Impurity in a granular gas under nonlinear Couette flow

We study in this work the transport properties of an impurity immersed in a granular gas under stationary nonlinear Couette flow. The starting point is a kinetic model for low-density granular mixtures recently proposed by the authors [Vega Reyes F et al. 2007 Phys. Rev. E 75 061306]. Two routes have been considered. First, a hydrodynamic or normal solution is found by exploiting a formal mapping between the kinetic equations for the gas particles and for the impurity. We show that the transport properties of the impurity are characterized by the ratio between the temperatures of the impurity and gas particles and by five generalized transport coefficients: three related to the momentum flux (a nonlinear shear viscosity and two normal stress differences) and two related to the heat flux (a nonlinear thermal conductivity and a cross coefficient measuring a component of the heat flux orthogonal to the thermal gradient). Second, by means of a Monte Carlo simulation method we numerically solve the kinetic equations and show that our hydrodynamic solution is valid in the bulk of the fluid when realistic boundary conditions are used. Furthermore, the hydrodynamic solution applies to arbitrarily (inside the continuum regime) large values of the shear rate, of the inelasticity, and of the rest of parameters of the system. Preliminary simulation results of the true Boltzmann description show the reliability of the nonlinear hydrodynamic solution of the kinetic model. This shows again the validity of a hydrodynamic description for granular flows, even under extreme conditions, beyond the Navier-Stokes domain.Comment: 23 pages, 11 figures; v2: Preliminary DSMC results from the Boltzmann equation included, Fig. 11 is ne

Computer simulations of an impurity in a granular gas under planar Couette flow

We present in this work results from numerical solutions, obtained by means of the direct simulation Monte Carlo (DSMC) method, of the Boltzmann and Boltzmann--Lorentz equations for an impurity immersed in a granular gas under planar Couette flow. The DSMC results are compared with the exact solution of a recent kinetic model for the same problem. The results confirm that, in steady states and over a wide range of parameter values, the state of the impurity is enslaved to that of the host gas: it follows the same flow velocity profile, its concentration (relative to that of the granular gas) is constant in the bulk region, and the impurity/gas temperature ratio is also constant. We determine also the rheological properties and nonlinear hydrodynamic transport coefficients for the impurity, finding a good semi-quantitative agreement between the DSMC results and the theoretical predictions.Comment: 23 pages, 11 figures; v2: minor change

Effect of inelasticity on the phase transitions of a thin vibrated granular layer

We describe an experimental and computational investigation of the ordered and disordered phases of a vibrating thin, dense granular layer composed of identical metal spheres. We compare the results from spheres with different amounts of inelasticity and show that inelasticity has a strong effect on the phase diagram. We also report the melting of an ordered phase to a homogeneous disordered liquid phase at high vibration amplitude or at large inelasticities. Our results show that dissipation has a strong effect on ordering and that in this system ordered phases are absent entirely in highly inelastic materials.Comment: 5 pages, 5 figures, published in Physical Review E. Title of first version slightly change

An exact solution of the inelastic Boltzmann equation for the Couette flow with uniform heat flux

In the steady Couette flow of a granular gas the sign of the heat flux gradient is governed by the competition between viscous heating and inelastic cooling. We show from the Boltzmann equation for inelastic Maxwell particles that a special class of states exists where the viscous heating and the inelastic cooling exactly compensate each other at every point, resulting in a uniform heat flux. In this state the (reduced) shear rate is enslaved to the coefficient of restitution $\alpha$, so that the only free parameter is the (reduced) thermal gradient $\epsilon$. It turns out that the reduced moments of order $k$ are polynomials of degree $k-2$ in $\epsilon$, with coefficients that are nonlinear functions of $\alpha$. In particular, the rheological properties ($k=2$) are independent of $\epsilon$ and coincide exactly with those of the simple shear flow. The heat flux ($k=3$) is linear in the thermal gradient (generalized Fourier's law), but with an effective thermal conductivity differing from the Navier--Stokes one. In addition, a heat flux component parallel to the flow velocity and normal to the thermal gradient exists. The theoretical predictions are validated by comparison with direct Monte Carlo simulations for the same model.Comment: 16 pages, 4 figures,1 table; v2: minor change

Barreras identificadas por el personal de salud para el tamizaje del virus de inmunodeficiencia humana en población indígena de la Amazonía peruana

Objetivos. Describir las barreras identificadas por el personal de salud para el tamizaje de virus de inmunodeficiencia humana (VIH) a población indígena en establecimientos de salud de cuatro regiones de la Amazonía peruana. Materiales y métodos. Estudio mixto. La fase cualitativa fue de tipo exploratoria y la fase cuantitativa se basó en un estudio descriptivo transversal de carácter exploratorio. Se seleccionaron 87 establecimientos en los que se aplicó una encuesta, además se realizaron 51 entrevistas a personal de salud de algunos de los establecimientos. Las entrevistas fueron codificadas mediante el software Atlas.ti 7.0. Para las encuestas se realizó un análisis estadístico descriptivo. Resultados. Las barreras identificadas fueron geográficas, socioculturales, y vinculadas al sistema de salud. Fue más frecuente encontrar desabastecimiento de insumos o reactivos en puestos de salud (40,0%) en comparación con los centros de salud (25,0%) y hospitales (11,1%). El principal aspecto que limitó la realización de campañas de tamizaje para el acceso a la población indígena fue la falta de presupuesto (59,3%), la falta de recursos humanos, el que las personas no acepten la toma de muestra de sangre, así como la falta de abastecimiento de insumos o reactivos para las pruebas de diagnóstico. Conclusiones. Las principales barreras identificadas por el personal de salud son en su mayoría las vinculadas al sistema de salud y de índole sociocultural. Se recomienda considerarlas para la mejora de la oferta de los servicios, así como para su adecuación cultural

Memory effects in a gas of viscoelastic particles

We study a granular gas of viscoelastic particles (kinetic energy loss upon collision is a function of the particles' relative velocities at impact) subject to a stochastic thermostat. We show that the system displays anomalous cooling and heating rates during thermal relaxation processes, this causing the emergence of thermal memory. In particular, a significant Mpemba effect is present, i.e., an initially hotter/cooler granular gas can cool down/heat up faster than an in comparison cooler/hotter granular gas. Moreover, a Kovacs effect is also observed, i.e., a nonmonotonic relaxation of the granular temperature¿if the gas undergoes certain sudden temperature changes before fixing its value. Our results show that both memory effects have distinct features, very different and eventually opposed to those reported in theory for granular fluids under simpler collisional models. We study our system via three independent methods: approximate solution of the kinetic equation time evolution and computer simulations (both molecular dynamics simulations and direct simulation Monte Carlo method), finding good agreement between them.This work has been partially funded by the Spanish Ministerio de Ciencia, Innovación y Universidades and the Agencia Estatal de Investigación through Grant Nos. MTM2017-84446-C2–2-R (A.L., E.M., and A.T.), FIS2017-84440-C2–2-P (A.T.), and FIS2016-76359-P. (F.V.R.). M.A.L.C. and F.V.R. also acknowledge support from the regional Extremadura Government through Project Nos. GR18079 and IB16087. Computing facilities from the Extremadura Research Centre for Advanced Technologies (CETA-CIEMAT) are also acknowledged. All grants and facilities were provided with partial support from the ERDF