Biomedical subjects

A Lanotte

Publications and source records attributed to A Lanotte.

8 recordsLinked to original sources

PubMed2004

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.

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PubMed2002

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.

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PubMed2001

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.

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PubMed2000

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.

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PubMed2000

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.

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PubMed1999

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.

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PubMed1999

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.

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PubMed1999

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.

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