Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session A36: Magnetohydrodynamics |
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Chair: Andre Thess, Institute of Engineering Thermodynamics, Stuttgart Room: Alcove A |
Sunday, November 23, 2014 8:00AM - 8:13AM |
A36.00001: Flow regimes in an electromagnetically forced circular Couette system Jean Boisson, Vincent Padilla, Fran\c{c}ois Daviaud, S\'{e}bastien Auma\^Itre We present an experimental study of a liquid metal flow electromagnetically forced in a large aspect ratio coaxial cylindrical geometry with and without a free surface. An azimuthal Lorentz force is applied on the liquid metal gap, through a radial current and an axial magnetic field. Using ultrasonic velocity measurements and direct visualisation of the free surface, we focus on the effect of these two parameters on the flow properties. We show that, depending on the strength of the magnetic field and not only on the applied Lorentz force, dynamical states exist in the two geometries. We first observe a stationary structure at low forcing. Then, two dynamical regimes are exhibited at higher forcing. We characterize them by their different frequencies and speeds. Higher magnetic fields clearly promote the faster regimes. Connections with other magnetohydrodynamics instabilities will be discussed. [Preview Abstract] |
Sunday, November 23, 2014 8:13AM - 8:26AM |
A36.00002: Numerical simulation of partially ionized gas flows under the influence of electromagnetic fields Konstantinos Panourgias, John Ekaterinaris Partially ionized gases under the influence of electromagnetic fields are described through the coupled system of the compressible Navier-Stokes equations augmented by the equations of species in the mixture (electrons, ions, atoms) and the Maxwell equations. The coupled system is completed with an energy equation for electrons. Stiff source terms encompass the interactions of fluid flow with electromagnetic fields and resulting system of equations is solved numerically. The discontinuous Galerkin finite element method is used for the numerical solution of the above system.For the Maxwell equations, DG method is performed using a divergence free vector basis for the magnetic field in order to preserve zero divergence in the element and retain the global implicit constraint of a divergence free magnetic field vector down to very low levels. In order to avoid severe time step limitations for the Maxwell system, implicit time marching is used with high order implicit Rugne-Kutta methods. The coupled system of the Navier-Stokes and the Maxwell equations is advanced in time simultaneously to avoid wrong wave shapes and propagation speeds that are obtained when the coupling source terms are lagged in time.The method is applied for supersonic plasma flows in strong electromagnetic fields. [Preview Abstract] |
Sunday, November 23, 2014 8:26AM - 8:39AM |
A36.00003: Inverse cascades and the evolution of decaying magnetohydrodynamic turbulence Moritz Linkmann, Arjun Berera Ensemble averaged high resolution direct numerical simulations of inverse cascade are presented, extending on the many single realization numerical studies done up to now. This identifies inverse cascade as a statistical property of magnetohydrodynamic turbulence and thus permits reliable numerical exploration of its dynamics. Our results show that at early times during the decay the properties of the ensemble average are represented by one realization, as the deviations between realizations are small. In contrast, at late times we measure significant deviations between realizations, thus the ensemble average cannot be avoided in this time frame. This is important for measurements of the magnetic energy decay exponent, which has been determined from these ensemble runs to be $n_E = (0.47 \pm 0.03) + (13.9 \pm 0.8)/R_{\lambda}$ for initially helical magnetic fields. We show for the first time that even after removing the Lorentz force term in the momentum equation, thus decoupling it from the induction equation, inverse cascade persists. The induction equation is now a linear partial differential equation with an externally imposed velocity field, thus amenable to numerous analysis techniques. A new door has opened for analyzing inverse cascade, with various ideas discussed. [Preview Abstract] |
Sunday, November 23, 2014 8:39AM - 8:52AM |
