Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session N5: The Generation of Magnetic Fields in the Cosmos and the Role of Turbulence |
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Sponsoring Units: DFD Chair: Annick Pouquet, National Center for Atmospheric Research Room: Colorado Convention Center Korbel 1A-1B |
Wednesday, March 7, 2007 8:00AM - 8:36AM |
N5.00001: Laboratory models of the Earth's outer core Invited Speaker: We construct liquid sodium experiments as models of the Earth's core. Key to understanding these several experimental devices is knowing how turbulence is effected by rotation and magnetic fields. In the approach to the planetary regeme, several remarkable behaviors appear [1]. As rotation and magnetic fields add some measure of elasticity to the flows, several types of driven planetary modes are observed depending on the force balances involved. Ordering the Coriolis, Lorentz, and inertial forces is key to understanding the complicated states observed. While these experiments are undertaken in part to understand the geodynamo, they have led to a number of different first observations, including the magneto-rotational instability [2] and inertial waves in spherical Couette flow. These different approaches to using laboratory experiments are opening up a new direction to understanding the dynamics of the Earth's outer core, other Planetary interiors, and a host of astrophysical objects. [1] W.L. Shew and D.P. Lathrop, ``Liquid sodium model of geophysical core convection,'' Phys. Earth and Planetary Interiors, 153, 136-149 (2005). [2] D.R. Sisan, N. Mujica, W.A. Tillotson, Y.-M. Huang, W.Dorland, A.B. Hassam, T.M. Antonsen, and D.P. Lathrop, ``Experimental Observation and Characterization of the Magnetorotational Instability,'' Phys. Rev. Lett. 93, 114502 (2004). [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 9:12AM |
N5.00002: Making Magnetic Fields on Cosmic Scales Invited Speaker: Magnetic fields are observed very early in the evolution of structure of the universe. It is not known how or when these fields were created. I will discuss the various theories of the field origin and the fluid mechanical issues that arise. Small scale fields of observable amplitudes are relatively easy to create on short time-scales by turbulent flows or compact objects. The central issue is the the creation of the observed long scale coherence in the field. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:48AM |
N5.00003: Generation of magnetic field by dynamo action in a turbulent flow of liquid sodium Invited Speaker: Industrial dynamos routinely generate currents and magnetic fields from mechanical motions. In these devices, pioneered by Siemens, the path of the electrical currents and the geometry of the rotors are completely prescribed. As it cannot be the case for planets and stars, experiments aimed at studying dynamos in the laboratory have evolved towards relaxing these constraints. Solid rotor experiments showed that a dynamo state could be reached with prescribed motions but currents free to self-organize. A landmark was reached in 2000, when the experiments in Riga and Karlsruhe showed that fluid dynamos could be generated by organizing favourable sodium flows, the electrical currents being again free to self-organize. For these experiments, the self-sustained dynamo fields had simple time dynamics (a steady field in Karlsruhe and an oscillatory field in Riga). No further dynamical regimes where reached. We report the observation of dynamo action in swirling flows for which turbulence is fully developed. The flows are generated in the gap between counter-rotating impellers (the von Karman Sodium experiment -VKS). Dynamo action is reached at magnetic Reynolds number Rm$\approx $30. When the impellers are rotating at equal rates, the dynamo field is statistically steady, although the rms fluctuation level is of the order of the mean amplitude. For impellers rotated at different speeds, a variety of dynamical regimes are observed, including magnetic field reversals. We will describe and discuss the features of these dynamos, including the nature of the bifuraction, the scaling of the self-sustained fields, the excess mechanical power, etc. Some regimes have geomorphic characteristics, while others may be relevant in the astrophysical context. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:24AM |
N5.00004: Dynamo action in penetrative Boussinesq convection Invited Speaker: Dynamo action in any highly turbulent, electrically-conducting fluid medium is plausible. Dynamo amplification of the magnetic fields on the scales of the velocity patterns might be expected if the effects of diffusion and packing of fields are not too drastic. An interesting question is whether magnetic fields can be generated on scales {\sl larger} than the velocity scale. We investigate the generation of magnetic fields in a Boussinesq convecting layer, and examine the effects of including a convectively-stable layer of fluid below, of rotation, and of adding a forced shear. We examine the efficiency of the dynamo and the relative production of small-scale and large-scale magnetic fields. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 11:00AM |
N5.00005: Scale interactions in MHD turbulence and dynamo action Invited Speaker: In recent years the increase in computing power, as well as the development of subgrid models for magnetohydrodynamic (MHD) turbulence has allowed the study of a numerically almost unexplored territory in MHD flows: the regime of low magnetic Prandtl number ($P_M$). This regime is of particular importance since several astrophysical and geophysical problems are characterized by $P_M<1$, as for example in the liquid core of planets such as the Earth, or in the convection zone of stars as the Sun. Liquid metals used in dynamo experiments to generate magnetic fields are also characterized by $P_M<1$. In this talk we will review some studies of dynamo action and MHD turbulence for $P_M \leq 1$, down to $P_M \sim 5 \times 10^{-3}$. In particular, we will focus on cases where a large scale flow is present and turbulence develops as the result of an instability. Interactions between scales will be discussed, and evidence of non-local interactions involving disparate scales in simulations of MHD turbulence with resolutions up to $1536^3$ grid points will be presented. The implications of these results for universality will be briefly discussed. [Preview Abstract] |
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