Pablo D. Mininni

Interactive desktop analysis of high resolution simulations: application to turbulent plume dynamics and current sheet formation

The ever increasing processing capabilities of the supercomputers available to computational scientists today, combined with the need for higher and higher resolution computational grids, has resulted in deluges of simulation data. Yet the computational resources and tools required to make sense of these vast numerical outputs through subsequent analysis are often far from adequate, makingsuchanalysisofthedataapainstaking,ifnotahopeless,task.Inthispaper, we describe a new tool for the scientific investigation of massive computational datasets. This tool (VAPOR) employs data reduction, advanced visualization, and quantitative analysis operations to permit the interactive exploration of vast datasets using only a desktop PC equipped with a commodity graphics card. We describe VAPORs use in the study of two problems. The first, motivated by stellar envelope convection, investigates the hydrodynamic stability of compressible thermal starting plumes as they descend through a stratified layer of increasing density with depth. The second looks at current sheet formation in an incompressible helical magnetohydrodynamic flow to understand the early spontaneous development of quasi two-dimensional (2D) structures embedded within the 3D solution. Both of the problems were studied at sufficiently high spatial resolution, a grid of 504 2 by 2048 points for the first and 1536 3 points for the second, to overwhelm the interactive capabilities of typically available analysis resources.

Numerical study of dynamo action at low magnetic Prandtl numbers.

We present a three-pronged numerical approach to the dynamo problem at low magnetic Prandtl numbers P(M). The difficulty of resolving a large range of scales is circumvented by combining direct numerical simulations, a Lagrangian-averaged model and large-eddy simulations. The flow is generated by the Taylor-Green forcing; it combines a well defined structure at large scales and turbulent fluctuations at small scales. Our main findings are (i) dynamos are observed from P(M)=1 down to P(M)=10(-2), (ii) the critical magnetic Reynolds number increases sharply with P(M)(-1) as turbulence sets in and then it saturates, and (iii) in the linear growth phase, unstable magnetic modes move to smaller scales as P(M) is decreased. Then the dynamo grows at large scales and modifies the turbulent velocity fluctuations.

Shell-to-shell energy transfer in magnetohydrodynamics. I. Steady state turbulence

We investigate the transfer of energy from large scales to small scales in fully developed forced three-dimensional magnetohydrodynamics (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 large scales to kinetic energy at smaller scales and the transfer of magnetic energy from 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 the Kolmogorov theory of turbulence. However, the transfer of energy from the velocity field to the magnetic field is a highly nonlocal process in Fourier space. Energy from the velocity field at large scales can be transferred directly into small-scale magnetic fields without the participation of intermediate scales. Some implications of our results to MHD turbulence modeling are also discussed.

Jet/cloud collision, 3D gasdynamic simulations of HH 110

We present 3D, gasdynamic simulations of jet/cloud collisions, with the purpose of modelling the HH 270/110 system. From the models, we obtain predictions of Hα and H_2 1–0 s(1) emission line maps, which qualitatively reproduce some of the main features of the corresponding observations of HH 110. We find that the model that better reproduces the observed structures corresponds to a jet that was deflected at the surface of the cloud ~1000 yr ago, but is now boring a tunnel directly into the cloud. This model removes the apparent contradiction between the jet/cloud collision model and the lack of detection of molecular emission in the crossing region of the HH 270 and HH 110 axes.

A hybrid MPI–OpenMP scheme for scalable parallel pseudospectral computations for fluid turbulence

Abstract A hybrid scheme that utilizes MPI for distributed memory parallelism and OpenMP for shared memory parallelism is presented. The work is motivated by the desire to achieve exceptionally high Reynolds numbers in pseudospectral computations of fluid turbulence on emerging petascale, high core-count, massively parallel processing systems. The hybrid implementation derives from and augments a well-tested scalable MPI-parallelized pseudospectral code. The hybrid paradigm leads to a new picture for the domain decomposition of the pseudospectral grids, which is helpful in understanding, among other things, the 3D transpose of the global data that is necessary for the parallel fast Fourier transforms that are the central component of the numerical discretizations. Details of the hybrid implementation are provided, and performance tests illustrate the utility of the method. It is shown that the hybrid scheme achieves good scalability up to ∼20,000 compute cores with a maximum efficiency of 89%, and a mean of 79%. Data are presented that help guide the choice of the optimal number of MPI tasks and OpenMP threads in order to maximize code performance on two different platforms.

