Enhancement of Parametric Effects in Polariton Waveguides Induced by Dipolar Interactions.

Exciton-polaritons are hybrid light-matter excitations arising from the nonperturbative coupling of a photonic mode and an excitonic resonance. Behaving as interacting photons, they show optical third-order nonlinearities providing effects such as optical parametric oscillation or amplification. It has been suggested that polariton-polariton interactions can be greatly enhanced by inducing aligned electric dipoles in their excitonic part. However, direct evidence of a true particle-particle interaction, such as superfluidity or parametric scattering, is still missing. In this Letter, we demonstrate that dipolar interactions can be used to enhance parametric effects such as self-phase modulation in waveguide polaritons. By quantifying these optical nonlinearities, we provide a reliable experimental measurement of the direct dipolar enhancement of polariton-polariton interactions.

Universality and saturation of intermittency in passive scalar turbulence

The statistical properties of a scalar field advected by the nonintermittent Navier-Stokes flow arising from a two-dimensional inverse energy cascade are investigated. The universality properties of the scalar field are probed by comparing the results obtained with two types of injection mechanisms. Scaling properties are shown to be universal, even though anisotropies injected at large scales persist down to the smallest scales and local isotropy is not fully restored. Scalar statistics is strongly intermittent and scaling exponents saturate to a constant for sufficiently high orders. This is observed also for the advection by a velocity field rapidly changing in time, pointing to the genericity of the phenomenon.

Anomalous and dimensional scaling in anisotropic turbulence.

We present a numerical study of anisotropic statistical fluctuations in homogeneous turbulent flows. We give a new argument to predict the dimensional scaling exponents, zeta(j)(d)(p)=(p+j)/3, for the projections of the pth order structure function in the jth sector of the rotational group. We show that the measured exponents are anomalous, showing a clear deviation from the dimensional prediction. Dimensional scaling is subleading and connected to the dynamical fluctuations without phase correlations. Universality of the observed anomalous scaling is discussed both theoretically and by means of numerical simulations at different Reynolds numbers and with different forcing.

Sarcopenia and its relationship with osteoarthritis: risk factor or direct consequence?

PURPOSE: The aim of this review is to evaluate the clinical role of sarcopenia in patients affected by osteoarthritis (OA) of major joints. METHODS: An online database research was performed, in order to retrieve all articles investigating the relationship between sarcopenia and OA. No peer-reviewed journal was excluded. Papers in English, French, Spanish and Italian language were considered. After consulting the full-text article, five studies have been included in the review. RESULTS: Of the five studies included, four are prospective studies and only one is a cross-sectional study which retrieved data retrospectively. A total of 4,231 patients, presenting a mean age of 62.0 years, were studied. Three studies evaluated only female subjects. Weight, height and body mass index were reported in all the articles. Knee OA has been assessed using the Kellgren-Lawrence grading system and the American college of rheumatology criteria. Alterations of the lean body mass on body weight ratio have been reported to be as a significant predictive parameter in two studies. Relationship between radiographic OA and an increase in the leg lean mass has been observed in one study. CONCLUSIONS: We cannot support neither the thesis of a direct effect of sarcopenia on OA development nor the opposite relation, because the up-to-date literature lacks basic science studies concerning these topics. The absence of clinical studies regarding measurements and tools to compare sarcopenia and OA do not allow to definitely clarify this relationship.

Universal intermittent properties of particle trajectories in highly turbulent flows.

We present a collection of eight data sets from state-of-the-art experiments and numerical simulations on turbulent velocity statistics along particle trajectories obtained in different flows with Reynolds numbers in the range R{lambda}in[120:740]. Lagrangian structure functions from all data sets are found to collapse onto each other on a wide range of time lags, pointing towards the existence of a universal behavior, within present statistical convergence, and calling for a unified theoretical description. Parisi-Frisch multifractal theory, suitably extended to the dissipative scales and to the Lagrangian domain, is found to capture the intermittency of velocity statistics over the whole three decades of temporal scales investigated here.

Heavy particle concentration in turbulence at dissipative and inertial scales.

Spatial distributions of heavy particles suspended in an incompressible isotropic and homogeneous turbulent flow are investigated by means of high resolution direct numerical simulations. In the dissipative range, it is shown that particles form fractal clusters with properties independent of the Reynolds number. Clustering is there optimal when the particle response time is of the order of the Kolmogorov time scale tau(eta). In the inertial range, the particle distribution is no longer scale invariant. It is, however, shown that deviations from uniformity depend on a rescaled contraction rate, which is different from the local Stokes number given by dimensional analysis. Particle distribution is characterized by voids spanning all scales of the turbulent flow; their signature in the coarse-grained mass probability distribution is an algebraic behavior at small densities.

