8,578 research outputs found
Turing pattern formation in the Brusselator system with nonlinear diffusion
In this work we investigate the effect of density dependent nonlinear
diffusion on pattern formation in the Brusselator system. Through linear
stability analysis of the basic solution we determine the Turing and the
oscillatory instability boundaries. A comparison with the classical linear
diffusion shows how nonlinear diffusion favors the occurrence of Turing pattern
formation. We study the process of pattern formation both in 1D and 2D spatial
domains. Through a weakly nonlinear multiple scales analysis we derive the
equations for the amplitude of the stationary patterns. The analysis of the
amplitude equations shows the occurrence of a number of different phenomena,
including stable supercritical and subcritical Turing patterns with multiple
branches of stable solutions leading to hysteresis. Moreover we consider
traveling patterning waves: when the domain size is large, the pattern forms
sequentially and traveling wavefronts are the precursors to patterning. We
derive the Ginzburg-Landau equation and describe the traveling front enveloping
a pattern which invades the domain. We show the emergence of radially symmetric
target patterns, and through a matching procedure we construct the outer
amplitude equation and the inner core solution.Comment: Physical Review E, 201
Cardiovascular autonomic function and MCI in Parkinson's disease
Introduction: dysautonomic dysfunction and cognitive impairment represent the most disabling non-motor features of Parkinson's Disease (PD). Recent evidences suggest the association between Orthostatic Hypotension (OH) and PD-Dementia. However, little is known on the interactions between cardiovascular dysautonomia and Mild Cognitive Impairment (MCI). We aimed to evaluate the association between cardiovascular dysautonomia and MCI in patients with PD. Methods: non-demented PD patients belonging to the PACOS cohort underwent a comprehensive instrumental neurovegetative assessment including the study of both parasympathetic and sympathetic function (30:15 ratio, Expiratory-Inspiratory ratio [E-I] and presence of Orthostatic Hypotension [OH]). Diagnosis of MCI was made according to the MDS criteria level II. Results: we enrolled 185 PD patients of whom 102 (55.1%) were men, mean age was 64.6 ± 9.7 years, mean disease duration of 5.6 ± 5.5 years with a mean UPDRS-ME score of 31.7 ± 10.9. MCI was diagnosed in 79 (42.7%) patients. OH was recorded in 52 (28.1%) patients, altered 30:15 ratio was recorded in 39 (24.1%) patients and an altered E-I ratio was found in 24 (19.1%) patients. Presence of MCI was associated with an altered 30:15 ratio (adjOR 2.83; 95%CI 1.25â6.40) but not with an altered E-I ratio, while OH was associated only with the amnestic MCI subgroup (OR 2.43; 95% CI 1.05â5.06). Conclusion: in our study sample, MCI was mainly associated with parasympathetic dysfunction in PD
The Global sphere reconstruction (GSR) - Demonstrating an independent implementation of the astrometric core solution for Gaia
Context. The Gaia ESA mission will estimate the astrometric and physical data
of more than one billion objects, providing the largest and most precise
catalog of absolute astrometry in the history of Astronomy. The core of this
process, the so-called global sphere reconstruction, is represented by the
reduction of a subset of these objects which will be used to define the
celestial reference frame. As the Hipparcos mission showed, and as is inherent
to all kinds of absolute measurements, possible errors in the data reduction
can hardly be identified from the catalog, thus potentially introducing
systematic errors in all derived work. Aims. Following up on the lessons
learned from Hipparcos, our aim is thus to develop an independent sphere
reconstruction method that contributes to guarantee the quality of the
astrometric results without fully reproducing the main processing chain.
Methods. Indeed, given the unfeasibility of a complete replica of the data
reduction pipeline, an astrometric verification unit (AVU) was instituted by
the Gaia Data Processing and Analysis Consortium (DPAC). One of its jobs is to
implement and operate an independent global sphere reconstruction (GSR),
parallel to the baseline one (AGIS, namely Astrometric Global Iterative
Solution) but limited to the primary stars and for validation purposes, to
compare the two results, and to report on any significant differences. Results.
