82 research outputs found
Influence of correlations on the velocity statistics of scalar granular gases
The free evolution of inelastic particles in one dimension is studied by
means of Molecular Dynamics (MD), of an inelastic pseudo-Maxwell model and of a
lattice model, with emphasis on the role of spatial correlations. We present an
exact solution of the 1d granular pseudo-Maxwell model for the scaling
distribution of velocities and discuss how this model fails to describe
correctly the homogeneous cooling stage of the 1d granular gas. Embedding the
pseudo-Maxwell gas on a lattice (hence allowing for the onset of spatial
correlations), we find a much better agreement with the MD simulations even in
the inhomogeneous regime. This is seen by comparing the velocity distributions,
the velocity profiles and the structure factors of the velocity field.Comment: Latex file: 6 pages, 5 figures (.eps). See also
http://axtnt3.phys.uniroma1.it/Maxwel
Measurement report: Receptor modeling for source identification of urban fine and coarse particulate matter using hourly elemental composition
The elemental composition of the fine (PM2.5) and coarse
(PM2.5−10) fraction of atmospheric particulate matter was measured at an hourly time resolution by the use of a streaker sampler during a winter
period at a Central European urban background site in Warsaw, Poland. A
combination of multivariate (Positive Matrix Factorization) and wind-
(Conditional Probability Function) and trajectory-based (Cluster Analysis)
receptor models was applied for source apportionment. It allowed for the
identification of five similar sources in both fractions, including sulfates, soil dust, road salt, and traffic- and industry-related sources. Another two sources, i.e., Cl-rich and wood and coal combustion, were solely identified in the fine fraction. In the fine fraction, aged sulfate aerosol related to emissions from domestic solid fuel combustion in the outskirts of the city was the largest contributing source to fine elemental mass (44 %), while traffic-related sources, including soil dust mixed with road dust, road dust, and traffic emissions, had the biggest contribution to the coarse elemental mass (together accounting for 83 %). Regional transport of aged aerosols and more local impact of the rest of the identified sources played a crucial role in aerosol formation over the city. In addition, two intensive Saharan dust outbreaks were registered on 18 February and 8 March 2016. Both episodes were characterized by the long-range transport of dust at 1500 and 3000 m over Warsaw and the concentrations of the soil component being 7 (up to 3.5 µg m−3) and 6 (up to 6.1 µg m−3) times higher than the mean concentrations observed during non-episodes days (0.5 and 1.1 µg m−3) in the fine and coarse fractions, respectively. The set of receptor models applied to the high time resolution data allowed us to follow, in detail, the daily evolution of the aerosol elemental composition and to identify distinct sources contributing to the concentrations of the different PM fractions, and it revealed the multi-faceted nature of some elements with diverse origins in the fine and coarse fractions. The hourly resolution of meteorological conditions and air mass back trajectories allowed us to follow the transport pathways of the aerosol as well.</p
Velocity Correlations in Driven Two-Dimensional Granular Media
Simulations of volumetrically forced granular media in two dimensions produce
s tates with nearly homogeneous density. In these states, long-range velocity
correlations with a characteristic vortex structure develop; given sufficient
time, the correlations fill the entire simulated area. These velocity
correlations reduce the rate and violence of collisions, so that pressure is
smaller for driven inelastic particles than for undriven elastic particles in
the same thermodynamic state. As the simulation box size increases, the effects
of veloc ity correlations on the pressure are enhanced rather than reduced.Comment: 12 pages, 6 figures, 21 reference
Steady state properties of a driven granular medium
We study a two-dimensional granular system where external driving force is
applied to each particle in the system in such a way that the system is driven
into a steady state by balancing the energy input and the dissipation due to
inelastic collision between particles. The velocities of the particles in the
steady state satisfy the Maxwellian distribution. We measure the
density-density correlation and the velocity-velocity correlation functions in
the steady state and find that they are of power-law scaling forms. The
locations of collision events are observed to be time-correlated and such a
correlation is described by another power-law form. We also find that the
dissipated energy obeys a power-law distribution. These results indicate that
the system evolves into a critical state where there are neither characteristic
spatial nor temporal scales in the correlation functions. A test particle
exhibits an anomalous diffusion which is apparently similar to the Richardson
law in a three-dimensional turbulent flow.Comment: REVTEX, submitted to Phys. Rev.
Transport Coefficients for Granular Media from Molecular Dynamics Simulations
Under many conditions, macroscopic grains flow like a fluid; kinetic theory
pred icts continuum equations of motion for this granular fluid. In order to
test the theory, we perform event driven molecular simulations of a
two-dimensional gas of inelastic hard disks, driven by contact with a heat
bath. Even for strong dissipation, high densities, and small numbers of
particles, we find that continuum theory describes the system well. With a bath
that heats the gas homogeneously, strong velocity correlations produce a
slightly smaller energy loss due to inelastic collisions than that predicted by
kinetic theory. With an inhomogeneous heat bath, thermal or velocity gradients
are induced. Determination of the resulting fluxes allows calculation of the
thermal conductivity and shear viscosity, which are compared to the predictions
of granular kinetic theory, and which can be used in continuum modeling of
granular flows. The shear viscosity is close to the prediction of kinetic
theory, while the thermal conductivity can be overestimated by a factor of 2;
in each case, transport is lowered with increasing inelasticity.Comment: 14 pages, 17 figures, 39 references, submitted to PRE feb 199
Removing the societal and legal impediments to the HIV response: An evidence-based framework for 2025 and beyond.
Societal and legal impediments inhibit quality HIV prevention, care, treatment and support services and need to be removed. The political declaration adopted by UN member countries at the high-level meeting on HIV and AIDS in June 2021, included new societal enabler global targets for achievement by 2025 that will address this gap. Our paper describes how and why UNAIDS arrived at the societal enabler targets adopted. We conducted a scoping review and led a participatory process between January 2019 and June 2020 to develop an evidence-based framework for action, propose global societal enabler targets, and identify indicators for monitoring progress. A re-envisioned framework called the '3 S's of the HIV response: Society, Systems and Services' was defined. In the framework, societal enablers enhance the effectiveness of HIV programmes by removing impediments to service availability, access and uptake at the societal level, while service and system enablers improve efficiencies in and expand the reach of HIV services and systems. Investments in societal enabling approaches that remove legal barriers, shift harmful social and gender norms, reduce inequalities and improve institutional and community structures are needed to progressively realize four overarching societal enablers, the first three of which fall within the purview of the HIV sector: (i) societies with supportive legal environments and access to justice, (ii) gender equal societies, (iii) societies free from stigma and discrimination, and (iv) co-action across development sectors to reduce exclusion and poverty. Three top-line and 15 detailed targets were recommended for monitoring progress towards their achievement. The clear articulation of societal enablers in the re-envisioned framework should have a substantial impact on improving the effectiveness of core HIV programmes if implemented. Together with the new global targets, the framework will also galvanize advocacy to scale up societal enabling approaches with proven impact on HIV outcomes
Long-term outcome of autistic spectrum disorder: a retrospective case study in a southern italian region
- …