Alessandra Lanotte | Searcharxiv

arXiv2003

Effects of forcing in three dimensional turbulent flows

We present the results of a numerical investigation of three-dimensional homogeneous and isotropic turbulence, stirred by a random forcing with a power law spectrum, $E_f(k)\sim k^{3-y}$. Numerical simulations are performed at different resolutions up to $512^3$. We show that at varying the spectrum slope $y$, small-scale turbulent fluctuations change from a {\it forcing independent} to a {\it forcing dominated} statistics. We argue that the critical value separating the two behaviours, in three dimensions, is $y_c=4$. When the statistics is forcing dominated, for $y y_c$, we find the same anomalous scaling measured in flows forced only at large scales. We connect these results with the issue of {\it universality} in turbulent flows.

nlin.CD↗

arXiv1999

Anisotropic non-perturbative zero modes for passively advected magnetic fields

A first analytic assessment of the role of anisotropic corrections to the isotropic anomalous scaling exponents is given for the $d$-dimensional kinematic magneto-hydrodynamics 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 $ξ$. Inertial-range anisotropic contributions to the scaling exponents, $ζ_j$, of second-order magnetic correlations are associated to zero modes and have been calculated non-perturbatively. For $d=3$, the limit $ξ\mapsto 0$ yields $\protect{ζ_j=j-2+ ξ(2j^3 +j^2 -5 j - 4)/[2(4 j^2 - 1)]} $ where $j$ ($j\geq 2$) 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.

chao-dyn↗