

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[Tonsillar angiofibroma]. / Angiofibroma amigdalar.

We report in this paper a case of a tonsillar angiofibroma, whose histopathological features did not differ from its nasopharyngeal counterpart. In an exhaustive review done we have only found another one reference, in the german literature of such location.

[Mixed bezoar]. / Bezoar mixto.

The case of an adolescent psychopath who, following traumatic psychological experiences, ingested her own hair and cotton string over a 6 to 8 month period, is presented. This resulted in the formation of a mixed bezoar (hair and cotton string) with subsequent deterioration of her general condition. The diagnosis was made roetgenographically. Curative surgery was performed and psychotherapy was initiated. A review of the current literature is made and the frequency, clinical characteristics, treatment, management and recommended prophylactic considerations are discussed.