Bulletin of the American Physical Society
2005 47th Annual Meeting of the Division of Plasma Physics
Monday–Friday, October 24–28, 2005; Denver, Colorado
Session FZ1: Mini-conference on Reconnection and Turbulence in Fluids and Plasmas I |
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Chair: David Montgomery, Dartmouth College Room: Adam's Mark Hotel Plaza Ballroom D |
Tuesday, October 25, 2005 9:30AM - 10:00AM |
FZ1.00001: Multiscale Problems in Fluid and MHD: Combining direct numerical simulations and models Annick Pouquet, Pablo Mininni, David Montgomery Geophysical flows, with a huge number of interacting scales, cannot be studied with direct numerical simulations (DNS) without proper modeling. In this context, DNS, Lagragian-averaged (LAMHD) and Large-Eddy Simulations (LES) runs of magnetohydrodynamics are presented. The models allow for a significant reduction of computer resources at given Reynolds numbers; e.g., with LAMHD, one reproduces the growth rate of magnetic energy and captures the saturation level of the dynamo instability. Combining DNS, LAMHD and LES, low magnetic Prandlt number dynamos have then been explored. Several forcing, from Beltrami to fully non-helical, are used and give similar though not identical results. In the case of the Taylor-Green vortex with a well defined structure at large scales and strong turbulent fluctuations, dynamos are observed down to the lowest PM=0.01 that can be modeled accurately; the critical magnetic Reynolds number increases sharply with PM as turbulence sets in and then saturates; in the linear phase, the most unstable magnetic modes move to small scales as PM is decreased; a Kazantsev 3/2 spectrum develops with strong non-local nonlinear transfer. [Preview Abstract] |
Tuesday, October 25, 2005 10:00AM - 10:30AM |
FZ1.00002: Current and Vortex Singularities: Drivers of Impulsive Reconnection in Plasmas and Fluids A. Bhattacharjee, K. Germaschewski, C.S. Ng Reconnection in nature is often impulsive or bursty, characterized not only by a fast growth rate but a rapid change in the time-derivative of the growth rate. We present analytical and numerical results, obtained by asymptotic analyses and high-resolution numerical simulations (using AMR) of the Hall MHD and Euler equations. Within the framework of Hall MHD, we consider a two-dimensional collisionless reconnection model in which electron inertia provides the mechanism for breaking field lines, and the electron pressure gradient plays a crucial role in controlling magnetic island dynamics. Current singularities tend to form in finite time and drive fast and impulsive reconnection. By a combination of analysis and simulations, we determine the scaling of the reconnection rate in the nonlinear regime, and demonstrate its dependence on the electron and the ion skin depth, plasma beta, and system size. We also present new results on the analogous problem of finite-time vortex singularity in a high-symmetry Kida flow containing null points of the flow. It is found that while the system tends to a self-similar vortex collapse solution in the early nonlinear stage, in the late nonlinear stage the self-similarity is broken and the solution becomes exponential in time. Eventually viscosity intervenes, producing fast vortex reconnection. [Preview Abstract] |
Tuesday, October 25, 2005 10:30AM - 10:45AM |
FZ1.00003: BREAK
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Tuesday, October 25, 2005 10:45AM - 11:15AM |
FZ1.00004: Dynamics of Vortex Lines in Turbulent 3D Stratified Flows Philip Marcus Within the context of examining the flows within ProtoPlanetary Disks (PPDs), we have numerically computed long-lived, 3D, spatially-compact vortices. These vortices are robust; they last many turn-around times, and like-signed vortices interact with each other by stretching and merging. The vorticity in the mid-plane of each vortex is primarily aligned with the rotation axis of the nearly-Keplerian PPD. Because the vortices have limited vertical extent and because the vortex lines cannot terminate, an open question has been what is the geometry of the lines outside the region normally associated with the vortex. Do vortex lines pass through multiple vortices? Do they become chaotically tangled in the turbulence exterior to the vortex? When vortices merge, is there breaking and reconnection of the lines? We show that most of the lines form dipolar field and that entanglement is the exception rather than the rule in vortex merger. [Preview Abstract] |
