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  • 1.
    Brandenburg, Axel
    et al.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Rogachevskii, Igor
    Special Issue: From Mean-Field to Large-Scale Dynamos Introduction2013In: Geophysical and Astrophysical Fluid Dynamics, ISSN 0309-1929, E-ISSN 1029-0419, Vol. 107, no 1-2, p. 1-2Article in journal (Other academic)
  • 2. Fowler, A. C.
    et al.
    Rust, Alison C.
    Vynnycky, Michael
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Casting of Metals. MACSI, University of Limerick, Ireland.
    The formation of vesicular cylinders in pahoehoe lava flows2015In: Geophysical and Astrophysical Fluid Dynamics, ISSN 0309-1929, E-ISSN 1029-0419, Vol. 109, no 1, p. 39-61Article in journal (Refereed)
    Abstract [en]

    Vertical cylinders of bubble-enriched, chemically evolved volcanic rock are found in many inflated pahoehoe lava flows. We provide a putative theoretical explanation for their formation, based on a description of a crystallising three-phase (liquid, solid, gas) crystal pile in which the water-saturated silicate melt exsolves steam and becomes more silica-rich as it crystallises anhydrous minerals. These cylinders resemble pipes that form in solidifying binary alloys as a result of sufficiently vigorous porous medium convection within the mush. A convection model with the addition of gas bubbles that provide the buoyancy source indicates that the effective Rayleigh number is too low for convection to occur in the mush of a basalt lava flow. However, the formation of gas bubbles during crystallisation means that the base state includes fluid migration up through the crystal mush even without convection. Stability considerations suggest that it is plausible to form a positive feedback where increased local porosity causes increased upwards fluid flow, which brings more silicic melt up and lowers the liquidus temperature, promoting locally higher porosity. Numerical solutions show that there are steady solutions in which cylinders form, and we conclude that this model provides a viable explanation for vesicular cylinder formation in inflated basalt lava flows.

  • 3. Kapyla, P. J.
    et al.
    Viviani, M.
    Kapyla, M. J.
    Brandenburg, Axel
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Spada, F.
    Effects of a subadiabatic layer on convection and dynamos in spherical wedge simulations2019In: Geophysical and Astrophysical Fluid Dynamics, ISSN 0309-1929, E-ISSN 1029-0419, Vol. 113, no 1-2, p. 149-183Article in journal (Refereed)
    Abstract [en]

    We consider the effect of a subadiabatic layer at the base of the convection zone on convection itself and the associated large-scale dynamos in spherical wedge geometry. We use a heat conduction prescription based on the Kramers opacity law which allows the depth of the convection zone to dynamically adapt to changes in the physical characteristics such as rotation rate and magnetic fields. We find that the convective heat transport is strongly concentrated towards the equatorial and polar regions in the cases without a substantial radiative layer below the convection zone. The presence of a stable layer below the convection zone significantly reduces the anisotropy of radial enthalpy transport. Furthermore, the dynamo solutions are sensitive to subtle changes in the convection zone structure. We find that the kinetic helicity changes sign in the deeper parts of the convection zone at high latitudes in all runs. This region expands progressively towards the equator in runs with a thicker stably stratified layer.

  • 4.
    Käpylä, Petri J.
    et al.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Mantere, M. J.
    Brandenburg, Axel
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Oscillatory large-scale dynamos from Cartesian convection simulations2013In: Geophysical and Astrophysical Fluid Dynamics, ISSN 0309-1929, E-ISSN 1029-0419, Vol. 107, no 1-2, p. 244-257Article in journal (Refereed)
    Abstract [en]

    We present results from compressible Cartesian convection simulations with and without imposed shear. In the former case the dynamo is expected to be of 2 type, which is generally expected to be relevant for the Sun, whereas the latter case refers to 2 dynamos that are more likely to occur in more rapidly rotating stars whose differential rotation is small. We perform a parameter study where the shear flow and the rotational influence are varied to probe the relative importance of both types of dynamos. Oscillatory solutions are preferred both in the kinematic and saturated regimes when the negative ratio of shear to rotation rates, qS/, is between 1.5 and 2, i.e. when shear and rotation are of comparable strengths. Other regions of oscillatory solutions are found with small values of q, i.e. when shear is weak in comparison to rotation, and in the regime of large negative qs, when shear is very strong in comparison to rotation. However, exceptions to these rules also appear so that for a given ratio of shear to rotation, solutions are non-oscillatory for small and large shear, but oscillatory in the intermediate range. Changing the boundary conditions from vertical field to perfect conductor ones changes the dynamo mode from oscillatory to quasi-steady. Furthermore, in many cases an oscillatory solution exists only in the kinematic regime whereas in the nonlinear stage the mean fields are stationary. However, the cases with rotation and no shear are always oscillatory in the parameter range studied here and the dynamo mode does not depend on the magnetic boundary conditions. The strengths of total and large-scale components of the magnetic field in the saturated state, however, are sensitive to the chosen boundary conditions.

