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Turbulent fluctuations in magnetohydrodynamic flows are known to become anisotropic under the action of a sufficiently strong magnetic field. We investigate this phenomenon in the case of low magnetic Reynolds number using direct numerical simulations and large eddy simulations of a forced flow in a periodic box. A series of simulations is performed with different strengths of the magnetic field, varying Reynolds number, and two types of forcing, one of which is isotropic and the other limited to two-dimensional flow modes. We find that both the velocity anisotropy ͑difference in the relative amplitude of the velocity components͒ and the anisotropy of the velocity gradients are predominantly determined by the value of the magnetic interaction parameter. The effects of the Reynolds number and the type of forcing are much weaker. We also find that the anisotropy varies only slightly with the length scale.

We give the complete list of all first-order consistent interaction vertices for a set of exterior form gauge fields of form degree >1, described in the free limit by the standard Maxwell-like action. A special attention is paid to the interactions that deform the gauge transformations. These are shown to be necessarily of the Noether form "conserved antisymmetric tensor" times "p-form potential" and exist only in particular spacetime dimensions. Conditions for consistency to all orders in the coupling constant are given. For illustrative purposes, the analysis is carried out explicitly for a system of forms with two different degrees p and q (1

The characteristic cohomology H k char (d) for an arbitrary set of free p-form gauge fields is explicitly worked out in all form degrees k < n − 1, where n is the spacetime dimension. It is shown that this cohomology is finite-dimensional and completely generated by the forms dual to the field strengths. The gauge invariant characteristic cohomology is also computed. The results are extended to interacting p-form gauge theories with gauge invariant interactions. Implications for the BRST cohomology are mentioned.

Using numerical methods, we systematically study in the framework of ideal MHD the effect of magnetic fields on heat transfer within a turbulent gas. We measure the rates of passive scalar diffusion within magnetized fluids and make the comparisons a) between MHD and hydro simulations, b) between different MHD runs with different values of the external magnetic field (up to the energy equipartition value), c) between thermal conductivities parallel and perpendicular to magnetic field. We do not find apparent suppression of diffusion rates by the presence of magnetic fields, which implies that magnetic fields do not suppress heat diffusion by turbulent motions.Comment: 4 pages; 2 figures; submitted to Ap

This article reviews the main established ideas on the influence of a magnetic field on turbulence in electrically conducting fluids. We limit our discussion to the asymptotic range of very small values of the magnetic Reynolds number, characterized by the fact that the induced magnetic field remains very small in comparison with the applied magnetic field. We consider three kinds of flows here. The simplest one is freely decaying homogeneous turbulence, which serves as a test bed to analyze the development of anisotropy resulting from the linear damping by the Lorentz force. We then discuss flows between walls perpendicular to the magnetic field and emphasize the influence of the Hartmann layers that develop in their vicinity. We then review the main features of the possible quasi-two-dimensional regime that can arise in that context. Finally, we consider magnetohydrodynamic turbulent shear flows. These are frequent in industrial applications involving molten metals, such as in metal processing or in the blanket of future nuclear fusion reactors. We pay particular attention to recent attempts to develop specific RANS (Reynolds-averaged Navier-Stokes) models for these flows.

Chemical reactions can accelerate, slow down or even be at the very origin of the development of dissolution-driven convection in partially miscible stratifications when they impact the density profile in the host fluid phase. We numerically analyze the dynamics of this reactive convective dissolution in the fully developed non-linear regime for a phase A dissolving into a host layer containing a dissolved reactant B. We show for a general A + B → C reaction in solution, that the dynamics vary with the Rayleigh numbers of the chemical species, i.e. with the nature of the chemicals in the host phase. Depending on whether the reaction slows down, accelerates or is at the origin of the development of convection, the spatial distributions of species A, B or C, the dissolution flux and the reaction rate are different. We show that chemical reactions can enhance the steady-state flux as they consume A and can induce more intense convection than in the non-reactive case. This result is important in the context of CO geological sequestration where quantifying the storage rate of CO dissolving into the host oil or aqueous phase is crucial to assess the efficiency and the safety of the project.

A spectral analysis of anisotropic magnetohydrodynamic turbulence, in presence of a constant magnetic field, is presented using high-resolution direct numerical simulations. A method of decomposing the spectral space into ring structures is presented and the energy transfers between such rings are studied. This decomposition method takes into account the angular dependency of energy transfers in anisotropic systems, while it allows one to recover easily the known shell-to-shell energy transfers in the limit of isotropic turbulence. For large values of the constant magnetic field, the total-energy transfer appears to be most dominant in the direction perpendicular to the mean magnetic field. The linear transfer due to the constant magnetic also appears to be important in redistributing the energy between the velocity and the magnetic fields.