Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session L21: Magnetohydrodynamics: Flows |
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Chair: Weili Luo, University of Central Florida Room: D139-140 |
Monday, November 21, 2016 4:30PM - 4:43PM |
L21.00001: Flow Structure Determined Enhancing and Inhibiting Convective Heat Transfers in Quasi 1D Magnetic Fluid Weili Luo, Jun Huang, Tianshu Liu We have found previously [1] that the convective flow in magnetic fluid responds to applied magnetic fields differently, depending on the relative direction of the gradient of temperature to that of the field. In this work we report the velocity profiles from these flows obtained from optical flow method. The peculiar magnetic driving force as well as the special configurations give rise to unique flow patterns, distinctly depends on the specific relative orientation of the temperature to that of field. The streamline plots indicate formation of local or global flow structures that explain the different effects of field on the heat transfer in the sample. For one configuration, the magneto-thermo convection causing the ``heat'' to be localized, stopping the equilibration process in the system [2]. We will discuss the different responses to the applied magnetic fields for two different sample configurations in terms of relative orientation of the temperature and field gradients. [1] Jun Huang and Weili Luo ``Heat Transfer Through Convection in a Quasi-One-Dimensional Magnetic Fluid\textbf{.'' }Journal of Thermal Analysis {\&} Calorimetry, 113, p449 (2013). \textbf{[2] }Jun Huang, T. Liu, and Weili Luo, preprint 2016 [Preview Abstract] |
Monday, November 21, 2016 4:43PM - 4:56PM |
L21.00002: Thin film ferromagnets acting like a compressible fluid Ezio Iacocca, Thomas Silva, Mark Hoefer Spin dynamics in ferromagnetic materials are mathematically described by the Landau-Lifshitz equation of motion. Recently, it has been shown that this equation can be exactly rewritten as a system of hydrodynamic equations [1] that are analogues of the isentropic Euler equations of compressible gas dynamics. These equations exhibit intriguing features such as a velocity-dependent pressure law and broken Galilean invariance, implying that the ferromagnet’s fluid-like physics are reference-frame dependent. A magnetic Mach number is defined from which subsonic and supersonic conditions are identified. By introducing finite-sized obstacles, we numerically observe laminar flow or the nucleation of ordered vortex-antivortex pairs in the subsonic regime; and the formation of a Mach cone, wavefronts, and irregular vortex-antivortex pairs in the supersonic regime. Our approach identifies a deep connection between ferromagnetism and fluid dynamics, enabling new predictions for thin film ferromagnets and opening up a new paradigm for magnetic research. References: [1] Iacocca, Silva, and Hoefer, arXiv:1606.01565 (2016) [Preview Abstract] |
(Author Not Attending)
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L21.00003: On-Off intermittency detected at the onset of turbulence in magnetized ionized gases Thiery Pierre The transition to turbulence is investigated in a rotating linear magnetized plasma column (MISTRAL device) and the role of the noise is emphasized. The destabilization is induced by the injection of electrons on the axis of the device biasing the anode of the source plasma. Starting from a rotating plasma (laminar regime), the slight increase of the potential of the source plasma leads to the onset of intermittent bursts in the edge corresponding to a subcritical (hysteretic) bifurcation and to the transient destruction of the stable rotating plasma column. The statistical analysis of the time series of the density at the onset of the intermittency is performed and the recurrence time of the turbulent bursts and distribution of the duration of the laminar phases are analyzed. At the threshold, a power law is found with critical exponent -3/2. This dynamical behavior is similar to On-off intermittency (Platt, Spiegel, Tresser, PRL 70, 279,1993) induced by Gaussian noise superimposed on the control parameter. When the control parameter is increased, the distribution evolves towards an exponential decay law. [Preview Abstract] |
Monday, November 21, 2016 5:09PM - 5:22PM |
L21.00004: Transition of energy transfer from MHD turbulence to kinetic plasma Yan Yang, William Matthaeus, Tulasi Parashar, Yipeng Shi, Minping Wan, Shiyi Chen The classical energy cascade scenario is of great importance in explaining the heating of corona and solar wind. One can envision that energy residing in large-scale fluctuations is transported to smaller scales where dissipation occurs and finally drives kinetic processes that absorb the energy flux and energize charged particles. Here we inquire how the cascade operates in a compressible plasma, and how the characteristics of energy transfer vary going from MHD to kinetic scales. When filtering MHD equations, we can get an apparent inertial range over which the conservative energy cascade occurs and the scale locality of energy transfer is similar to the cases of incompressible MHD turbulence. Pervasive shocks not only make a significant difference on energy cascade and magnetic amplification, but can also introduce considerable pressure dilation, a complement of viscous and ohmic dissipation that can trigger an alternative channel of the conversion between kinetic and internal energy. The procedure can also be applied to the Vlasov equation and kinetic simulation, in comparison with MHD turbulence, and is a good candidate to investigate the energy cascade process and the analogous role of the (tensor) pressure dilation in collisionless plasma. [Preview Abstract] |
