arXiv subjects

A. Celani

Publications and source records attributed to A. Celani.

45 records · Page 3Linked to original sources

arXiv1998

Large scale instabilities in two-dimensional magnetohydrodynamics

The stability of a sheared magnetic field is analyzed in two-dimensional magnetohydrodynamics with resistive and viscous dissipation. Using a multiple-scale analysis, it is shown that at large enough Reynolds numbers the basic state describing a motionless fluid and a layered magnetic field, becomes unstable with respect to large scale perturbations. The exact expressions for eddy-viscosity and eddy-resistivity are derived in the nearby of the critical point where the instability sets in. In this marginally unstable case the nonlinear phase of perturbation growth obeys to a Cahn-Hilliard-like dynamics characterized by coalescence of magnetic islands leading to a final new equilibrium state. High resolution numerical simulations confirm quantitatively the predictions of multiscale analysis.

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arXiv1998

Pair dispersion in synthetic fully developed turbulence

The Lagrangian statistics of relative dispersion in fully developed turbulence is numerically investigated. A scaling range spanning many decades is achieved by generating a synthetic velocity field with prescribed Eulerian statistical features. When the velocity field obeys Kolmogorov similarity, the Lagrangian statistics is self similar too, and in agreement with Richardson's predictions. For an intermittent velocity field the scaling laws for the Lagrangian statistics are found to depend on Eulerian intermittency in agreement with a multifractal description. As a consequence of the Kolmogorov law the Richardson law for the variance of pair separation is not affected by intermittency corrections. A new analysis method, based on fixed scale averages instead of usual fixed time statistics, is shown to give much wider scaling range and should be preferred for the analysis of experimental data.

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arXiv1998

Nonlocal bottleneck effect in two-dimensional turbulence

The bottleneck pileup in the energy spectrum is investigated for several two-dimensional (2D) turbulence systems by numerical simulation using high-order diffusion terms to amplify the effect, which is weak for normal diffusion. For 2D magnetohydrodynamic (MHD) turbulence, 2D electron MHD (EMHD) turbulence and 2D thermal convection, which all exhibit direct energy cascades, a nonlocal behavior is found resulting in a logarithmic enhancement of the spectrum.

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arXiv1998

Bridge relations in Navier-Stokes turbulence

The correlation between inertial range velocity fluctuations and energy dissipation in fully developed turbulence is studied using high resolution direct numerical simulation. Runs with microscale Reynolds number up to ${\cal R}_λ \simeq 300$ have been performed in order to achieve the maximum extent of the scaling range compatible with a proper resolution of the dissipative scales. The bridge relations, a set of scaling relations which express quantitatively the statistical dependence of dissipation-scale velocity fluctuations on inertial-range ones, are verified. Corrections to the bridge relations due to the finite size of the scaling range are investigated.

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arXiv1998

Multi-time, multi-scale correlation functions in turbulence and in turbulent models

A multifractal-like representation for multi-time multi-scale velocity correlation in turbulence and dynamical turbulent models is proposed. The importance of subleading contributions to time correlations is highlighted. The fulfillment of the dynamical constraints due to the equations of motion is thoroughly discussed. The prediction stemming from this representation are tested within the framework of shell models for turbulence.

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arXiv1998

Relative dispersion in fully developed turbulence: from Eulerian to Lagrangian statistics in synthetic flows

The effect of Eulerian intermittency on the Lagrangian statistics of relative dispersion in fully developed turbulence is investigated. A scaling range spanning many decades is achieved by generating a multi-affine synthetic velocity field with prescribed intermittency features. The scaling laws for the Lagrangian statistics are found to depend on Eulerian intermittency in agreement with a multifractal description. As a consequence of the Kolmogorov's law, the Richardson's law for the variance of pair separation is not affected by intermittency corrections.

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arXiv1997

Mimicking a turbulent signal: sequential multiaffine processes

An efficient method for the construction of a multiaffine process, with prescribed scaling exponents, is presented. At variance with the previous proposals, this method is sequential and therefore it is the natural candidate in numerical computations involving synthetic turbulence. The application to the realization of a realistic turbulent-like signal is discussed in detail. The method represents a first step towards the realization of a realistic spatio-temporal turbulent field.

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arXiv1997

Dispersion of passive tracers in closed basins: beyond the diffusion coefficient

We investigate the spreading of passive tracers in closed basins. If the characteristic length scale of the Eulerian velocities is not very small compared with the size of the basin the usual diffusion coefficient does not give any relevant information about the mechanism of spreading. We introduce a finite size characteristic time $τ(δ)$ which describes the diffusive process at scale $δ$. When $δ$ is small compared with the typical length of the velocity field one has $τ(δ) \sim λ^{-1}$, where $λ$ is the maximum Lyapunov exponent of the Lagrangian motion. At large $δ$ the behavior of $τ(δ)$ depends on the details of the system, in particular the presence of boundaries, and in this limit we have found a universal behavior for a large class of system under rather general hypothesis. The method of working at fixed scale $δ$ makes more physical sense than the traditional way of looking at the relative diffusion at fixed delay times. This technique is displayed in a series of numerical experiments in simple flows.

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arXiv1997

Kolmogorov's law for two-dimensional electron-magnetohydrodynamic turbulence

The analogue of the Kolmogorov's four-fifths law is derived for two-dimensional, homogeneous, isotropic EMHD turbulence in the energy cascade inertial range. Direct numerical simulations for the freely decaying case show that this relation holds true for different values of the adimensional electron inertial length scale, $d_e$. The energy spectrum is found to be close to the expected Kolmogorov spectrum.

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