Tests performed on simulated data show that GSR is able to reproduce at the
sub-as level the results of the AGIS demonstration run presented in
Lindegren et al. (2012). Conclusions. Further development is ongoing to improve
on the treatment of real data and on the software modules that compare the AGIS
and GSR solutions to identify possible discrepancies above the tolerance level
set by the accuracy of the Gaia catalog.Comment: Accepted for publication on Astronomy & Astrophysic
Radiation hardened transistor characteristics for applications at LHC and beyond
The high radiation environment at the LHC will require the use of radiation hardened microelectronics for the readout of inner detectors. Two such technologies are a Harris bulk CMOS process and the DMILL mixed technology process. Transistors have been fabricated in both of these and have been tested before and after irradiation to 10 Mrads, the total dose expected in the innermost silicon microstrip layers. Several processing runs of Harris transistors have been carried out and samples from one have also been irradiated to 100 Mrads. A preamplifier-shaper circuit, to be used for readout of the CMS microstrip tracker, has been tested and the noise performance is compared with individual transistors
Route to chaos in the weakly stratified Kolmogorov flow
We consider a two-dimensional fluid exposed to Kolmogorovâs forcing cos(ny) and heated from above. The stabilizing effects of temperature are taken into account using the Boussinesq approximation. The fluid with no temperature stratification has been widely studied and, although relying on strong simplifications, it is considered an important tool for the theoretical and experimental study of transition to turbulence. In this paper, we are interested in the set of transitions leading the temperature stratified fluid from the laminar solution [Uâcos(ny),0, T â y] to more complex states until the onset of chaotic states. We will consider Reynolds numbers 0 < Re †30, while the Richardson numbers shall be kept in the regime of weak stratifications (Ri †5 Ă 10 â3 ). We shall first review the non-stratified Kolmogorov flow and find a new period-tripling bifurcation as the precursor of chaotic states. Introducing the stabilizing temperature gradient, we shall observe that higher Re are required to trigger instabilities. More importantly, we shall see new states and phenomena: the newly discovered period-tripling bifurcation is supercritical or subcritical according to Ri; more period-tripling and doubling bifurcations may depart from this new state; strong enough stratifications trigger new regions of chaotic solutions and, on the drifting solution branch, non-chaotic bursting solutions
Hydrodynamical Models of Superfluid Turbulence
This review paper puts together some of our results concerning the application of non equilibrium Thermodynamics to superfluid liquid helium. Two of the most important situations of this quantum fluid are rotating superfluid and superfluid turbulence, both characterized by the presence of quantized vortices (vortex lines
whose core is about 1 Angstrom and the quantum of circulation is , being the Plank's constant and the mass of helium atom). In the first part of
the work a non-standard model of superfluid helium, which considers heat flux as independent variable, is briefly recalled,
and compared with the well known two-fluid model, in absence of vortices, proposed by Tisza and Landau more than half a century
ago. The model is generalized taking into account the presence of vortices in different cases of physical interest: rotating superfluids, counterflow superfluid turbulence (a particular situation in which no mass flux but only heat flux is present) and combined situations of counterflow and rotation. Since vortices are not fixed when all the hydrodynamical fields change, an additional scalar quantity, the averaged vortex line density per unit volume , {\it line density} for short, is introduced in the model as a new field variable and an evolution equation is written for it, both in linear and in nonlinear regimes, via Extended Thermodynamics. Finally, to encompass more general situations, the
model is further extended considering the flux of vortex line density as an independent new variable. In all these models the propagation of harmonic waves is studied, motivated by the fact that vortex lines density is experimentally detected via the attenuation of second sound. A new kind of waves, vortex density waves, is also dealt with
Usefulness of regional right ventricular and right atrial strain for prediction of early and late right ventricular failure following a left ventricular assist device implant: A machine learning approach
Background: Identifying candidates for left ventricular assist device surgery at risk of right ventricular failure remains difficult. The aim was to identify the most accurate predictors of right ventricular failure among clinical, biological, and imaging markers, assessed by agreement of different supervised machine learning algorithms. Methods: Seventy-four patients, referred to HeartWare left ventricular assist device since 2010 in two Italian centers, were recruited. Biomarkers, right ventricular standard, and strain echocardiography, as well as cath-lab measures, were compared among patients who did not develop right ventricular failure (N = 56), those with acuteâright ventricular failure (N = 8, 11%) or chronicâright ventricular failure (N = 10, 14%). Logistic regression, penalized logistic regression, linear support vector machines, and naĂŻve Bayes algorithms with leave-one-out validation were used to evaluate the efficiency of any combination of three collected variables in an âall-subsetsâ approach. Results: Michigan risk score combined with central venous pressure assessed invasively and apical longitudinal systolic strain of the right ventricularâfree wall were the most significant predictors of acuteâright ventricular failure (maximum receiver operating characteristicâarea under the curve = 0.95, 95% confidence interval = 0.91â1.00, by the naĂŻve Bayes), while the right ventricularâfree wall systolic strain of the middle segment, right atrial strain (QRS-synced), and tricuspid annular plane systolic excursion were the most significant predictors of Chronic-RVF (receiver operating characteristicâarea under the curve = 0.97, 95% confidence interval = 0.91â1.00, according to naĂŻve Bayes). Conclusion: Apical right ventricular strain as well as right atrial strain provides complementary information, both critical to predict acuteâright ventricular failure and chronicâright ventricular failure, respectively
Rilevamento delle sorgenti doppler della ionosfera tramite radar-HF
Il presente lavoro contiene un insieme di concetti di base utili per comprendere la cosiddetta tecnica della âinterferometria dopplerâ, usata, nellâambito degli studi sulla ionosfera, per ricavare informazioni sulla conformazione e velocitĂ degli strati riflettenti (operazione chiamata anche sky mapping). Ă utile ricordare, infatti, che il sondaggio ionosferico tradizionale consente solo la determinazione dellâaltezza virtuale di ogni strato, pensato come un unico oggetto riflettente piano. Tale determinazione Ăš giunta nel tempo ad avere una risoluzione molto spinta, dellâordine di qualche chilometro, tuttavia una ionosonda tradizionale non possiede la capacitĂ di individuare la struttura degli strati riflettenti
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