Dispersion and collapse in stochastic velocity fields on a cylinder

The dynamics of fluid particles on cylindrical manifolds is investigated. The velocity field is obtained by generalizing the isotropic Kraichnan ensemble, and is therefore Gaussian and decorrelated in time. The degree of compressibility is such that when the radius of the cylinder tends to infinity the fluid particles separate in an explosive way. Nevertheless, when the radius is finite the transition probability of the two-particle separation converges to an invariant measure. This behavior is due to the large-scale compressibility generated by the compactification of one dimension of the space

Uniform shrinking and expansion under isotropic Brownian flows

We study some finite time transport properties of isotropic Brownian flows. Under a certain nondegeneracy condition on the potential spectral measure, we prove that uniform shrinking or expansion of balls under the flow over some bounded time interval can happen with positive probability. We also provide a control theorem for isotropic Brownian flows with drift. Finally, we apply the above results to show that under the nondegeneracy condition the length of a rectifiable curve evolving in an isotropic Brownian flow with strictly negative top Lyapunov exponent converges to zero as $t\to \infty$ with positive probability

In an Ising model with spin-exchange dynamics damage always spreads

We investigate the spreading of damage in Ising models with Kawasaki spin-exchange dynamics which conserves the magnetization. We first modify a recent master equation approach to account for dynamic rules involving more than a single site. We then derive an effective-field theory for damage spreading in Ising models with Kawasaki spin-exchange dynamics and solve it for a two-dimensional model on a honeycomb lattice. In contrast to the cases of Glauber or heat-bath dynamics, we find that the damage always spreads and never heals. In the long-time limit the average Hamming distance approaches that of two uncorrelated systems. These results are verified by Monte-Carlo simulations.Comment: 5 pages REVTeX, 4 EPS figures, final version as publishe

Ergodic properties of a model for turbulent dispersion of inertial particles

We study a simple stochastic differential equation that models the dispersion of close heavy particles moving in a turbulent flow. In one and two dimensions, the model is closely related to the one-dimensional stationary Schroedinger equation in a random delta-correlated potential. The ergodic properties of the dispersion process are investigated by proving that its generator is hypoelliptic and using control theory

Stopping power of hot QCD plasma

The partonic energy loss has been calculated taking both the hard and soft contributions for all the $2 \to 2$ processes, revealing the importance of the individual channels. Cancellation of the intermediate separation scale has been exhibited. Subtleties related to the identical final state partons have properly been taken into account. The estimated collisional loss is compared with its radiative counter part. We show that there exists a critical energy ($E_c$) below which the collisional loss is more than its radiative counterpart. In addition, we present closed form formulas for both the collision probabilities and the stopping power ($dE/dx$)Comment: revised version, section added, 9pages with 5 figure

Validité discriminante d'épreuves de dépistage de la dyslexie chez des enfants de CE2-CM1

La dyslexie est un trouble spécifique du langage écrit. Les recherches menées sur l'origine de la dyslexie ont conduit à de multiples hypothèses (i.e. hypothèse phonologique, hypothèse du traitement auditif temporel, hypothèse cérébelleux, etc.). Cette diversité des hypothèses a engendré des traitements multiples et variés et, aujourd'hui, les praticiens expriment le besoin d'un outil d'aide au diagnostic de la dyslexie prenant en compte l'ensemble des déficits de l'enfant afin de proposer une rééducation adaptée. Ce papier présente la conception d'un test préliminaire contenant les épreuves d'évaluation de la dyslexie les plus représentatives de la littérature et les premiers résultats concernant la validité discriminante de ce test préliminaire chez des enfants en âge scolaire (8-10 ans)

Stochastic Lagrangian Particle Approach to Fractal Navier-Stokes Equations

In this article we study the fractal Navier-Stokes equations by using stochastic Lagrangian particle path approach in Constantin and Iyer \cite{Co-Iy}. More precisely, a stochastic representation for the fractal Navier-Stokes equations is given in terms of stochastic differential equations driven by L\'evy processes. Basing on this representation, a self-contained proof for the existence of local unique solution for the fractal Navier-Stokes equation with initial data in \mW^{1,p} is provided, and in the case of two dimensions or large viscosity, the existence of global solution is also obtained. In order to obtain the global existence in any dimensions for large viscosity, the gradient estimates for L\'evy processes with time dependent and discontinuous drifts is proved.Comment: 19 page

Urinary Neutrophil Gelatinase-Associated Lipocalin Measured on Admission to the Intensive Care Unit Accurately Discriminates between Sustained and Transient Acute Kidney Injury in Adult Critically Ill Patients

Background: First we aimed to evaluate the ability of neutrophil gelatinase-associated lipocalin (NGAL) and cystatin-C (CyC) in plasma and urine to discriminate between sustained, transient and absent acute kidney injury (AKI), and second to evaluate their predictive performance for sustained AKI in adult intensive care unit (ICU) patients. Methods: A prospective cohort study of 700 patients was studied. Sample collection was performed over 8 time points starting on admission. Results: After exclusion 510 patients remained for the analysis. All biomarkers showed significant differentiation between sustained and no AKI at all time points (p ≤ 0.0002) except for urine CyC (uCyC) on admission (p = 0.06). Urine NGAL (uNGAL) was the only biomarker significantly differentiating sustained from transient AKI on ICU admission (p = 0.02). Individually, uNGAL performed better than the other biomarkers (area under the curves, AUC = 0.80, 95% confidence interval, CI = 0.72–0.88) for the prediction of sustained AKI. The combination with plasma NGAL (pNGAL) showed a nonsignificant improvement (AUC = 0.83, 95% CI = 0.75–0.91). The combination of individual markers with a model of clinical characteristics (MDRD eGFR, HCO3– and sepsis) did not improve its performance significantly. However, the integrated discrimination improvement showed significant improvement when uNGAL was added (p = 0.04). Conclusions: uNGAL measured on ICU admission differentiates patients with sustained AKI from transient or no-AKI patients. Combining biomarkers such as pNGAL, uNGAL and plasma CyC with clinical characteristics adds some value to the predictive model

Interacting Arrays of Steps and Lines in Random Media

The phase diagram of two interacting planar arrays of directed lines in random media is obtained by a renormalization group analysis. The results are discussed in the contexts of the roughening of reconstructed crystal surfaces, and the pinning of flux line arrays in layered superconductors. Among the findings are a glassy flat phase with disordered domain structures, a novel second-order phase transition with continuously varying critical exponents, and the generic disappearance of the glassy ``super-rough'' phases found previously for a single array.Comment: 4 pages, REVTEX 3.0, uses epsf,multicol, 3 .eps-figures, submitted to PR

Polymer transport in random flow

The dynamics of polymers in a random smooth flow is investigated in the framework of the Hookean dumbbell model. The analytical expression of the time-dependent probability density function of polymer elongation is derived explicitly for a Gaussian, rapidly changing flow. When polymers are in the coiled state the pdf reaches a stationary state characterized by power-law tails both for small and large arguments compared to the equilibrium length. The characteristic relaxation time is computed as a function of the Weissenberg number. In the stretched state the pdf is unstationary and exhibits multiscaling. Numerical simulations for the two-dimensional Navier-Stokes flow confirm the relevance of theoretical results obtained for the delta-correlated model.Comment: 28 pages, 6 figure