833 research outputs found
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
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
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
Performance characterization and near-realtime monitoring of MUSE adaptive optics modes at Paranal
The Multi Unit Spectroscopic Explorer (MUSE) is an integral field
spectrograph on the Very Large Telescope Unit Telescope 4, capable of laser
guide star assisted and tomographic adaptive optics using the GALACSI module.
Its observing capabilities include a wide field (1 square arcmin), ground layer
AO mode (WFM-AO) and a narrow field (7.5"x7.5"), laser tomography AO mode
(NFM-AO). The latter has had several upgrades in the 4 years since
commissioning, including an optimisation of the control matrices for the AO
system and a new sub-electron noise detector for its infra-red low order
wavefront sensor. We set out to quantify the NFM-AO system performance by
analysing 230 spectrophotometric standard star observations taken over
the last 3 years. To this end we expand upon previous work, designed to
facilitate analysis of the WFM-AO system performance. We briefly describe the
framework that will provide a user friendly, semi-automated way for system
performance monitoring during science operations. We provide the results of our
performance analysis, chiefly through the measured Strehl ratio and full width
at half maximum (FWHM) of the core of the point spread function (PSF) using two
PSF models, and correlations with atmospheric conditions. These results will
feed into a range of applications, including providing a more accurate
prediction of the system performance as implemented in the exposure time
calculator, and the associated optimization of the scientific output for a
given set of limiting atmospheric conditions.Comment: SPIE proceedings (2022), Observatory Operations: Strategies,
Processes, and Systems I
The dynamics of thin vibrated granular layers
We describe a series of experiments and computer simulations on vibrated
granular media in a geometry chosen to eliminate gravitationally induced
settling. The system consists of a collection of identical spherical particles
on a horizontal plate vibrating vertically, with or without a confining lid.
Previously reported results are reviewed, including the observation of
homogeneous, disordered liquid-like states, an instability to a `collapse' of
motionless spheres on a perfect hexagonal lattice, and a fluctuating,
hexagonally ordered state. In the presence of a confining lid we see a variety
of solid phases at high densities and relatively high vibration amplitudes,
several of which are reported for the first time in this article. The phase
behavior of the system is closely related to that observed in confined
hard-sphere colloidal suspensions in equilibrium, but with modifications due to
the effects of the forcing and dissipation. We also review measurements of
velocity distributions, which range from Maxwellian to strongly non-Maxwellian
depending on the experimental parameter values. We describe measurements of
spatial velocity correlations that show a clear dependence on the mechanism of
energy injection. We also report new measurements of the velocity
autocorrelation function in the granular layer and show that increased
inelasticity leads to enhanced particle self-diffusion.Comment: 11 pages, 7 figure
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