Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session A03: Magnetohydrodynamics |
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Chair: Douglas Kelly, University of Rochester Room: Georgia World Congress Center B204 |
Sunday, November 18, 2018 8:00AM - 8:13AM |
A03.00001: Decay of turbulence in a duct with transverse magnetic field Oleg Zikanov, Dmitry Krasnov, Thomas Boeck, Semion Sukoriansky Decay of honeycomb-generated turbulence in a duct with a static transverse magnetic field is studied via high-resolution direct numerical simulations. The work follows the experimental study (Sukoriansky et al, Exp. Fluids 1986), in which the paradoxical observation has been made that high-amplitude velocity fluctuations exist in the far-downstream part of the duct when the honeycomb exit is located in the zone of strong magnetic field. It is shown in the simulations that such fluctuations are caused by large-scale quasi-two-dimensional structures forming in the flow at the initial stages of the decay and surviving the magnetic suppression. Turbulence properties, in particular the two-point correlation statistics and typical length scales are computed. The study demonstrates that turbulence decay in the presence of a magnetic field is a complex phenomenon critically affected by the state of the flow at the moment the field is introduced. |
Sunday, November 18, 2018 8:13AM - 8:26AM |
A03.00002: On the ideal two-fluid plasma equations and magnetohydrodynamics Naijian Shen, Yaun Li, Dale I. Pullin, Ravi Samtaney, Vincent Wheatley The ideal two-ﬂuid plasma equations, obtained from truncating moments of the Vlasov-Boltzmann equation, are increasingly used to describe an ion-electron plasma whose transport phenomena occur on a time scale slower, and length scale longer than those of particle collisions. A similar treatment under more stringent constraints gives the well-known single-ﬂuid, ideal magnetohydrodynamic (MHD) equations for low-frequency macroscopic processes. Here we perform a sequence of formal asymptotic expansions for the dimensionless, ideal two-ﬂuid plasma equations, with respect to limiting values of the speed-of-light c, the ion-to-electron mass ratio M and the plasma skin depth d_{S}. Three different closed MHD systems result including the well-known Hall-MHD and single-fluid MHD equations. It is shown that while the zeroth-order description corresponding to the infinite c limit (with M, d_{S} fixed) is strictly charge neutral, it nonetheless uniquely determines the perturbation charge non neutrality at first order. Further, the additional M -> ∞ limit is found to be not required to obtain the single-fluid MHD equations, despite being essential for the Hall-MHD system. |
Sunday, November 18, 2018 8:26AM - 8:39AM |
A03.00003: Magnetoconvection in vertical channels with a liquid metal downflow and a strong transverse magnetic field Yaroslav Listratov, Xuan Zhang, Ivan Belyaev, Oleg Zikanov Downward flows in vertical ducts and pipes with non-uniform wall heating and transverse magnetic field are studied experimentally and numerically. The work is motivated by the design of liquid metal blankets for tokamak fusion reactors. The configuration considered is related to the blanket module with poloidal channels, in which liquid metal is pumped downward and the main component of the magnetic field is perpendicular to the flow direction. Strong heating flux is deposited at the wall facing the reaction chamber. A peculiar manifestation of the magnetoconvection effect, which also has signiﬁcant implications for the blanket design is that of anomalous high-amplitude low-frequency ﬂuctuations of temperature that appear at strong heating and strong magnetic field. They have been observed in the recent experiments for various orientations toward gravity and magnetic ﬁeld. The analysis of experimental data and DNS shows that in a vertical channel the fluctuations are caused by growth and quasi-periodic breakdown of the pairs of ascending and descending jets related to the elevator-mode thermal convection. |
Sunday, November 18, 2018 8:39AM - 8:52AM |
A03.00004: Many-particle dispersion in anisotropic magnetohydrodynamic turbulence Jane L Pratt, Angela Busse, Wolf-Christian Müller Magnetohydrodynamic (MHD) turbulence is an essential aspect of a wide range of astrophysical systems, among them convection in stellar interiors, mixing of material from accretion disks into stars, and the explosive dynamics in stellar atmospheres that drive stellar winds. MHD turbulence differs structurally from hydrodynamic turbulence. For example, in hydrodynamic turbulence small-scale vorticity tends to be organized in the form of vortex filaments, but in MHD turbulence vortex sheets form. We investigate the structure of MHD turbulence from the Lagrangian viewpoint based on the trajectories of many Lagrangian tracer particles. We evaluate standard Lagrangian statistics such as single-particle diffusion curves and velocity autocorrelation functions. We also demonstrate that the convex hull of a group of dense Lagrangian tracer particles can provide new information about anisotropic dispersion, derived from the surface area and volume of a cluster of particles. |
Sunday, November 18, 2018 8:52AM - 9:05AM |
A03.00005: Simple model of the slosh instability in aluminium reduction cells Ibrahim Mohammad, Douglas H Kelley The “slosh” instability occurs when Lorentz forces drive gravity waves by exciting coupled gravitational wave modes. This instability occurs in Hall-Heroult reduction cells which are used to manufacture aluminium. These cells consist of a horizontally large and shallow layer of molten aluminium under a layer of molten electrolyte, with both layers in between large carbon electrodes. Due to the high electrical resistivity of the electrolyte, a slight tilting of the interface between the aluminium and the electrolyte substantially redistributes the current in the cell. As a result, horizontal currents appear in the aluminum layer and interact with vertical ambient magnetic fields, giving rise to a Lorentz Force which drives the instability. Davidson and Lindsay proposed a simple mechanical analogue that captures the important physics of the instability ("Stability of Interfacial Waves in Aluminium Reduction Cells" JFM, 1998). In this talk, I will present the proposed mechanical model through discussing simplifying assumptions and deriving governing equations. I will also show numerical results illustrating instability at parameters matching the transition criterion derived in Davidson’s paper. |
Sunday, November 18, 2018 9:05AM - 9:18AM |
A03.00006: Numerical investigation of heat transport in a MHD flow past a wedge over a stretching surface in different nanofluids Mittu Walia The present article theoretically discusses the numerical study of thermally driven fluid past a wedge, MHD flow of nanofluids over a stretching surface. We examined the properties of the solution profiles for flow regime for all possible flows. Continuum conservation laws are the basis of this investigation. To solve the governing incompressible Navier-Stokes equation along with the energy we introduced the analytical method. Computations are carried out for fluid flows and temperature profiles for different nanofluids taking metals and oxides as nano-particles with water as a base fluid. The effects of ascending power of nanoparticle volume fraction on the velocity and temperature field are captured and discussed. A comparative analysis is obtained for distinct volume fractions of nano-particles, Hartmann, and Hartee numbers. The fluid flow approach towards the accelerated and decelerated flows followed by multiple solutions is also observed. |
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