  • 5.
    Pol, Alberto Roper
    et al.
    Univ Colorado, Dept Aerosp Engn Sci, Boulder, CO 80309 USA.;Univ Colorado, Lab Atmospher & Space Phys, Boulder, CO 80309 USA.;Ilia State Univ, Abastumani Astrophys Observ, Tbilisi, Georgia..
    Brandenburg, Axel
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Univ Colorado, Lab Atmospher & Space Phys, Boulder, CO 80309 USA.;Univ Colorado, JILA, Boulder, CO 80309 USA.;Univ Colorado, Dept Astrophys & Planetary Sci, Boulder, CO 80309 USA.;Stockholm Univ, Stockholm, Sweden.;Stockholm Univ, Dept Astron, NORDITA, Stockholm, Sweden.;Carnegie Mellon Univ, McWilliams Ctr Cosmol, Pittsburgh, PA 15213 USA.;Carnegie Mellon Univ, Dept Phys, Pittsburgh, PA 15213 USA..
    Kahniashvili, Tina
    Ilia State Univ, Abastumani Astrophys Observ, Tbilisi, Georgia.;Carnegie Mellon Univ, McWilliams Ctr Cosmol, Pittsburgh, PA 15213 USA.;Carnegie Mellon Univ, Dept Phys, Pittsburgh, PA 15213 USA.;Laurentian Univ, Dept Phys, Sudbury, ON, Canada..
    Kosowsky, Arthur
    Univ Pittsburgh, Dept Phys & Astron, Pittsburgh, PA 15260 USA.;Pittsburgh Particle Phys Astrophys & Cosmol Ctr P, Pittsburgh, PA USA..
    Mandal, Sayan
    Ilia State Univ, Abastumani Astrophys Observ, Tbilisi, Georgia.;Carnegie Mellon Univ, McWilliams Ctr Cosmol, Pittsburgh, PA 15213 USA.;Carnegie Mellon Univ, Dept Phys, Pittsburgh, PA 15213 USA..
    The timestep constraint in solving the gravitational wave equations sourced by hydromagnetic turbulence2019In: Geophysical and Astrophysical Fluid Dynamics, ISSN 0309-1929, E-ISSN 1029-0419Article in journal (Refereed)
    Abstract [en]

    Hydromagnetic turbulence produced during phase transitions in the early universe can be a powerful source of stochastic gravitational waves (GWs). GWs can be modelled by the linearised spatial part of the Einstein equations sourced by the Reynolds and Maxwell stresses. We have implemented two different GW solvers into the Pencil Code - a code which uses a third order timestep and sixth order finite differences. Using direct numerical integration of the GW equations, we study the appearance of a numerical degradation of the GW amplitude at the highest wavenumbers, which depends on the length of the timestep - even when the Courant-Friedrichs-Lewy condition is ten times below the stability limit. This degradation leads to a numerical error, which is found to scale with the third power of the timestep. A similar degradation is not seen in the magnetic and velocity fields. To mitigate numerical degradation effects, we alternatively use the exact solution of the GW equations under the assumption that the source is constant between subsequent timesteps. This allows us to use a much longer timestep, which cuts the computational cost by a factor of about ten.

  • 6.
    Qian, Chengeng
    et al.
    Beijing Inst Technol, State Key Lab Explos Sci & Technol, Beijing, Peoples R China..
    Wang, Cheng
    Beijing Inst Technol, State Key Lab Explos Sci & Technol, Beijing, Peoples R China..
    Liu, JianNan
    Taiyuan Univ Technol, Coll Min Engn, Taiyuan, Shanxi, Peoples R China..
    Brandenburg, Axel
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Stockholm, Sweden.;Stockholm Univ, Dept Astron, Stockholm, Sweden.;Univ Colorado, JILA, Boulder, CO 80309 USA.;Univ Colorado, Lab Atmospher & Space Phys, Boulder, CO 80309 USA..
    Haugen, Nils E. L.
    SINTEF Energy Res, Trondheim, Norway..
    Liberman, Michael A.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Stockholm, Sweden..
    Convergence properties of detonation simulations2019In: Geophysical and Astrophysical Fluid Dynamics, ISSN 0309-1929, E-ISSN 1029-0419Article in journal (Refereed)
    Abstract [en]

    We present a high-resolution convergence study of detonation initiated by a temperature gradient in a stoichiometric hydrogen-oxygen mixture using the PENCIL CODE and compare with a code that employs a fifth order weighted essentially non-oscillating (WENO) scheme. With Mach numbers reaching 10-30, a certain amount of shock viscosity is needed in the PENCIL CODE to remove or reduce numerical pressure oscillations on the grid scale at the position of the shock. Detonation is found to occur for intermediate values of the shock viscosity parameter. At fixed values of this parameter, the numerical error associated with those small wiggles in the pressure profile is found to decrease with decreasing mesh width like down to . With the WENO scheme, solutions are smooth at , but no detonation is obtained for . This is argued to be an artifact of a decoupling between pressure and reaction fronts.

