David C. Montgomery

Publications (6)11.88 Total impact

• Article: Intrinsic Rotation of Toroidally Confined Magnetohydrodynamics.

  Jorge A Morales, Wouter J T Bos, Kai Schneider, David C Montgomery
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  ABSTRACT: The spatiotemporal self-organization of viscoresistive magnetohydrodynamics in a toroidal geometry is studied. Curl-free toroidal magnetic and electric fields are imposed. It is observed in our simulations that a flow is generated, which evolves from dominantly poloidal to toroidal when the Lundquist numbers are increased. It is shown that this toroidal organization of the flow is consistent with the tendency of the velocity field to align with the magnetic field. Up-down asymmetry of the geometry causes the generation of a nonzero toroidal angular momentum.
  Physical Review Letters 10/2012; 109(17):175002. · 7.37 Impact Factor
• Source

  Available from: Pablo Mininni

  Article: Low magnetic Prandtl number dynamos with helical forcing.

  Pablo D Mininni, David C Montgomery
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  ABSTRACT: We present direct numerical simulations of dynamo action in a forced Roberts flow. The behavior of the dynamo is followed as the mechanical Reynolds number is increased, starting from the laminar case until a turbulent regime is reached. The critical magnetic Reynolds for dynamo action is found, and in the turbulent flow it is observed to be nearly independent on the magnetic Prandtl number in the range from approximately 0.3 to approximately 0.1. Also the dependence of this threshold with the amount of mechanical helicity in the flow is studied. For the different regimes found, the configuration of the magnetic and velocity fields in the saturated steady state are discussed.
  Physical Review E 12/2005; 72(5 Pt 2):056320. · 2.26 Impact Factor
• Source

  Available from: Pablo Mininni

  Article: Numerical solutions of the three-dimensional magnetohydrodynamic alpha model.

  Pablo D Mininni, David C Montgomery, Annick Pouquet
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  ABSTRACT: We present direct numerical simulations and alpha -model simulations of four familiar three-dimensional magnetohydrodynamic (MHD) turbulence effects: selective decay, dynamic alignment, inverse cascade of magnetic helicity, and the helical dynamo effect. The MHD alpha model is shown to capture the long-wavelength spectra in all these problems, allowing for a significant reduction of computer time and memory at the same kinetic and magnetic Reynolds numbers. In the helical dynamo, not only does the alpha model correctly reproduce the growth rate of magnetic energy during the kinematic regime, it also captures the nonlinear saturation level and the late generation of a large scale magnetic field by the helical turbulence.
  Physical Review E 05/2005; 71(4 Pt 2):046304. · 2.26 Impact Factor
• Source

  Available from: Annick Pouquet

  Article: An alternative interpretation for the Holm "alpha model"

  David C Montgomery, Annick Pouquet
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  ABSTRACT: By re-interpreting a recent successful closure procedure in terms of local spatial averaging and the neglect of fluctuations about that average, it is shown how the results of that closure scheme (the ''alpha model'') may be more simply derived.
  08/2002;
• Source

  Available from: arxiv.org

  Article: Apparent suppression of turbulent magnetic dynamo action by a dc magnetic field

  David C. Montgomery, W. H. Matthaeus, L J Milano, P Dmitruk
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  ABSTRACT: Numerical studies of the effect of a dc magnetic field on dynamo action (development of magnetic fields with large spatial scales), due to helically-driven magnetohydrodynamic turbulence, are reported. The apparent effect of the dc magnetic field is to suppress the dynamo action, above a relatively low threshold. However, the possibility that the suppression results from an improper combination of rectangular triply spatially-periodic boundary conditions and a uniform dc magnetic field is addressed: heretofore a common and convenient computational convention in turbulence investigations. Physical reasons for the observed suppression are suggested. Other geometries and boundary conditions are offered for which the dynamo action is expected not to be suppressed by the presence of a dc magnetic field component. Comment: To appear in Physics of Plasmas
  02/2002;
• Source

  Available from: arxiv.org

  Article: Pressure determinations for incompressible fluids and magnetofluids

  Brian T. Kress, David C. Montgomery
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  ABSTRACT: Certain unresolved ambiguities surround pressure determinations for incompressible flows, both Navier-Stokes and magnetohydrodynamic. For uniform-density fluids with standard Newtonian viscous terms, taking the divergence of the equation of motion leaves a Poisson equation for the pressure to be solved. But Poisson equations require boundary conditions. For the case of rectangular periodic boundary conditions, pressures determined in this way are unambiguous. But in the presence of "no-slip" rigid walls, the equation of motion can be used to infer both Dirichlet and Neumann boundary conditions on the pressure P, and thus amounts to an over-determination. This has occasionally been recognized as a problem, and numerical treatments of wall-bounded shear flows usually have built in some relatively ad hoc dynamical recipe for dealing with it, often one which appears to "work" satisfactorily. Here we consider a class of solenoidal velocity fields which vanish at no-slip walls, have all spatial derivatives, but are simple enough that explicit analytical solutions for P can be given. Satisfying the two boundary conditions separately gives two pressures, a "Neumann pressure" and a "Dirichlet pressure" which differ non-trivially at the initial instant, even before any dynamics are implemented. We compare the two pressures, and find that in particular, they lead to different volume forces near the walls. This suggests a reconsideration of no-slip boundary conditions, in which the vanishing of the tangential velocity at a no-slip wall is replaced by a local wall-friction term in the equation of motion.
  12/2000;