| 1. |
- Brandenburg, Axel, et al.
(author)
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Introduction
- 2020
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In: Geophysical and Astrophysical Fluid Dynamics. - : Taylor & Francis. - 0309-1929 .- 1029-0419. ; 114:1-2, s. 1-7
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Journal article (other academic/artistic)
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| 2. |
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| 3. |
- Brandenburg, Axel, et al.
(author)
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The time step constraint in radiation hydrodynamics
- 2020
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In: Geophysical and Astrophysical Fluid Dynamics. - : Taylor & Francis. - 0309-1929 .- 1029-0419. ; 114:1-2, s. 162-195
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Journal article (peer-reviewed)abstract
- Explicit radiation hydrodynamic simulations of the atmospheres of massive stars and of convection in accretion discs around white dwarfs suffer from prohibitively short time steps due to radiation. This constraint is related to the cooling time rather than the radiative pressure, which also becomes important in hot stars and discs. We show that the radiative time step constraint is governed by the minimum of the sum of the optically thick and thin contributions rather than the smaller one of the two. In simulations with the Pencil Code, their weighting fractions are found empirically. In three-dimensional convective accretion disc simulations, the Deardorff term is found to be the main contributor to the enthalpy flux rather than the superadiabatic gradient. We conclude with a discussion of how the radiative time step problem could be mitigated in certain types of investigations.
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| 4. |
- Brandenburg, Axel, et al.
(author)
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Yoshizawa's cross-helicity effect and its quenching
- 2013
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In: Geophysical and Astrophysical Fluid Dynamics. - : Informa UK Limited. - 0309-1929 .- 1029-0419. ; 107:1-2, s. 207-217
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Journal article (peer-reviewed)abstract
- A central quantity in mean-field magnetohydrodynamics is the mean electromotive force , which in general depends on the mean magnetic field. It may however also have a part independent of the mean magnetic field. Here we study an example of a rotating conducting body of turbulent fluid with non-zero cross-helicity, in which a contribution to proportional to the angular velocity occurs (Yoshizawa, A., Self-consistent turbulent dynamo modeling of reversed field pinches and planetary magnetic fields. Phys. Fluids B 1990, 2, 15891600). If the forcing is helical, it also leads to an effect, and large-scale magnetic fields can be generated. For not too rapid rotation, the field configuration is such that Yoshizawa's contribution to is considerably reduced compared to the case without effect. In that case, large-scale flows are also found to be generated.
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| 5. |
- Chatterjee, Piyali, et al.
(author)
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Can catastrophic quenching be alleviated by separating shear and α effect?
- 2010
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In: Geophysical and Astrophysical Fluid Dynamics. - : Informa UK Limited. - 0309-1929 .- 1029-0419. ; 104:5&6, s. 591-599
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Journal article (peer-reviewed)abstract
- The small-scale magnetic helicity produced as a by-product of the large-scale dynamo is believed to play a major role in dynamo saturation. In a mean-field model the generation of small-scale magnetic helicity can be modelled by using the dynamical quenching formalism. Catastrophic quenching refers to a decrease of the saturation field strength with increasing Reynolds number. It has been suggested that catastrophic quenching only affects the region of non-zero helical turbulence (i.e. where the kinematic alpha operates) and that it is possible to alleviate catastrophic quenching by separating the region of strong shear from the alpha layer. We perform a systematic study of a simple axisymmetric two-layer alpha-omega dynamo in a spherical shell for Reynolds numbers in the range 1 < Rm < 10^5. In the framework of dynamical quenching we show that this may not be the case, suggesting that magnetic helicity fluxes would be necessary.
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| 6. |
- Fowler, A. C., et al.
(author)
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The formation of vesicular cylinders in pahoehoe lava flows
- 2015
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In: Geophysical and Astrophysical Fluid Dynamics. - : Informa UK Limited. - 0309-1929 .- 1029-0419. ; 109:1, s. 39-61
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Journal article (peer-reviewed)abstract
- 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.
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| 7. |
- Hubbard, Alexander, et al.
(author)
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Magnetic helicity fluxes in an α2 dynamo embedded in a halo
- 2010
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In: Geophysical and Astrophysical Fluid Dynamics. - : Informa UK Limited. - 0309-1929 .- 1029-0419. ; 104:5-6, s. 577-590
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Journal article (peer-reviewed)abstract
- We present the results of simulations of forced turbulence in a slab where the mean kinetic helicity has a maximum near the mid-plane, generating gradients of magnetic helicity of both large and small-scale fields. We also study systems that have poorly conducting buffer zones away from the midplane in order to assess the effects of boundaries. The dynamical alpha quenching phenomenology requires that the magnetic helicity in the small-scale fields approaches a nearly static, gauge independent state. To stress-test this steady state condition we choose a system with a uniform sign of kinetic helicity, so that the total magnetic helicity can reach a steady state value only through fluxes through the boundary, which are themselves suppressed by the velocity boundary conditions. Even with such a set up, the small-scale magnetic helicity is found to reach a steady state. In agreement with earlier work, the magnetic helicity fluxes of small-scale fields are found to be turbulently diffusive. By comparing results with and without halos, we show that artificial constraints on magnetic helicity at the boundary do not have a significant impact on the evolution of the magnetic helicity, except that "softer" (halo) boundary conditions give a lower energy of the saturated mean magnetic field.
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| 8. |
- Käpylä, Petri J., et al.
(author)
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Effects of a subadiabatic layer on convection and dynamos in spherical wedge simulations
- 2019
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In: Geophysical and Astrophysical Fluid Dynamics. - : Informa UK Limited. - 0309-1929 .- 1029-0419. ; 113:1-2, s. 149-183
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Journal article (peer-reviewed)abstract
- 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.
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| 9. |
- Käpylä, Petri J., et al.
(author)
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Oscillatory large-scale dynamos from Cartesian convection simulations
- 2013
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In: Geophysical and Astrophysical Fluid Dynamics. - : Informa UK Limited. - 0309-1929 .- 1029-0419. ; 107:1-2, s. 244-257
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Journal article (peer-reviewed)abstract
- 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.
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| 10. |
- Käpylä, Petri, et al.
(author)
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Sensitivity to luminosity, centrifugal force, and boundary conditions in spherical shell convection
- 2020
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In: Geophysical and Astrophysical Fluid Dynamics. - : Taylor & Francis. - 0309-1929 .- 1029-0419. ; 114:1-2, s. 8-34
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Journal article (peer-reviewed)abstract
- We test the sensitivity of hydrodynamic and magnetohydrodynamic turbulent convection simulations with respect to Mach number, thermal and magnetic boundary conditions, and the centrifugal force. We find that varying the luminosity, which also controls the Mach number, has only a minor effect on the large-scale dynamics. A similar conclusion can also be drawn from the comparison of two formulations of the lower magnetic boundary condition with either vanishing electric field or current density. The centrifugal force has an effect on the solutions, but only if its magnitude with respect to acceleration due to gravity is by two orders of magnitude greater than in the Sun. Finally, we find that the parameterisation of the photospheric physics, either by an explicit cooling term or enhanced radiative diffusion, is more important than the thermal boundary condition. In particular, runs with cooling tend to lead to more anisotropic convection and stronger deviations from the Taylor-Proudman state. In summary, the fully compressible approach taken here with the Pencil Code is found to be valid, while still allowing the disparate timescales to be taken into account.
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