arXiv subjects

Annick Pouquet

Publications and source records attributed to Annick Pouquet.

41 records · Page 3Linked to original sources

arXiv2005

Geophysical-astrophysical spectral-element adaptive refinement (GASpAR): Object-oriented h-adaptive code for geophysical fluid dynamics simulation

We present an object-oriented geophysical and astrophysical spectral-element adaptive refinement (GASpAR) code for application to turbulent flows. Like most spectral-element codes, GASpAR combines finite-element efficiency with spectral-method accuracy. It is also designed to be flexible enough for a range of geophysics and astrophysics applications where turbulence or other complex multiscale problems arise. For extensibility and flexibilty the code is designed in an object-oriented manner. The computational core is based on spectral-element operators, which are represented as objects. The formalism accommodates both conforming and nonconforming elements and their associated data structures for handling interelement communications in a parallel environment. Many aspects of this code are a synthesis of existing methods; however, we focus on a new formulation of dynamic adaptive refinement (DARe) of nonconforming h-type. This paper presents the code and its algorithms; we do not consider parallel efficiency metrics or performance. As a demonstration of the code we offer several two-dimensional test cases that we propose as standard test problems for comparable DARe codes. The suitability of these test problems for turbulent flow simulation is considered.

math.NA↗

arXiv2005

On spectral scaling laws for incompressible anisotropic MHD turbulence

A heuristic model is given for anisotropic magnetohydrodynamics (MHD) turbulence in the presence of a uniform external magnetic field $B_0 {\bf {\hat e}_{\pa}}$. The model is valid for both moderate and strong $B_0$ and is able to describe both the strong and weak wave turbulence regimes as well as the transition between them. The main ingredient of the model is the assumption of constant ratio at all scales between \add{the} linear wave period and \add{the} nonlinear turnover timescale. Contrary to the model of critical balance introduced by Goldreich and Sridhar [P. Goldreich and S. Sridhar, ApJ {\bf 438}, 763 (1995)], it is not assumed in addition that this ratio be equal to unity at all scales which allows us to use the Iroshnikov-Kraichnan phenomenology. It is then possible to recover the widely observed anisotropic scaling law $\kpa \propto \kpe^{2/3}$ between parallel and perpendicular wavenumbers (with reference to $B_0 {\bf {\hat e}_{\pa}}$) and to obtain the universal prediction, $3α+ 2β= 7$, for the total energy spectrum $E(\kpe,\kpa) \sim \kpe^{-α} \kpa^{-β}$. In particular, with such a prediction the weak Alfvén wave turbulence constant-flux solution is recovered and, for the first time, a possible explanation to its precursor found numerically by Galtier et al [S. Galtier et al., J. Plasma Phys. {\bf 63}, 447 (2000)] is given.

physics.plasm-ph↗

arXiv2005

Shell to shell energy transfer in MHD, Part I: steady state turbulence

We investigate the transfer of energy from large scales to small scales in fully developed forced three-dimensional MHD-turbulence by analyzing the results of direct numerical simulations in the absence of an externally imposed uniform magnetic field. Our results show that the transfer of kinetic energy from the large scales to kinetic energy at smaller scales, and the transfer of magnetic energy from the large scales to magnetic energy at smaller scales, are local, as is also found in the case of neutral fluids, and in a way that is compatible with Kolmogorov (1941) theory of turbulence. However, the transfer of energy from the velocity field to the magnetic field is a highly non-local process in Fourier space. Energy from the velocity field at large scales can be transfered directly into small scale magnetic fields without the participation of intermediate scales. Some implications of our results to MHD turbulence modeling are also discussed.

physics.flu-dyn↗

arXiv2005

Shell to shell energy transfer in MHD, Part II: Kinematic dynamo

We study the transfer of energy between different scales for forced three-dimensional MHD turbulent flows in the kinematic dynamo regime. Two different forces are examined: a non-helical Taylor Green flow with magnetic Prandtl number P_M=0.4, and a helical ABC flow with P_M=1. This analysis allows us to examine which scales of the velocity flow are responsible for dynamo action, and identify which scales of the magnetic field receive energy directly from the velocity field and which scales receive magnetic energy through the cascade of the magnetic field from large to small scales. Our results show that the turbulent velocity fluctuations are responsible for the magnetic field amplification in the small scales (small scale dynamo) while the large scale field is amplified mostly due to the large scale flow. A direct cascade of the magnetic field energy from large to small scales is also present and is a complementary mechanism for the increase of the magnetic field in the small scales. Input of energy from the velocity field in the small magnetic scales dominates over the energy that is cascaded down from the large scales until the large-scale peak of the magnetic energy spectrum is reached. At even smaller scales, most of the magnetic energy input is from the cascading process.

physics.flu-dyn↗

arXiv2002

An alternative interpretation for the Holm "alpha model"

By re-interpreting a recent successful closure procedure in terms of local spatial averaging and the neglect of fluctuations about that average, it is shown how the results of that closure scheme (the ''alpha model'') may be more simply derived.

physics.flu-dyn↗