A36.00004: Azimuthal magnetorotational instabilities to non-axisymmetric perturbations Yasuhide Fukumoto, Rong Zou Short-wavelength stability analysis is made of axisymmetric rotating flows of a perfectly conducting fluid subjected to external azimuthal magnetic field, to non-axisymmetric as well as axisymmetric perturbations. The instability caused by the azimuthal magnetic field is referred to as the azimuthal magnetorotational instability (AMRI). We determine the range of unstable angular-velocity distribution and the overall maximum growth rate for the AMRI. Non-axisymmetric perturbations, when coupled to azimuthal magnetic field, widen the instability range of angular-velocity profiles of rotating flows. For strong external field, the maximum growth rate increases, beyond the Oort A-value, without bound in proportion to the strength of the external field. The effect of the electric resistivity is also considered in the limit of very low magnetic Prandtl number. [Preview Abstract] |
Sunday, November 23, 2014 8:52AM - 9:05AM |
A36.00005: Spectra and correlations in the solar wind from Voyager 2 around 5 AU Luca Gallana, Federico Fraternale, Michele Iovieno, Enrico Magli, Sophie Fosson, Merav Opher, John Richardson, Daniela Tordella Solar wind spectra deduced from the data recorded by the Voyager 2 mission during 1979 at about 5 astronomical units from the sun are considered. The data are time series which contain voids that typically become larger and irregularly sparse as the craft moves away from the sun (45\%\ missing data in 1979). By extracting complete subsets and filling gaps with different techniques (polynomial interpolation, Rybicki (AJ 1992) and compressed sensing (e.g. Candes et al. CPAM 2006) reconstruction methods, global DFT for irregularly spaced data) we obtain velocity and magnetic field fluctuations between $10^{-5}$ and $10^{-2}$ Hz in the MHD inertial range of solar wind. Spectra of all variables show a power law scaling with exponents in between -1.5 and -1.8. PDFs and correlations indicate that the flow has a significant intermittency. The reliability of the reconstruction methods used is analyzed by introducing the same sequence of gaps observed in the Voyager data into a reference dataset extracted from direct numerical simulations of incompressible Navier-Stokes turbulence as well as from synthetic turbulence, and then by comparing the statistics obtained with those of the complete reference dataset. [Preview Abstract] |
Sunday, November 23, 2014 9:05AM - 9:18AM |
A36.00006: Turbulent convection in a horizontal duct with strong axial magnetic field Xuan Zhang, Oleg Zikanov Convection in a horizontal duct with one heated wall is studied computationally. The work is motivated by the concept of a blanket for fusion reactors, according to which liquid metal slowly flows in toroidal ducts aligned with the main component of the magnetic field. We first assume that the magnetic field is sufficiently strong for the flow to be purely two-dimensional and analyze chaotic flow regimes at very high Grashof numbers. Furthermore, three-dimensional perturbations are considered and the relation between the length of the duct and the critical Hartmann number, below which the flow becomes three-dimensional, is determined. [Preview Abstract] |
Sunday, November 23, 2014 9:18AM - 9:31AM |
A36.00007: ABSTRACT WITHDRAWN |
Sunday, November 23, 2014 9:31AM - 9:44AM |
A36.00008: On the dynamic behavior of the flow past a magnetic obstacle Alberto Beltran, Roberto Dominguez-Lozoya, Joel Roman, Eduardo Ramos, Sergio Cuevas We study numerically the duct flow of an electrically conducting incompressible viscous fluid (a liquid metal) past a a localized magnetic field, namely, a {\it magnetic obstacle}. We use a quasi-two-dimensional model based on a formulation that includes the induced magnetic field as electromagnetic variable ($B$-formulation) and analyze the stability of the flow in the parametric space of the Hartmann and Reynolds numbers. We find that even though for a given strength of the localized braking Lorentz force (characterized by the Hartmann number) the flow may become unstable and give rise to a time-periodic wake, when a critical Reynolds number is reached, a further increase in the Reynolds number may result in the flow becoming steady again. Evidently, this behavior is not observed in the flow past a solid obstacle. Experimental observations carried out in a liquid metal (GaInSn) duct flow suggest that this prediction is correct. [Preview Abstract] |
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