Do magnetic fields enhance turbulence at low magnetic Reynolds number?

Imposing a magnetic field on a turbulent flow of electrically conducting fluid incurs the Joule effect. A current paradigm is that the corresponding dissipation increases with the intensity of the magnetic field and as a result turbulent fluctuations are all the more damped as the magnetic field is strong. While this idea finds apparent support in the phenomenology of decaying turbulence, measurements of turbulence in duct flows and other, more complex configurations have produced seemingly contradicting results. The root of the controversy is that magnetic fields promote sufficient scale-dependent anisotropy to profoundly reorganize the structure of turbulence, so their net effect cannot be understood in terms of the additional dissipation only. Here we show that when turbulence is forced in a magnetic field that acts on turbulence itself rather than on the mechanisms that generate it, the field promotes large, nearly two-dimensional structures capturing sufficient energy to offset the loss due to Joule dissipation, with the net effect of increasing the intensity of turbulent fluctuations. This change of paradigm potentially carries important consequences for systems as diverse as the liquid cores of planets, accretion disks, and a wide range of metallurgical and nuclear engineering applications.<br/

Why, how and when MHD turbulence at low Rm becomes three-dimensional

MHD turbulence at low Magnetic Reynolds number is experimentally investigated by studying a liquid metal flow in a cubic domain. We focus on the mechanisms that determine whether the flow is quasi-2D, 3D or in any intermediate state. To this end, forcing is applied by injecting a DC current $I$ through one wall of the cube only, to drive vortices spinning along the magnetic field. Depending on the intensity of the externally applied magnetic field, these vortices extend part or all of the way through the cube. Driving the flow in this way allows us to precisely control not only the forcing intensity but also its dimensionality. A comparison with the theoretical analysis of this configuration singles out the influences of the walls and of the forcing on the flow dimensionality, which is characterised in several ways. First, when inertia drives three-dimensionality, the velocity near the wall where current is injected scales as $U_b\sim I^{2/3}$. Second, when the distance $l_z$ over which momentum diffuses under the action of the Lorentz force reaches the channel width $h$, the velocity near the opposite wall $U_t$ follows a similar law with a correction factor $(1-h/l_z)$. When $l_z<h$, by contrast, the opposite wall has less influence on the flow and $U_t\sim I^{1/2}$. The central role played by the ratio $l_z/h$ is confirmed by experimentally verifying Sommeria & Moreau (1982)'s scaling $l_z\sim N^{1/2}$ ($N$ is the interaction parameter) and finally, the nature of the three-dimensionality is further clarified by distinguishing weak and strong three-dimensionalities. It is found that both vanish only asymptotically in the limit $N\rightarrow\infty$. This provides evidence that because of the no-slip walls, 1) the transition between quasi-2D and 3D turbulence does not result from a global instability of the flow, and 2) it doesn't occur simultaneously at all scales

About the origin of Chipiona ingots. The contribution of lead isotope analyses

Este artículo presenta los resultados de los análisis isotópicos del plomo realizados en algunos de los 22 lingotes romanos de plomo y de cobre de Chipiona (Cádiz), estudiados en un primer artículo publicado en 2010 por la revista "Habis". El objetivo de dichos análisis era determinar la o las zonas de fabricación de los lingotes, que su estudio arqueológico y epigráfico orientaba hacia la Sierra Morena oriental para el plomo, la Faja Pirítica para el cobre. La arqueometría sólo confirma en parte estas conclusiones, designando una misma área, la Sierra Morena oriental, como lugar de procedencia del conjunto de lingotes.The paper presents the results of the lead isotope analyses made on some of the 22 Roman lead and copper ingots found in Chipiona (Cádiz), yet studied in a first article “Habis” published in 2010. The aim of such analyses was to precise the geographical origin of the ingots, as the archaeological and epigraphic study seems to indicate the Eastern part of Sierra Morena as the area of provenience of the lead, and the Piritic Belt as the origin of the copper. The archaeometry only partly confirms these conclusions; it appoints the Eastern Sierra Morena as the unique raw source for the whole lead and copper ingots

La industria de hortalizas y frutales

Tomate-Solanum lycopersicumGuayaba-Psidium guajav

Note sur deux lingots de cuivre romains trouvés en Méditerranée

El trabajo vuelve sobre dos lingotes ya conocidos, salidos a la luz recientemente, su epigrafía y sobre los análisis de isotopos de plomo que les atribuyen una procedencia hispánica y mas concretamente del Cinturón pirítico del suroeste peninsular. Son dos testimonios concretos de la vitalidad del comercio marítimo del cobre de esta zona de la Península durante el siglo II de nuestra era.

Radio to infrared spectra of late-type galaxies with Planck and WMAP data

We use the Planck Early Release Compact Source Catalogue combined with WMAP and other archival measurements to construct continuum spectra of three nearby dusty star-forming galaxies: Messier 82, NGC 253 and NGC 4945. We carry out a least-squares fit to the spectra using a combination of simple synchrotron, free-free and thermal dust models, and look for evidence of anomalous microwave emission (AME). We find that the radio spectra of all three galaxies are consistent with steep spectrum synchrotron emission, with a significant amount of free-free emission required to explain the Planck and WMAP data points in the frequency range 30-150 GHz. This brings the star-formation rate based on free-free emission into better agreement with that from the non-thermal emission. We place limits on the presence of AME in these galaxies, finding that it is lower than expectations based on the ratio of far infrared to AME from the Galaxy. Nevertheless, the shape of the spectrum of NGC 4945 hints at the presence of AME with a peak around 30 GHz. Future Planck data will let us look more closely at these galaxies, as well as to extend the analysis to many more galaxies.Comment: 5 pages, 1 figure (6 panels), 1 table. Submitted to MNRAS letter

Controlling the dimensionality of low-Rm MHD turbulence experimentally

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