Monday, November 21, 2016 5:22PM - 5:35PM |
L21.00005: Ferrofluid patterns in Hele-Shaw cells: Exact, stable, stationary shape solutions Sergio Lira, Jose Miranda We investigate a quasi-two-dimensional system composed by an initially circular ferrofluid droplet surrounded by a nonmagnetic fluid of higher density. These immiscible fluids flow in a rotating Hele-Shaw cell, under the influence of an in-plane radial magnetic field. We focus on the situation in which destabilizing bulk magnetic field effects are balanced by stabilizing centrifugal forces. In this framing, we consider the interplay of capillary and magnetic normal traction effects in determining the fluid-fluid interface morphology. By employing a vortex-sheet formalism we have been able to find a family of exact stationary $N$-fold polygonal shape solutions for the interface. A weakly nonlinear theory is then used to verify that such exact interfacial solutions are in fact stable. [Preview Abstract] |
Monday, November 21, 2016 5:35PM - 5:48PM |
L21.00006: Flow channeling in magnetohydrodynamic jets and mixing layers Divya Sri Praturi, Sharath Girimaji Incompressible hydrodynamic (HD) jets and mixing layers satisfy Rayleigh criterion, and hence are naturally susceptible to Kelvin-Helmholtz (KH) instability. However, for magnetohydrodynamic (MHD) jets and mixing layers at high magnetic field strengths we show by means of linear analysis and numerical simulations that the interaction of shear with Alfv\'en waves ceases KH instability to exist. Linear analysis shows that the behavior of velocity perturbation along the direction of shear is crucial in the evolution of KH instability. We prove that the evolution of this velocity perturbation becomes oscillatory at high magnetic field strengths. This oscillatory behavior results in the confinement of the fluid particle displacement to a narrow region in the flow. We label this phenomenon "flow channeling" to indicate the confinement of the fluid particles without needing any physical boundaries. [Preview Abstract] |
Monday, November 21, 2016 5:48PM - 6:01PM |
L21.00007: Energy interactions in homogeneously sheared magnetohydrodynamic flows Diane Collard, Divya Sri Praturi, Sharath Girimaji We investigate the behavior of homogeneously sheared magnetohydrodynamic (MHD) flows subject to perturbations in various directions. We perform rapid distortion theory (RDT) analysis and direct numerical simulations (DNS) to examine the interplay between magnetic, kinetic, and internal energies. For perturbation wavevectors oriented along the spanwise direction, RDT analysis shows that the magnetic and velocity fields are decoupled. In the case of streamwise wavevectors, the magnetic and velocity fields are tightly coupled. The coupling is "harmonic" in nature. DNS is then used to confirm the RDT findings. Computations of spanwise perturbations indeed exhibit behavior that is impervious to the magnetic field. Computed streamwise perturbations exhibit oscillatory evolution of kinetic and magnetic energies for low magnetic field strength. As the strength of magnetic field increases, the oscillatory behavior intensifies even as the energy magnitude decays, indicating strong stabilization. [Preview Abstract] |
Monday, November 21, 2016 6:01PM - 6:14PM |
L21.00008: A Gas-kinetic Scheme for the Two-Fluid MHD Equations with Resistivity Steven Anderson, Sharath Girimaji, Eduardo Da Silva, Diogo Siebert, Juan Salazar The two-fluid MHD equations are a simplified model of plasma flow wherein a mixture of two species (electrons and ions) is considered. In this model, unlike single-fluid MHD, quasi-neutrality is not enforced, Ohm's Law is not used, and the fluids are not in thermal equilibrium - thus both fluids assume their own density, velocity, and temperature. Here we present a numerical scheme to solve the two-fluid MHD equations based on an extension of the gas-kinetic method. In contrast to previous implementations of the gas-kinetic scheme for MHD, the solution of the non-equilibrium distribution function for each gas at the cell interface is extended to include the effect of the electromagnetic forces as well as the inter-species collisions (resistivity). Closure of the fluid equations with the electromagnetic fields is obtained through Maxwell's equations, and physically correct divergences are enforced via correction potentials. Maxwell's equations are integrated via a simple Lax-Friedrichs type flux-splitting. To separate integration of the source and flux terms in the governing equations we use Strang splitting. Some numerical results are presented to demonstrate accuracy of the scheme and we discuss advantages and potential applications of the scheme. [Preview Abstract] |
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