4,005 research outputs found
Time irreversibility in reversible shell models of turbulence
Turbulent flows governed by the Navier-Stokes equations (NSE) generate an
out-of-equilibrium time irreversible energy cascade from large to small scales.
In the NSE, the energy transfer is due to the nonlinear terms that are formally
symmetric under time reversal. As for the dissipative term: first it explicitly
breaks time reversibility; second it produces a small-scale sink for the energy
transfer that remains effective even in the limit of vanishing viscosity. As a
result, it is not clear how to disentangle the time irreversibility originating
from the non-equilibrium energy cascade from the explicit time-reversal
symmetry breaking due to the viscous term. To this aim, in this paper we
investigate the properties of the energy transfer in turbulent Shell models by
using a reversible viscous mechanism, avoiding any explicit breaking of the symmetry. We probe time-irreversibility by studying the
statistics of Lagrangian power, which is found to be asymmetric under time
reversal also in the time-reversible model. This suggests that the turbulent
dynamics converges to a strange attractor where time-reversibility is
spontaneously broken and whose properties are robust for what concerns purely
inertial degrees of freedoms, as verified by the anomalous scaling behavior of
the velocity structure functions.Comment: 13 pages, 7 figure
Equivalence of nonequilibrium ensembles in turbulence models
Understanding under which conditions it is possible to construct equivalent ensembles is key to advancing our ability to connect microscopic and macroscopic properties of non-equilibrium statisti- cal mechanics. In the case of fluid dynamical systems, a first issue is to test whether different models for viscosity lead to the same macroscopic properties of the fluid systems in different regimes. Such models include, besides the standard choice of constant viscosity, also cases where the time symme- try of the evolution equations is exactly preserved, as it must be in the corresponding microscopic systems, when available. Here a time-reversible dynamics is obtained by imposing the conservation of global observables. We test the equivalence of reversible and irreversible ensembles for the case of a multiscale shell model of turbulence. We verify that the equivalence is obeyed for the mean-values of macroscopic observables, up to an error that vanishes as the system becomes more and more chaotic
Nasal histamine responses in nonallergic rhinitis with eosinophilic syndrome
Background: Nonallergic rhinitis with eosinophilic syndrome (NARES) is persistent, without atopy, but with ≥25% nasal eosinophilia. Hypereosinophilia seems to contribute to nasal mucosa dysfunction.
Objectives: This analytical case-control study aimed at assessing the presence and severity of nonspecific nasal hyperactivity and at finding out whether eosinophilia may be correlated with the respiratory and mucociliary clearance functions.
Materials: The symptom score was assessed in 38 patients and 15 controls whose nasal smear was also tested for eosinophils and mucociliary transport (MCT). Nonspecific nasal provocation tests (NSNPT) with histamine were also carried out, and total nasal resistance (TNR) was determined.
Results: The symptom score of NARES after NSNPT were not significantly different from the control group, and there was poor or no correlation among the single symptoms and the differences studied for every nasal reactivity class. This correlation improved when using the composite symptom score. The most severe eosinophilia was observed in high reactivity groups, and it was correlated with an increase in TNR. MCT worsened as eosinophilia and nasal reactivity increased. Unlike controls, a significant correlation was observed between the increase in MCT and TNR.
Conclusions: In NARES, nonspecific nasal hyperreactivity is the result of epithelial damage produced by eosinophilic inflammation, which causes MCT slow down, an increase in TNR, and nasal reactivity classes, with possible impact on classification, prognosis, and treatment control
Clustering of vertically constrained passive particles in homogeneous, isotropic turbulence
We analyze the dynamics of small particles vertically confined, by means of a
linear restoring force, to move within a horizontal fluid slab in a
three-dimensional (3D) homogeneous isotropic turbulent velocity field. The
model that we introduce and study is possibly the simplest description for the
dynamics of small aquatic organisms that, due to swimming, active regulation of
their buoyancy, or any other mechanism, maintain themselves in a shallow
horizontal layer below the free surface of oceans or lakes. By varying the
strength of the restoring force, we are able to control the thickness of the
fluid slab in which the particles can move. This allows us to analyze the
statistical features of the system over a wide range of conditions going from a
fully 3D incompressible flow (corresponding to the case of no confinement) to
the extremely confined case corresponding to a two-dimensional slice. The
background 3D turbulent velocity field is evolved by means of fully resolved
direct numerical simulations. Whenever some level of vertical confinement is
present, the particle trajectories deviate from that of fluid tracers and the
particles experience an effectively compressible velocity field. Here, we have
quantified the compressibility, the preferential concentration of the
particles, and the correlation dimension by changing the strength of the
restoring force. The main result is that there exists a particular value of the
force constant, corresponding to a mean slab depth approximately equal to a few
times the Kolmogorov length scale, that maximizes the clustering of the
particles
The strength of crystalline color superconductors
We present a study of the shear modulus of the crystalline color
superconducting phase of quark matter, showing that this phase of dense, but
not asymptotically dense, quark matter responds to shear stress as a very rigid
solid. This phase is characterized by a gap parameter that is
periodically modulated in space and therefore spontaneously breaks
translational invariance. We derive the effective action for the phonon fields
that describe space- and time-dependent fluctuations of the crystal structure
formed by , and obtain the shear modulus from the coefficients of the
spatial derivative terms. Within a Ginzburg-Landau approximation, we find shear
moduli which are 20 to 1000 times larger than those of neutron star crusts.
