1,475 research outputs found
Electron inertia and quasi-neutrality in the Weibel instability
While electron kinetic effects are well known to be of fundamental importance
in several situations, the electron mean-flow inertia is often neglected when
lengthscales below the electron skin depth become irrelevant. This has led to
the formulation of different reduced models, where electron inertia terms are
discarded while retaining some or all kinetic effects. Upon considering general
full-orbit particle trajectories, this paper compares the dispersion relations
emerging from such models in the case of the Weibel instability. As a result,
the question of how lengthscales below the electron skin depth can be neglected
in a kinetic treatment emerges as an unsolved problem, since all current
theories suffer from drawbacks of different nature. Alternatively, we discuss
fully kinetic theories that remove all these drawbacks by restricting to
frequencies well below the plasma frequency of both ions and electrons. By
giving up on the lengthscale restrictions appearing in previous works, these
models are obtained by assuming quasi-neutrality in the full Maxwell-Vlasov
system.Comment: 25pages; 7 figures. Submitted to J. Plasma Phys. Special issue
contribution, on the occasion of the Vlasovia 2016 conferenc
The dissipation of solar wind turbulent fluctuations at electron scales
We present two-dimensional fully-kinetic Particle-in-Cell simulations of
decaying electromagnetic fluctuations. The computational box is such that
wavelengths ranging from electron to ion gyroradii are resolved. The parameters
used are realistic for the solar wind, and the ion to electron mass ratio is
physical. The understanding of the dissipation of turbulent fluctuations at
small scales is thought to be a crucial mechanism for solar wind acceleration
and coronal heating. The computational results suggest that a power law cascade
of magnetic fluctuations could be sustained up to scales of the electron Larmor
radius and smaller. We analyse the simulation results in the light of the
Vlasov linear theory, and we comment on the particle heating. The dispersion
curves of lightly damped modes in this regime suggest that a linear mechanism
could be responsible for the observed steepening of power spectra at electron
scales, but a straightforward identification of turbulent fluctuations as an
ensemble of linear modes is not possibleComment: to appear in Astrophysical Journa
Transient growth in stable collisionless plasma
The first kinetic study of transient growth for a collisionless homogeneous
Maxwellian plasma in a uniform magnetic field is presented. A system which is
linearly stable may display transient growth if the linear operator describing
its evolution is non-normal, so that its eigenvectors are non-orthogonal. In
order to include plasma kinetic effects a Landau fluid model is employed. The
linear operator of the model is shown to be non-normal and the results suggest
that the nonnormality of a collisionless plasma is intrinsically related to its
kinetic nature, with the transient growth being more accentuated for smaller
scales and higher plasma beta. The results based on linear spectral theory have
been confirmed with nonlinear simulations.Comment: accepted as a Letter in Physics of Plasma
Impact of atrial fibrillation on the cardiovascular system through a lumped-parameter approach
Atrial fibrillation (AF) is the most common arrhythmia affecting millions of
people in the Western countries and, due to the widespread impact on the
population and its medical relevance, is largely investigated in both clinical
and bioengineering sciences. However, some important feedback mechanisms are
still not clearly established. The present study aims at understanding the
global response of the cardiovascular system during paroxysmal AF through a
lumped-parameter approach, which is here performed paying particular attention
to the stochastic modeling of the irregular heartbeats and the reduced
contractility of the heart. AF can be here analyzed by means of a wide number
of hemodynamic parameters and avoiding the presence of other pathologies, which
usually accompany AF. Reduced cardiac output with correlated drop of ejection
fraction and decreased amount of energy converted to work by the heart during
blood pumping, as well as higher left atrial volumes and pressures are some of
the most representative results aligned with the existing clinical literature
and here emerging during acute AF. The present modeling, providing new insights
on cardiovascular variables which are difficult to measure and rarely reported
in literature, turns out to be an efficient and powerful tool for a deeper
comprehension and prediction of the arrythmia impact on the whole
cardiovascular system.Comment: 16 pages, 8 figures, 2 tables, Medical & Biological Engineering &
Computing, 2014, Print ISSN: 0140-0118, Online ISSN: 1741-044
Process and plastic transformation. Contemporary architecture in mediterranean historical centers
Ponència presentada a: Session 5: Del diseño a la construcción. Sostenibilidad (temas de diseño ambiental) / From design to construction. Sustainability (environmental design issues
Process and plastic transformation. Contemporary architecture in mediterranean historical centers
Ponència presentada a la sessió 5Historical urban fabrics of Mediterranean cities have well‐defined characters: they are a temporary synthesis of
continuous and physical transformations, more or less homogeneous, recognizable and readable through the study of
stratification and fusions that characterized their typical formative process, in relation with masonry material they are
composed/constructed. Their conservation results positively effective if paradoxically they continue to transform themselves.
How to make such transformations coherent, constructively and expressively, compared to the inherited reality they fit into? Through a certain kind of typological studies, it was possible to read and codify the type of process that has led to the formation, often only apparently complex, of historical centers that we are now protecting against any type of intervention.
This work‐in‐progress paper intends to reflect on the coherent and processual use of reinforced concrete in the transformation process of historic centers. A material that has undergone a formative process that was concluded in its full and complete ‘elastic’ development, born and realized in the technologically‐advanced areas of central and northern Europe, which ended up invading Mediterranean areas. The same material possesses a ‘plastic’ potential that, if architecturally well‐used, makes coherent, necessary and congruent, interventions for the continuity of the life of such historical centers. Spanish ‘plastic’ contemporary architecture could represents, in my opinion, an interesting example to study
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