Tuesday, October 25, 2005 11:15AM - 11:45AM |
FZ1.00005: Baroclinic Turbulence in Accelerated Inhomogeneous Flows Norman Zabusky This talk presents an overview and recent understanding of \textit{accelerated inhomogeneous} \textit{flows} (AIFs) or shock-accelerated Richtmyer-Meshkov flows. We use the vortex paradigm and the visiometric approach. We ``project'' data to lower dimensions to quantify, validate simulations of and model phenomena involving coherent space-time events. We emphasize our recent work,\footnote{Shuang Zhang , Jian Chen and Norman J. Zabusky ``Turbulent Decay and Mixing of accelerated inhomogeneous Flows via a Feature Based Analysis'' SIAM J. Sci. Comput. $'04 $ \textbf{26,}, pp. 86--10.}$^{,}$\footnote{Shuang Zhang, Norman J. Zabusky, Gao-Zhu Peng , ``Vortex dynamics and baroclinically forced inhomogeneous turbulence~for shock - planar heavy curtain interactions'' J. of Turbulence, `05.} including vortex induced \textit{secondary baroclinic circulation generation} which yields more positive and negative circulation through intermediate times than the original shock-accelerated vortex deposition. In addition we quantify the effects of the initial \textit{thickness} of the interfacial transition layer and the ubiquity of vortex projectiles and transition to turbulence for the shock-curtain interaction. [Preview Abstract] |
Tuesday, October 25, 2005 11:45AM - 12:15PM |
FZ1.00006: Laboratory sodium experiments modeling astrophysical and geophysical MHD flows Daniel Lathrop Numerous systems are effected by rotation and magnetic fields. These include astrophysical and geophysical settings: stellar convective zones, planetary interiors, accretion disks, and galaxies as a whole, as well as laboratory plasma devices. Experiments in liquid sodium are becoming important to understanding instabilities leading to turbulence and enhanced transport where rotation and magnetic fields interact. There is theoretical and computational evidence for the importance of small magnetic fields in destabilizing differential rotation. This talk will describe the first direct observation of this instability, and important future directions in understanding MHD instabilities and turbulent transport. Beyond confirming established theory, these experiments are important to stimulate and benchmark new theoretical and computational tools to enable prediction of turbulent transport at physically realistic parameters. [Preview Abstract] |
Tuesday, October 25, 2005 12:15PM - 12:35PM |
FZ1.00007: Rayleigh-Taylor turbulent mixing of immiscible, miscible and stratified fluids Snezhana I. Abarzhi, Andrey Gorobets, Katepalli R. Sreenivasan We present a heuristic model describing the Rayleigh-Taylor(RT) turbulent mixing of immiscible, miscible, and stratified fluids. The model does not presume a single-scale character of the interface dynamics and distinguishes between the evolution of horizontal and vertical scales. For fluids with constant densities, the results obtained indicate two distinct mechanisms for the mixing development. The former is the traditional “merge” associated with the growth of horizontal scales. The latter is associated with the production of small- scale structures and with the growth of the vertical scale, which plays the role of the integral scale for energy dissipation. In RT mixing, the rate of momentum loss is the flow invariant, whereas the energy dissipation rate is not, and the fundamental scaling properties of the accelerated flow differ from those of the classical Kolmogorov turbulence. The model considers the influence of turbulent diffusion and stratification on mixing process. We show that turbulent diffusion calculated through temperature fluctuations does not stop mixing, but decreases its growth-rate significantly, makes it time-dependent and sensitive to the initial conditions. In a stratified density profile, the mixing process is terminated. [Preview Abstract] |
