arXiv subjects

Daniele Carati

Publications and source records attributed to Daniele Carati.

5 recordsLinked to original sources

arXiv2011

Dynamic Procedure for Filtered Gyrokinetic Simulations

Large Eddy Simulations (LES) of gyrokinetic plasma turbulence are investigated as interesting candidates to decrease the computational cost. A dynamic procedure is implemented in the GENE code, allowing for dynamic optimization of the free parameters of the LES models (setting the amplitudes of dissipative terms). Employing such LES methods, one recovers the free energy and heat flux spectra obtained from highly resolved Direct Numerical Simulations (DNS). Systematic comparisons are performed for different values of the temperature gradient and magnetic shear, parameters which are of prime importance in Ion Temperature Gradient (ITG) driven turbulence. Moreover, the degree of anisotropy of the problem, that can vary with parameters, can be adapted dynamically by the method that shows Gyrokinetic Large Eddy Simulation (GyroLES) to be a serious candidate to reduce numerical cost of gyrokinetic solvers.

physics.plasm-ph↗

arXiv2008

Chaotic traveling rolls in Rayleigh-Bénard convection

In this paper we investigate two-dimensional (2D) Rayleigh-Bénard convection using direct numerical simulation in Boussinesq fluids of Prandtl number $P = 6.8$ confined between thermally conducting plates. We show through the simulation that in a small range of reduced Rayleigh number $r$ ($770 < r < 890$) the 2D rolls move chaotically in a direction normal to the roll axis. The lateral shift of the rolls may lead to global flow reversal of the convective motion. The chaotic traveling rolls are observed in simulation with {\it free-slip} as well as {\it no-slip} boundary conditions on the velocity field. We show that the traveling rolls and the flow reversal are due to an interplay between the real and imaginary parts of the critical modes.

nlin.CD↗

arXiv2008

Dynamo Transition in Low-dimensional Models

Two low-dimensional magnetohydrodynamic models containing three velocity and three magnetic modes are described. One of them (nonhelical model) has zero kinetic and current helicity, while the other model (helical) has nonzero kinetic and current helicity. The velocity modes are forced in both these models. These low-dimensional models exhibit a dynamo transition at a critical forcing amplitude that depends on the Prandtl number. In the nonhelical model, dynamo exists only for magnetic Prandtl number beyond 1, while the helical model exhibits dynamo for all magnetic Prandtl number. Although the model is far from reproducing all the possible features of dynamo mechanisms, its simplicity allows a very detailed study and the observed dynamo transition is shown to bear similarities with recent numerical and experimental results.

nlin.CD↗

arXiv2005

The Effect of Coherent Structures on Stochastic Acceleration in MHD Turbulence

We investigate the influence of coherent structures on particle acceleration in the strongly turbulent solar corona. By randomizing the Fourier phases of a pseudo-spectral simulation of isotropic MHD turbulence (Re $\sim 300$), and tracing collisionless test protons in both the exact-MHD and phase-randomized fields, it is found that the phase correlations enhance the acceleration efficiency during the first adiabatic stage of the acceleration process. The underlying physical mechanism is identified as the dynamical MHD alignment of the magnetic field with the electric current, which favours parallel (resistive) electric fields responsible for initial injection. Conversely, the alignment of the magnetic field with the bulk velocity weakens the acceleration by convective electric fields $- \bfu \times \bfb$ at a non-adiabatic stage of the acceleration process. We point out that non-physical parallel electric fields in random-phase turbulence proxies lead to artificial acceleration, and that the dynamical MHD alignment can be taken into account on the level of the joint two-point function of the magnetic and electric fields, and is therefore amenable to Fokker-Planck descriptions of stochastic acceleration.

astro-ph↗

arXiv2004

Search for optimal 2D and 3D wave launching configurations for the largest acceleration of charged particles in a magnetized plasma, Resonant Moments Method

Optimal two-dimensional (2D), three-dimensional (3D) wave launching configurations are proposed for enhanced acceleration of charged particles in magnetized plasmas. A primary wave is launched obliquely with respect to the magnetic field and a secondary, low amplitude, wave is launched perpendicularly. The effect of both the launching angle of the primary wave, and the presence of the secondary wave is investigated. Theoretical predictions of the highest performances of the three-dimensional (3D) configurations are proposed using a Resonance Moments Method (RMM) based on estimates for the moments of the velocity distribution function calculated inside the resonance layers (RL). They suggest the existence of an optimal angle corresponding to non parallel launching. Direct statistical simulations show that it is possible to rise the mean electron velocity up to the order of magnitude as compared to the primary wave launching alone. It is a quite promising result because the amplitude of the secondary wave is ten times lower the one of the first wave. The parameters used are related to magnetic plasma fusion experiments in electron cyclotron resonance heating and electron acceleration in planetary ionospheres and magnetospheres.

physics.plasm-ph↗