Anisotropic nonperturbative zero modes for passively advected magnetic fields.

An analytic assessment of the role of anisotropic corrections to the isotropic anomalous scaling exponents is given for the d-dimensional kinematic magnetohydrodynamics problem in the presence of a mean magnetic field. The velocity advecting the magnetic field changes very rapidly in time and scales with a positive exponent xi. Inertial-range anisotropic contributions to the scaling exponents, zeta(j), of second-order magnetic correlations are associated with zero modes and have been calculated nonperturbatively. For d=3, the limit xi-->0 yields zeta(j)=j-2+xi(2j(3)+j(2)-5j-4)/[2(4j(2)-1)], where j (j>or=2) is the order in the Legendre polynomial decomposition applied to correlation functions. Conjectures on the fact that anisotropic components cannot change the isotropic threshold to the dynamo effect are also made.

Persistence of small-scale anisotropies and anomalous scaling in a model of magnetohydrodynamics turbulence

The problem of anomalous scaling in magnetohydrodynamics turbulence is considered within the framework of the kinematic approximation, in the presence of a large-scale background magnetic field. The velocity field is Gaussian, delta-correlated in time, and scales with a positive exponent xi. Explicit inertial-range expressions for the magnetic correlation functions are obtained; they are represented by superpositions of power laws with nonuniversal amplitudes and universal (independent of the anisotropy and forcing) anomalous exponents. The complete set of anomalous exponents for the pair correlation function is found nonperturbatively, in any space dimension d, using the zero-mode technique. For higher-order correlation functions, the anomalous exponents are calculated to O(xi) using the renormalization group. The exponents exhibit a hierarchy related to the degree of anisotropy; the leading contributions to the even correlation functions are given by the exponents from the isotropic shell, in agreement with the idea of restored small-scale isotropy. Conversely, the small-scale anisotropy reveals itself in the odd correlation functions: the skewness factor is slowly decreasing going down to small scales and higher odd dimensionless ratios (hyperskewness, etc.) dramatically increase, thus diverging in the r-->0 limit.

Passive scalar intermittency in compressible flow.

A compressible generalization of the Kraichnan model [Phys. Rev. Lett. 72, 1016 (1994)] of passive scalar advection is considered. The dynamical role of compressibility on the intermittency of the scalar statistics is investigated for the direct cascade regime. Simple physical arguments suggest that an enhanced intermittency should appear for increasing compressibility, due to the slowing down of Lagrangian trajectory separations. This is confirmed by a numerical study of the dependence of intermittency exponents on the degree of compressibility, by a Lagrangian method for calculating simultaneous N-point tracer correlations.

Coherent structures in random shell models for passive scalar advection.

A study of anomalous scaling in models of passive scalar advection in terms of singular coherent structures is proposed. The stochastic dynamical system considered is a shell model reformulation of Kraichnan model. We extend the method introduced by Daumont, Dombre, and Gilson (e-print archive chao-dyn/9905017) to the calculation of self-similar instantons and we show how such objects, being the most singular events, are appropriate to capture asymptotic scaling properties of the scalar field. Preliminary results concerning the statistical weight of fluctuations around these optimal configurations are also presented.

Multifractal statistics of Lagrangian velocity and acceleration in turbulence.

The statistical properties of velocity and acceleration fields along the trajectories of fluid particles transported by a fully developed turbulent flow are investigated by means of high resolution direct numerical simulations. We present results for Lagrangian velocity structure functions, the acceleration probability density function, and the acceleration variance conditioned on the instantaneous velocity. These are compared with predictions of the multifractal formalism, and its merits and limitations are discussed.

Inverse statistics of smooth signals: the case of two dimensional turbulence.

The problem of inverse statistics (statistics of distances for which the signal fluctuations are larger than a certain threshold) in differentiable signals with power law spectrum, E(k) approximately k(-alpha), 3< or =alpha<5, is discussed. We show that for these signals, with random phases, exit-distance moments follow a bifractal distribution. We also investigate two dimensional turbulent flows in the direct cascade regime, which display a more complex behavior. We give numerical evidences that the inverse statistics of 2D turbulent flows is described by a multifractal probability distribution; i.e., the statistics of laminar events is not simply captured by the exponent alpha characterizing the spectrum.