Bogdan Teaca

Energy transfers in forced MHD turbulence

The energy cascade in magnetohydrodynamics is studied using high resolution direct numerical simulations of forced isotropic turbulence. The magnetic Prandtl number is unity and the large scale forcing is a function of the velocity that injects a constant rate of energy without generating a mean flow. A shell decomposition of the velocity and magnetic fields is proposed and is extended to the Elsässer variables. The analysis of energy exchanges between these shell variables shows that the velocity and magnetic energy cascades are mainly local and forward, though non-local energy transfer does exist between the large, forced, velocity scales and the small magnetic structures. The possibility of splitting the shell-to-shell energy transfer into forward and backward contributions is also discussed.

Applications of large eddy simulation methods to gyrokinetic turbulence

The large eddy simulation (LES) approach—solving numerically the large scales of a turbulent system and accounting for the small-scale influence through a model—is applied to nonlinear gyrokinetic systems that are driven by a number of different microinstabilities. Comparisons between modeled, lower resolution, and higher resolution simulations are performed for an experimental measurable quantity, the electron density fluctuation spectrum. Moreover, the validation and applicability of LES is demonstrated through a series of diagnostics based on the free energetics of the system.

Locality properties of the energy flux in turbulence

The scale locality functions, originally introduced by Kraichnan, are computed from results of direct numerical simulations of forced isotropic turbulence. It is found that a curve with the classical, asymptotic scaling exponent of 4/3 provides the lower bound for the numerical data in the infrared limit and the upper bound in the ultraviolet limit, i.e., the nonlocality effects are stronger in the infrared and weaker in the ultraviolet than the theoretical result obtained for an infinite inertial range. The departures from the 4/3 scaling are substantial for fully resolved direct numerical simulations at lower Reynolds number but decrease for forced simulations that impose the −5/3 spectrum outside the energy containing range.

Gyrokinetic turbulence: between idealized estimates and a detailed analysis of nonlinear energy transfers

Using large resolution numerical simulations of GK turbulence, spanning an interval ranging from the end of the fluid scales to the electron gyroradius, we study the energy transfers in the perpendicular direction for a proton-electron plasma in a slab magnetic geometry. In addition, to aid our understanding of the nonlinear cascade, we use an idealized test representation for the energy transfers between two scales, mimicking the dynamics of turbulence in an infinite inertial range. For GK turbulence, a detailed analysis of nonlinear energy transfers that account for the separation of energy exchanging scales is performed. We show that locality functions associated with the energy cascade across dyadic (i.e. multiple of two) separated scales achieve an asymptotic state, recovering clear values for the locality exponents. We relate these exponents to the energy exchange between two scales, diagnostics that are less computationally intensive than the locality functions. It is the first time asymptotic locality is shown to exist for GK turbulence and the contributions made by highly non-local interactions, previously reported in the literature, are explained as very local transfers of energy that occur between wavenumbers within the same dyadic signal. The results presented here and their implications are discussed from the perspective of previous findings reported in the literature and the idea of universality of GK turbulence.

Energy Fluxes and Shell-to-Shell Transfers in MHD Turbulence

A spectral analysis of the energy cascade in magnetohydrodynamics (MHD) is presented using high resolution direct numerical simulations of both forced and decaying isotropic turbulence. The triad interactions between velocity and magnetic field modes are averaged into shell interactions between similar length scales phenomena. This is achieved by combining all the velocity Fourier modes that correspond to wave vectors with similar amplitude into a shell velocity variable. The same procedure is adopted for the magnetic field. The analysis of the interactions between these shell variables gives a global picture of the energy transfers between different length scales, as well as between the velocity and the magnetic fields. Also, two different attempts to separate the shell-to-shell interactions into forward and backward energy transfers are proposed. They provide diagnostics that can be used in order to assess subgrid-scale modelling in large-eddy simulation for turbulent MHD systems.

Charged-particle heating in imbalanced MHD turbulence

Astrophysical plasmas such as the solar wind often exhibit characteristics of imbalanced magnetohydrodynamic (MHD) turbulence, in which the fluctuations of the bulk velocity and the magnetic field are strongly correlated. As a result, turbulent heating is less efficient than in the more commonly studied case of balanced turbulence. We study the transport and acceleration properties of charged particles in imbalanced MHD turbulence by performing test-particle simulations. The cross-helicity level, measuring the degree of imbalance of the MHD steady-state, is controlled by using a correlated forcing scheme for velocity and magnetic fields. We discuss the decrease of the turbulent heating rate in systems with non-zero cross-helicity and compare its scaling with theoretical predictions, and show the pitchangle asymmetry of the scattering coefficient in cross-helical turbulence. Our results are relevant for any plasma in which turbulent heating is important, for example the heating of dust particles in the interstellar medium or the injection of thermal protons into the Fermi-II acceleration process in supernova remnant shocks.

Spectral analysis of energy transfers in anisotropic MHD turbulence

We investigate anisotropic magnetohydrodynamic turbulence, in presence of a constant magnetic field, using direct numerical simulations. A method of decomposing the spectral space into ring structures is presented and the energy transfers between such rings are studied. For large values of the constant magnetic field, the total energy transfer appears to be dominant in the direction perpendicular to the mean magnetic field. The linear transfer due to the constant magnetic field also appears to be important in redistributing the energy between the velocity and the magnetic fields.