9,550 research outputs found
ASHEE: a compressible, equilibrium-Eulerian model for volcanic ash plumes
A new fluid-dynamic model is developed to numerically simulate the
non-equilibrium dynamics of polydisperse gas-particle mixtures forming volcanic
plumes. Starting from the three-dimensional N-phase Eulerian transport
equations for a mixture of gases and solid particles, we adopt an asymptotic
expansion strategy to derive a compressible version of the first-order
non-equilibrium model, valid for low concentration regimes and small particles
Stokes . When the model reduces to the dusty-gas one. The
new model is significantly faster than the Eulerian model while retaining the
capability to describe gas-particle non-equilibrium. Direct numerical
simulation accurately reproduce the dynamics of isotropic turbulence in
subsonic regime. For gas-particle mixtures, it describes the main features of
density fluctuations and the preferential concentration of particles by
turbulence, verifying the model reliability and suitability for the simulation
of high-Reynolds number and high-temperature regimes. On the other hand,
Large-Eddy Numerical Simulations of forced plumes are able to reproduce their
observed averaged and instantaneous properties. The self-similar radial profile
and the development of large-scale structures are reproduced, including the
rate of entrainment of atmospheric air. Application to the Large-Eddy
Simulation of the injection of the eruptive mixture in a stratified atmosphere
describes some of important features of turbulent volcanic plumes, including
air entrainment, buoyancy reversal, and maximum plume height. Coarse particles
partially decouple from the gas within eddies, modifying the turbulent
structure, and preferentially concentrate at the eddy periphery, eventually
being lost from the plume margins due to the gravity. By these mechanisms,
gas-particle non-equilibrium is able to influence the large-scale behavior of
volcanic plumes.Comment: 29 pages, 22 figure
Effect of particle inertia on the turbulence in a suspension
We propose a one-fluid analytical model for a turbulently flowing dilute
suspension, based on modified Navier-Stokes equation with a -dependent
effective density of suspension, , and an additional damping
term , representing the fluid-particle friction
(described by Stokes law). The statistical description of turbulence within the
model is simplified by a modification of the usual closure procedure based on
the Richardson-Kolmogorov picture of turbulence with a differential
approximation for the energy transfer term. The resulting ordinary differential
equation for the energy budget is solved analytically for various important
limiting cases and numerically in the general case. In the inertial interval of
scales we describe analytically two competing effects: the energy suppression
due to the fluid particle friction and the energy enhancement during the
cascade process due to decrease of the effective density of the small scale
motions. An additional suppression or enhancement of the energy density may
occur in the viscous subrange, caused by the variation of the extent of the
inertial interval due to the combined effect of the fluid-particle friction and
the decrease of the kinematic viscosity of the suspensions. The analytical
description of the complicated interplay of these effects supported by
numerical calculations is presented. Our findings allow one to rationalize the
qualitative picture of the isotropic homogeneous turbulence of dilute
suspensions as observed in direct numerical simulations.Comment: 21 pages, 5 figues,included, PRE, submitte
DNS of compressible multiphase flows through the Eulerian approach
In this paper we present three multiphase flow models suitable for the study
of the dynamics of compressible dispersed multiphase flows. We adopt the
Eulerian approach because we focus our attention to dispersed (concentration
smaller than 0.001) and small particles (the Stokes number has to be smaller
than 0.2). We apply these models to the compressible ()
homogeneous and isotropic decaying turbulence inside a periodic
three-dimensional box ( cells) using a numerical solver based on the
OpenFOAM C++ libraries. In order to validate our simulations in the
single-phase case we compare the energy spectrum obtained with our code with
the one computed by an eighth order scheme getting a very good result (the
relative error is very small ). Moving to the bi-phase case,
initially we insert inside the box an homogeneous distribution of particles
leaving unchanged the initial velocity field. Because of the centrifugal force,
turbulence induce particle preferential concentration and we study the
evolution of the solid-phase density. Moreover, we do an {\em a-priori} test on
the new sub-grid term of the multiphase equations comparing them with the
standard sub-grid scale term of the Navier-Stokes equations.Comment: 10 pages, 5 figures, preprint. Direct and Large Eddy Simulations 9,
201
Deblocking of interacting particle assemblies: from pinning to jamming
A wide variety of interacting particle assemblies driven by an external force
are characterized by a transition between a blocked and a moving phase. The
origin of this deblocking transition can be traced back to the presence of
either external quenched disorder, or of internal constraints. The first case
belongs to the realm of the depinning transition, which, for example, is
relevant for flux-lines in type II superconductors and other elastic systems
moving in a random medium. The second case is usually included within the
so-called jamming scenario observed, for instance, in many glassy materials as
well as in plastically deforming crystals. Here we review some aspects of the
rich phenomenology observed in interacting particle models. In particular, we
discuss front depinning, observed when particles are injected inside a random
medium from the boundary, elastic and plastic depinning in particle assemblies
driven by external forces, and the rheology of systems close to the jamming
transition. We emphasize similarities and differences in these phenomena.Comment: 20 pages, 8 figures, submitted for a special issue of the Brazilian
Journal of Physics entitled: Statistical Mechanics of Irreversible Stochastic
Models - I
A statistical mechanics approach to mixing in stratified fluids
Predicting how much mixing occurs when a given amount of energy is injected
into a Boussinesq fluid is a longstanding problem in stratified turbulence. The
huge number of degrees of freedom involved in those processes renders extremely
difficult a deterministic approach to the problem. Here we present a
statistical mechanics approach yielding prediction for a cumulative, global
mixing efficiency as a function of a global Richardson number and the
background buoyancy profile.Comment: Accepted in Journal of Fluid Mechanic
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