  • 7. Schober, J.
    et al.
    Brandenburg, Axel
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Rogachevskii, l.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Kleeorin, Nathan
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Energetics of turbulence generated by chiral MHD dynamos2019In: Geophysical and Astrophysical Fluid Dynamics, ISSN 0309-1929, E-ISSN 1029-0419, Vol. 113, no 1-2, p. 107-130Article in journal (Refereed)
    Abstract [en]

    An asymmetry in the number density of left- and right-handed fermions is known to give rise to a new term in the induction equation that can result in a dynamo instability. At high temperatures, when a chiral asymmetry can survive for long enough, this chiral dynamo instability can amplify magnetic fields efficiently, which in turn drive turbulence via the Lorentz force. While it has been demonstrated in numerical simulations that this chiral magnetically driven turbulence exists and strongly affects the dynamics of the magnetic field, the details of this process remain unclear. The goal of this paper is to analyse the energetics of chiral magnetically driven turbulence and its effect on the generation and dynamics of the magnetic field using direct numerical simulations. We study these effects for different initial conditions, including a variation of the initial chiral chemical potential and the magnetic Prandtl number, . In particular, we determine the ratio of kinetic to magnetic energy, , in chiral magnetically driven turbulence. Within the parameter space explored in this study, reaches a value of approximately 0.064-0.074-independently of the initial chiral asymmetry and for . Our simulations suggest, that decreases as a power law when increasing by decreasing the viscosity. While the exact scaling depends on the details of the fitting criteria and the Reynolds number regime, an approximate result of is reported. Using the findings from our numerical simulations, we analyse the energetics of chiral magnetically driven turbulence in the early Universe.

  • 8.
    Singh, Nishant K.
    et al.
    Max Planck Inst Solar Syst Res, Gottingen, Germany..
    Raichur, Harsha
    Max Planck Inst Solar Syst Res, Gottingen, Germany..
    Kapyla, Maarit J.
    Max Planck Inst Solar Syst Res, Gottingen, Germany.;Aalto Univ, ReSoLVE Ctr Excellence, Dept Comp Sci, Aalto, Finland..
    Rheinhardt, Matthias
    Aalto Univ, ReSoLVE Ctr Excellence, Dept Comp Sci, Aalto, Finland..
    Brandenburg, Axel
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Stockholm, Sweden.;Stockholm Univ, AlbaNova Univ Ctr, Dept Astron, Stockholm, Sweden.;Univ Colorado, JILA, Boulder, CO 80309 USA.;Univ Colorado, Dept Astrophys & Planetary Sci, Boulder, CO 80309 USA.;Lab Atmospher & Space Phys, Boulder, CO USA..
    Kapyla, Petri J.
    Stockholm Univ, AlbaNova Univ Ctr, Dept Astron, Stockholm, Sweden.;Georg August Univ Gottingen, Inst Astrophys, Gottingen, Germany..
    f-mode strengthening from a localised bipolar subsurface magnetic field2019In: Geophysical and Astrophysical Fluid Dynamics, ISSN 0309-1929, E-ISSN 1029-0419Article in journal (Refereed)
    Abstract [en]

    Recent numerical work in helioseismology has shown that a periodically varying subsurface magnetic field leads to a fanning of the f-mode, which emerges from a density jump at the surface. In an attempt to model a more realistic situation, we now modulate this periodic variation with an envelope, giving thus more emphasis on localised bipolar magnetic structures in the middle of the domain. Some notable findings are: (i) compared to the purely hydrodynamic case, the strength of the f-mode is significantly larger at high horizontal wavenumbers k, but the fanning is weaker for the localised subsurface magnetic field concentrations investigated here than the periodic ones studied earlier; (ii) when the strength of the magnetic field is enhanced at a fixed depth below the surface, the fanning of the f-mode in the diagram increases proportionally in such a way that the normalised f-mode strengths remain nearly the same in different such cases; (iii) the unstable Bloch modes reported previously in case of harmonically varying magnetic fields are now completely absent when more realistic localised magnetic field concentrations are imposed beneath the surface, thus suggesting that the Bloch modes are unlikely to be supported during most phases of the solar cycle; (iv) the f-mode strength appears to depend also on the depth of magnetic field concentrations such that it shows a relative decrement when the maximum of the magnetic field is moved to a deeper layer. We argue that detections of f-mode perturbations such as those being explored here could be effective tracers of solar magnetic fields below the photosphere before these are directly detectable as visible manifestations in terms of active regions or sunspots.

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