ALBERT

Description: Magnetic fields pervade the entire universe and affect the formation and evolution of astrophysical systems from cosmological to planetary scales. The generation and dynamical amplification of extragalactic magnetic fields through cosmic times (up to microgauss levels reported in nearby galaxy clusters, near equipartition with kinetic energy of plasma motions, and on scales of at least tens of kiloparsecs) are major puzzles largely unconstrained by observations. A dynamo effect converting kinetic flow energy into magnetic energy is often invoked in that context; however, extragalactic plasmas are weakly collisional (as opposed to magnetohydrodynamic fluids), and whether magnetic field growth and sustainment through an efficient turbulent dynamo instability are possible in such plasmas is not established. Fully kinetic numerical simulations of the Vlasov equation in a 6D-phase space necessary to answer this question have, until recently, remained beyond computational capabilities. Here, we show by means of such simulations that magnetic field amplification by dynamo instability does occur in a stochastically driven, nonrelativistic subsonic flow of initially unmagnetized collisionless plasma. We also find that the dynamo self-accelerates and becomes entangled with kinetic instabilities as magnetization increases. The results suggest that such a plasma dynamo may be realizable in laboratory experiments, support the idea that intracluster medium turbulence may have significantly contributed to the amplification of cluster magnetic fields up to near-equipartition levels on a timescale shorter than the Hubble time, and emphasize the crucial role of multiscale kinetic physics in high-energy astrophysical plasmas.

Description: This paper provides a detailed study of turbulent statistics and scale-by-scale budgets in turbulent Rayleigh-Bénard convection. It aims at testing the applicability of Kolmogorov and Bolgiano theories in the case of turbulent convection and at improving the understanding of the underlying inertial-range scalings, for which a general agreement is still lacking. Particular emphasis is laid on anisotropic and inhomogeneous effects, which are often observed in turbulent convection between two differentially heated plates. For this purpose, the SO(3) decomposition of structure functions and a method of description of inhomogeneities are used to derive inhomogeneous and anisotropic generalizations of Kolmogorov and Yaglom equations applying to Rayleigh-Bénard convection, which can be extended easily to other types of anisotropic and/or inhomogeneous flows. The various contributions to these equations are computed in and off the central plane of a convection cell using data produced by a direct numerical simulation of turbulent Boussinesq convection at Ra = 106 and Pr = 1 with aspect ratio A=5. The analysis of the isotropic part of the Kolmogorov equation demonstrates that the shape of the third-order velocity structure function is significantly influenced by buoyancy forcing and large-scale inhomogeneities, while the isotropic part of the mixed third-order structure function 〈(Δ∈)2 Δu〉 appearing in the Yaglom equation exhibits a clear scaling exponent 1 in a small range of scales. The magnitudes of the various low l degree anisotropic components of the equations are also estimated and are shown to be comparable to their isotropic counterparts at moderate to large scales. The analysis of anisotropies notably reveals that computing reduced structure functions (structure functions computed at fixed depth for correlation vectors r lying in specific planes only) in order to reveal scaling exponents predicted by isotropic theories is misleading in the case of fully three-dimensional turbulence in the bulk of a convection cell, since such quantities involve linear combinations of different ℓ components which are not negligible in the flow. This observation also indicates that using single-point measurements together with the Taylor hypothesis in the particular direction of a mean flow to test the predictions of asymptotic dimensional isotropic theories of turbulence or to calculate intermittency corrections to these theories may lead to significant bias for mildly anisotropic three-dimensional flows. A qualitative analysis is finally used to show that the influence of buoyancy forcing at scales smaller than the Bolgiano scale is likely to remain important up to Ra = 109, thus preventing Kolmogorov scalings from showing up in convective flows at lower Rayleigh numbers. © 2006 Cambridge University Press.

Description: Large-scale motions in wall-bounded turbulent flows are frequently interpreted as resulting from an aggregation process of smaller-scale structures. Here, we explore the alternative possibility that such large-scale motions are themselves self-sustained and do not draw their energy from smaller-scale turbulent motions activated in buffer layers. To this end, it is first shown that large-scale motions in turbulent Couette flow at ReD2150 self-sustain, even when active processes at smaller scales are artificially quenched by increasing the Smagorinsky constant Cs in large-eddy simulations (LES). These results are in agreement with earlier results on pressure-driven turbulent channel flows. We further investigate the nature of the large-scale coherent motions by computing upper- and lower-branch nonlinear steady solutions of the filtered (LES) equations with a Newton-Krylov solver, and find that they are connected by a saddle-node bifurcation at large values of Cs. Upper-branch solutions for the filtered large-scale motions are computed for Reynolds numbers up to Re D 2187 using specific paths in the Re-Cs parameter plane and compared to large-scale coherent motions. Continuation to Cs D 0 reveals that these large-scale steady solutions of the filtered equations are connected to the Nagata-Clever-Busse-Waleffe branch of steady solutions of the Navier-Stokes equations. In contrast, we find it impossible to connect the latter to buffer-layer motions through a continuation to higher Reynolds numbers in minimal flow units. © 2015 Cambridge University Press.