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FZ1.00008: Mini-conference Poster: 3D Onset and Propagation of Magnetic Reconnection Giovanni Lapenta, P. Ricci, J.U. Brackbill, W. Daughton, G.L. Delzanno A fundamental problem in reconnection physics is how to relate 2D and 3D reconnection. The great majority of studies of reconnection focus on 2D configurations. The state of affair is best described by the cave allegory of Plato. In 2D we look at the reconnection process as if we were studying the real world through its reflections on the walls of a cave [1]. We propose to turn our head away from the familiar wall and face the real world. We do 3D fully kinetic (i.e. both electrons and ions are kinetic) simulations of reconnection. We use the implicit PIC code CELESTE to achieve parameter ranges and system sizes inaccessible to traditional explicit codes. Our results will clarify the fundamental issue: if reconnection is started at one location, does it propagate in the system? How? And what coupling of microscopic and macroscopic processes causes the reconnection onset in the first place? All these effects can only be studied in 3D [2]. The crucial physics missing in 2D simulations is the role of macroscopic equilibrium changes induced by the microinstabilities [3].\\ 1) Plato, Republic, Book VII, Hackett Pub. Co., 1992. \\ 2) P. Ricci, J.U. Brackbill, W.S. Daughton, G. Lapenta, Phys. Plasmas, 11, 4489, 2004.\newline 3) W. Daughton, G. Lapenta, P. Ricci, Phys. Rev. Lett., 93, 105004, 2004. [Preview Abstract] |
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FZ1.00009: Mini-conference Poster: Magnetic reconnection in nontoroidal plasmas Allen Boozer Magnetic reconnection in solar and astrophysical plasmas differs fundamentally from the formation of magnetic islands that is characteristic of reconnection in toroidal plasmas. At any instant a generic magnetic field has only point nulls, which can be shown to imply that the evolution of a generic field is consistent, near each spatial point, with being embedded in a perfectly conducting fluid {Phys. Rev. Lett. $<$88$>$, 215005 (2002)}. This result implies, in doubly periodic systems, that the nonideal evolution of the magnetic field lines is localized to surfaces on which the magnetic field lines close on themselves, the rational surfaces. That is, the rational surfaces split to form magnetic islands. Rational surfaces are not a credible explanation for reconnection in non-laboratory plasmas--different mechanisms are required. We have shown {Phys. Plasmas $<$12$>$, 070706, (2005)} that the exponentially increasing separation of neighboring magnetic field lines, which is generic, tends to produce rapid magnetic reconnection if the length of the field lines is greater than about 20 times the exponentiation, or Lyapunov, length. This derivation and the importance of this result will be discussed. [Preview Abstract] |
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FZ1.00010: Mini-conference Poster: Thin current sheets and plasma bubbles Mikhail Sitnov, Parvez Guzdar, Marc Swisdak The concept of underpopulated flux tubes or plasma bubbles has been introduced by Pontius and Wolf [1990] to resolve the convection crisis problem in the tail of Earth's magnetosphere. Plasma bubbles may convect much faster than the rest of the tail plasmas because of the buoyancy effect. The existence of bubbles and their relation to bursty bulk flows is confirmed by many observations. However, both formation and properties of bubbles remain insufficiently understood. We propose a simple self-consistent model, which describes properties of the tail current sheet after the formation of a small plasmoid on the closed field lines in the magnetotail and its quick tailward retreat. The model is based on the theory of forced magnetic reconnection, which is modified and re-interpreted, invoking stability properties of geomagnetotail plasmas. It is shown that the flux tube that contained a plasmoid becomes a plasma bubble, with the plasmoid being replaced by a thin current sheet embedded within the original thicker sheet. The model allows for an estimate of the decrease of the plasma entropy per unit magnetic flux within the bubble. It predicts that the bubble structure may also include bifurcated current sheets. The predictions are consistent with recent multi-probe observations of fast flows and atypical (embedded and bifurcated) current sheets from the Cluster mission. [Preview Abstract] |
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