188 research outputs found
Aplicación de dos técnicas de separación de Minerales del Grupo del Platino (EPD + HS y Superpanning) en muestras de cromitas y de placeres
El superpanning (SP) y la hidroseparación (HS) son dos técnicas de separación de minerales del grupo del platino (MGP) que permiten concentrar estos minerales sin contaminar la muestra. La HS se suele usar en combinación con la Desintegración por Impulsos Eléctricos (EPD), ya que ésta permite una trituración de la muestra sin sufrir daños en su estructura.
En este trabajo se pretende comparar la eficacia de ambos métodos para la concentración de MGP en muestras de placeres fluviales de Loma Cabrera y de cromititas en lateritas niquelíferas de Loma Peguera, ambas de la República Dominicana. Estudios previos (Zaccarini et al 2009) indican que en las cromititas existen una gran variedad de Elementos del Grupo del Platino (EGP) por lo que en un futuro estos depósitos podrían ser explotables.
Para las muestras de sedimentos fluviales se ha analizado, mediante microsonda electrónica, la composición de los granos de cromita, en primer lugar, para identificar el área fuente de los MGP y, en segundo lugar, para determinar en qué campo composicional se halla la cromita
Existence of global strong solutions to a beam-fluid interaction system
We study an unsteady non linear fluid-structure interaction problem which is
a simplified model to describe blood flow through viscoleastic arteries. We
consider a Newtonian incompressible two-dimensional flow described by the
Navier-Stokes equations set in an unknown domain depending on the displacement
of a structure, which itself satisfies a linear viscoelastic beam equation. The
fluid and the structure are fully coupled via interface conditions prescribing
the continuity of the velocities at the fluid-structure interface and the
action-reaction principle. We prove that strong solutions to this problem are
global-in-time. We obtain in particular that contact between the viscoleastic
wall and the bottom of the fluid cavity does not occur in finite time. To our
knowledge, this is the first occurrence of a no-contact result, but also of
existence of strong solutions globally in time, in the frame of interactions
between a viscous fluid and a deformable structure
Well-posedness of boundary layer equations for time-dependent flow of non-Newtonian fluids
We consider the flow of an upper convected Maxwell fluid in the limit of high
Weissenberg and Reynolds number. In this limit, the no-slip condition cannot be
imposed on the solutions. We derive equations for the resulting boundary layer
and prove the well-posedness of these equations. A transformation to Lagrangian
coordinates is crucial in the argument
Viscous-Inviscid Interactions in a Boundary-Layer Flow Induced by a Vortex Array
In this paper we investigate the asymptotic validity of boundary layer
theory. For a flow induced by a periodic row of point-vortices, we compare
Prandtl's solution to Navier-Stokes solutions at different numbers. We
show how Prandtl's solution develops a finite time separation singularity. On
the other hand Navier-Stokes solution is characterized by the presence of two
kinds of viscous-inviscid interactions between the boundary layer and the outer
flow. These interactions can be detected by the analysis of the enstrophy and
of the pressure gradient on the wall. Moreover we apply the complex singularity
tracking method to Prandtl and Navier-Stokes solutions and analyze the previous
interactions from a different perspective
On discretization in time in simulations of particulate flows
We propose a time discretization scheme for a class of ordinary differential
equations arising in simulations of fluid/particle flows. The scheme is
intended to work robustly in the lubrication regime when the distance between
two particles immersed in the fluid or between a particle and the wall tends to
zero. The idea consists in introducing a small threshold for the particle-wall
distance below which the real trajectory of the particle is replaced by an
approximated one where the distance is kept equal to the threshold value. The
error of this approximation is estimated both theoretically and by numerical
experiments. Our time marching scheme can be easily incorporated into a full
simulation method where the velocity of the fluid is obtained by a numerical
solution to Stokes or Navier-Stokes equations. We also provide a derivation of
the asymptotic expansion for the lubrication force (used in our numerical
experiments) acting on a disk immersed in a Newtonian fluid and approaching the
wall. The method of this derivation is new and can be easily adapted to other
cases
Time scales separation for dynamo action
The study of dynamo action in astrophysical objects classically involves two
timescales: the slow diffusive one and the fast advective one. We investigate
the possibility of field amplification on an intermediate timescale associated
with time dependent modulations of the flow. We consider a simple steady
configuration for which dynamo action is not realised. We study the effect of
time dependent perturbations of the flow. We show that some vanishing low
frequency perturbations can yield exponential growth of the magnetic field on
the typical time scale of oscillation. The dynamo mechanism relies here on a
parametric instability associated with transient amplification by shear flows.
Consequences on natural dynamos are discussed
Direct mass measurements of 19B, 22C, 29F, 31Ne, 34Na and other light exotic nuclei
We report on direct time-of-flight based mass measurements of 16 light
neutron-rich nuclei. These include the first determination of the masses of the
Borromean drip-line nuclei B, C and F as well as that of
Na. In addition, the most precise determinations to date for N
and Ne are reported. Coupled with recent interaction cross-section
measurements, the present results support the occurrence of a two-neutron halo
in C, with a dominant configuration, and a
single-neutron halo in Ne with the valence neutron occupying
predominantly the 2 orbital. Despite a very low two-neutron separation
energy the development of a halo in B is hindered by the 1
character of the valence neutrons.Comment: 5 page
The Navier wall law at a boundary with random roughness
We consider the Navier-Stokes equation in a domain with irregular boundaries.
The irregularity is modeled by a spatially homogeneous random process, with
typical size \eps \ll 1. In a parent paper, we derived a homogenized boundary
condition of Navier type as \eps \to 0. We show here that for a large class
of boundaries, this Navier condition provides a O(\eps^{3/2} |\ln
\eps|^{1/2}) approximation in , instead of O(\eps^{3/2}) for periodic
irregularities. Our result relies on the study of an auxiliary boundary layer
system. Decay properties of this boundary layer are deduced from a central
limit theorem for dependent variables
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