DYNAMO ACTION IN MAGNETOHYDRODYNAMICS AND HALL-MAGNETOHYDRODYNAMICS

The first direct numerical simulations of turbulent Hall dynamos are presented. The evolution of an initially weak and small-scale magnetic field in a system maintained in a stationary regime of hydrodynamic turbulence (by a stirring force at a macroscopic scale) is studied to explore the conditions for exponential growth of the magnetic energy. The Hall current is shown to have a profound effect on turbulent dynamo action; it can strongly enhance or suppress the generation of the large-scale magnetic energy depending on the relative values of the length scales of the system.

Small-Scale Structures in Three-Dimensional Magnetohydrodynamic Turbulence

We investigate using direct numerical simulations with grids up to 1536(3) points, the rate at which small scales develop in a decaying three-dimensional MHD flow both for deterministic and random initial conditions. Parallel current and vorticity sheets form at the same spatial locations, and further destabilize and fold or roll up after an initial exponential phase. At high Reynolds numbers, a self-similar evolution of the current and vorticity maxima is found, in which they grow as a cubic power of time; the flow then reaches a finite dissipation rate independent of the Reynolds number.

Rapid Alignment of Velocity and Magnetic Field in Magnetohydrodynamic Turbulence

We show that local directional alignment of the velocity and magnetic field fluctuations occurs rapidly in magnetohydrodynamics for a variety of parameters and is seen both in direct numerical simulations and in solar wind data. The phenomenon is due to an alignment between magnetic field and gradients of either pressure or kinetic energy, and is similar to alignment of velocity and vorticity in Navier-Stokes turbulence. This rapid and robust relaxation process leads to a local weakening of nonlinear terms.

Rotating helical turbulence. I. Global evolution and spectral behavior

We present results from two 15363 direct numerical simulations of rotating turbulence where both energy and helicity are injected into the flow by an external forcing. The dual cascade of energy and helicity toward smaller scales observed in isotropic and homogeneous turbulence is broken in the presence of rotation, with the development of an inverse cascade of energy now coexisting with direct cascades of energy and helicity. In the direct cascade range, the flux of helicity dominates over that of energy at low Rossby number. These cascades have several consequences for the statistics of the flow. The evolution of global quantities and of the energy and helicity spectra is studied, and comparisons with simulations at different Reynolds and Rossby numbers at lower resolution are done to identify scaling laws.

Energy transfer in Hall-MHD turbulence: cascades, backscatter, and dynamo action

Abstract. Scale interactions in Hall MHD are studied using both the mean fieldtheory derivation of transport coefficients, and direct numerical simulations in threespace dimensions. In the magnetically dominated regime, the eddy resistivity isfound to be negative definite, leading to large scale instabilities. A direct cascade ofthe total energyisobserved,althoughasthe amplitude ofthe Hall effect is increased,backscatterofmagneticenergytolargescalesis found, afeature notpresentin MHDflows. The coupling between the magnetic and velocity fields is different than in theMHD case, and backscatter of energy from small scale magnetic fields to large scaleflows is also observed. For the magnetic helicity, a strong quenching of its transferis found. We also discuss non-helical magnetically forced Hall-MHD simulationswhere growth of a large scale magnetic field is observed. 1. Introduction The relevance of two fluid effects has recently been pointed out in several stud-ies of astrophysical and laboratory plasmas (Balbus and Terquem, 2001; Sano andStone, 2002; Mirnov et al., 2003; Ding et al., 2004). The effect of adding the Hallcurrent to the dynamics of the flow was studied in several scenarios, particularly dy-namo action (Helmis, 1968; Galanti et al., 1995; Mininni et al., 2002, 2003a, 2005b)and reconnection (Birn et al., 2001; Shay et al., 2001; Wang et al., 2001; Moraleset al., 2005). Several of these works showed that the Hall currents increase the re-connection rate of magnetic field lines. However, most of the studies of magneticreconnection were done for particular configurations of current sheets. It was shownin particular by Smith et al. (2004) that when a turbulent background is present thereconnection rate is dominated by the amplitude of the turbulent fluctuations. Theprocess of magnetic reconnection is relevant in several astrophysical and geophys-ical scenarios, such as the magnetopause, the magnetotail, the solar atmosphere,or the interplanetary and interstellar medium. Reconnection can also play a rolein the generation of large scale magnetic fields by dynamo action Zeldovich et al.(1983).Some of the works in Hall-magnetohydrodynamics (Hall-MHD) present conflict-ing results, indicating in some cases that the Hall effect can help the growth of a