This phase of matter is thus more rigid than any known material in the
universe, but at the same time the crystalline color superconducting phase is
also superfluid. These properties raise the possibility that the presence of
this phase within neutron stars may have distinct implications for their
phenomenology. For example, (some) pulsar glitches may originate in crystalline
superconducting neutron star cores.Comment: 8 pages, proceedings of QCD@work 2007 International Workshop on QCD
Theory and Experiment- Martina Franca (Italy), June 200
Effect of trees on street canyon ventilation
Due to the overall growth of the world population and to the progressive shift from rural to urban centres, 70% of the world population is expected to live in urban areas in 2050. This trend is alarming when related to the constant decline of urban air quality at the global level. To cope with this rapid urbanization, solutions for sustainable cities are extensively sought. In this framework, the mitigation of air pollution in street canyons plays a crucial role. The street canyon (a street flanked by high buildings on both sides) is the fundamental unit of the urban tissue, as well as a vital public and residential space. Street canyons are particularly vulnerable to air pollution due to traffic emissions, low ventilation conditions, and the number of citizens exposed. Tree planting in street canyons is often used as a pollution mitigation strategy, due to the filtering effect of vegetation on airborne pollutants. However, from the aerodynamic point of view, trees can obstruct the wind flow thus reducing canyon ventilation and leading to higher pollutant concentrations. In this framework, we present the results of an experimental study aimed at evaluating how tree planting influences the flow and concentration fields within a street canyon. The study was carried out in a recirculating wind tunnel. An idealised urban district was simulated by an array of square blocks, whose orientation with respect to the incident wind was varied. Within this urban geometry, two rows of model trees were arranged at the sides of a street canyon. Three configurations with different spacing between the trees were considered. A passive scalar was injected from a line source placed at ground level to simulate traffic emissions. Concentration and flow field measurements were performed in several cross-sections of the street canyon. Results showed the effect of trees on the spatial distribution of pollutants. Moreover, a characteristic exchange velocity between the street canyon and the overlying atmosphere was estimated to quantify the overall canyon ventilation under several wind directions and different planting densities. These preliminary results provide city planners with first recommendations for the sustainable design of urban environments. Moreover, the experimental dataset is valuable in validating numerical simulations of air pollution in cities accounting for urban vegetation
Implantation of a poly-L-lactide GCSF-functionalized scaffold in a model of chronic myocardial infarction
A previously developed poly-l-lactide scaffold releasing granulocyte colony-stimulating factor (PLLA/GCSF) was tested in a rabbit chronic model of myocardial infarction (MI) as a ventricular patch. Control groups were constituted by healthy, chronic MI and nonfunctionalized PLLA scaffold. PLLA-based electrospun scaffold efficiently integrated into a chronic infarcted myocardium. Functionalization of the biopolymer with GCSF led to increased fibroblast-like vimentin-positive cellular colonization and reduced inflammatory cell infiltration within the micrometric fiber mesh in comparison to nonfunctionalized scaffold; PLLA/GCSF polymer induced an angiogenetic process with a statistically significant increase in the number of neovessels compared to the nonfunctionalized scaffold; PLLA/GCSF implanted at the infarcted zone induced a reorganization of the ECM architecture leading to connective tissue deposition and scar remodeling. These findings were coupled with a reduction in end-systolic and end-diastolic volumes, indicating a preventive effect of the scaffold on ventricular dilation, and an improvement in cardiac performance
Nasal, pharyngeal and laryngeal endoscopy proceduresduring COVID‐19 pandemic: available recommendations from national and international societies
The Severe Acute Respiratory Syndrome-CoronaVirus-2 (SARS-CoV-2), also known as COVID-19, pandemic is engaging clinicians around the world in an unprecedented effort to limit the viral spread and treat affected patients; the rapid diffusion of the disease represents a risk for healthcare providers who have a close contact with the upper aerodiges- tive tract during medical, diagnostic and surgical procedure
- …