| 1. |
- Brandenburg, Axel, et al.
(författare)
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Introduction
- 2020
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Ingår i: Geophysical and Astrophysical Fluid Dynamics. - : Taylor & Francis. - 0309-1929 .- 1029-0419. ; 114:1-2, s. 1-7
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 2. |
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| 3. |
- Brandenburg, Axel, et al.
(författare)
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The time step constraint in radiation hydrodynamics
- 2020
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Ingår i: Geophysical and Astrophysical Fluid Dynamics. - : Taylor & Francis. - 0309-1929 .- 1029-0419. ; 114:1-2, s. 162-195
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Tidskriftsartikel (refereegranskat)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.
(författare)
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Yoshizawa's cross-helicity effect and its quenching
- 2013
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Ingår i: Geophysical and Astrophysical Fluid Dynamics. - : Informa UK Limited. - 0309-1929 .- 1029-0419. ; 107:1-2, s. 207-217
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Tidskriftsartikel (refereegranskat)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.
(författare)
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Can catastrophic quenching be alleviated by separating shear and α effect?
- 2010
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Ingår i: Geophysical and Astrophysical Fluid Dynamics. - : Informa UK Limited. - 0309-1929 .- 1029-0419. ; 104:5&6, s. 591-599
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Tidskriftsartikel (refereegranskat)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.
(författare)
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The formation of vesicular cylinders in pahoehoe lava flows
- 2015
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Ingår i: Geophysical and Astrophysical Fluid Dynamics. - : Informa UK Limited. - 0309-1929 .- 1029-0419. ; 109:1, s. 39-61
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Tidskriftsartikel (refereegranskat)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.
(författare)
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Magnetic helicity fluxes in an α2 dynamo embedded in a halo
- 2010
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Ingår i: Geophysical and Astrophysical Fluid Dynamics. - : Informa UK Limited. - 0309-1929 .- 1029-0419. ; 104:5-6, s. 577-590
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Tidskriftsartikel (refereegranskat)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.
(författare)
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Effects of a subadiabatic layer on convection and dynamos in spherical wedge simulations
- 2019
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Ingår i: Geophysical and Astrophysical Fluid Dynamics. - : Informa UK Limited. - 0309-1929 .- 1029-0419. ; 113:1-2, s. 149-183
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Tidskriftsartikel (refereegranskat)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.
(författare)
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Oscillatory large-scale dynamos from Cartesian convection simulations
- 2013
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Ingår i: Geophysical and Astrophysical Fluid Dynamics. - : Informa UK Limited. - 0309-1929 .- 1029-0419. ; 107:1-2, s. 244-257
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Tidskriftsartikel (refereegranskat)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.
(författare)
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Sensitivity to luminosity, centrifugal force, and boundary conditions in spherical shell convection
- 2020
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Ingår i: Geophysical and Astrophysical Fluid Dynamics. - : Taylor & Francis. - 0309-1929 .- 1029-0419. ; 114:1-2, s. 8-34
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Tidskriftsartikel (refereegranskat)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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| 11. |
- Qian, Chengeng, et al.
(författare)
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Convergence properties of detonation simulations
- 2020
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Ingår i: Geophysical and Astrophysical Fluid Dynamics. - : Taylor & Francis. - 0309-1929 .- 1029-0419. ; 114:1-2, s. 58-76
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 12. |
- Rahm, Lars, 1948-, et al.
(författare)
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On thermal convection in stratified fluids
- 1979
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Ingår i: Geophysical and Astrophysical Fluid Dynamics. - : Informa UK Limited. - 0309-1929 .- 1029-0419. ; 13, s. 51-65
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Tidskriftsartikel (refereegranskat)abstract
- The study of the mechanisms controlling the stratification in closed fluid regions is an important branch of geophysical fluid dynamics. Part of this subject can be handled with a simple linear model, consisting of a buoyancy layer at the non-horizontal boundaries of a container and an advective-diffusive interior coupled by volume continuity. The model is valid under the following conditions: firstly, the buoyancy-frequency characterizing the solution must be sufficiently large to give rise to a flow pattern of boundary layer type and, secondly, the non-horizontal walls must not have too large thermal conductivity.The main purpose of the present paper is to summarise previous work done by the authors in this field and to present some consequences of their theory not previously discussed.Three important cases are discussed; certain stationary solutions, the decay of a given stratification and the build up of a stratification in a homogeneous fluid. The experimental results concerning the afore-mentioned cases are presented.
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| 13. |
- Roper Pol, Alberto, et al.
(författare)
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The timestep constraint in solving the gravitational wave equations sourced by hydromagnetic turbulence
- 2020
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Ingår i: Geophysical and Astrophysical Fluid Dynamics. - : Informa UK Limited. - 0309-1929 .- 1029-0419. ; 114:1-2, s. 130-161
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 14. |
- Schober, J., et al.
(författare)
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Chiral fermion asymmetry in high-energy plasma simulations
- 2020
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Ingår i: Geophysical and Astrophysical Fluid Dynamics. - : Informa UK Limited. - 0309-1929 .- 1029-0419. ; 114:1-2, s. 106-129
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Tidskriftsartikel (refereegranskat)abstract
- The chiral magnetic effect (CME) is a quantum relativistic effect that describes the appearance of an additional electric current along a magnetic field. It is caused by an asymmetry between the number densities of left- and right-handed fermions, which can be maintained at high energies when the chirality flipping rate can be neglected, for example in the early Universe. The inclusion of the CME in the Maxwell equations leads to a modified set of magnetohydrodynamical (MHD) equations. The CME is studied here in numerical simulations with the Pencil Code. We discuss how the CME is implemented in the code and how the time step and the spatial resolution of a simulation need to be adjusted in presence of a chiral asymmetry. The CME plays a key role in the evolution of magnetic fields, since it results in a dynamo effect associated with an additional term in the induction equation. This term is formally similar to the alpha effect in classical mean-field MHD. However, the chiral dynamo can operate without turbulence and is associated with small spatial scales that can be, in the case of the early Universe, orders of magnitude below the Hubble radius. A chiral alpha mu effect has also been identified in mean-field theory. It occurs in the presence of turbulence, but is not related to kinetic helicity. Depending on the plasma parameters, chiral dynamo instabilities can amplify magnetic fields over many orders of magnitude. These instabilities can potentially affect the propagation of MHD waves. Our numerical simulations demonstrate strong modifications of the dispersion relation for MHD waves for large chiral asymmetry. We also study the coupling between the evolution of the chiral chemical potential and the ordinary chemical potential, which is proportional to the sum of the number densities of left- and right-handed fermions. An important consequence of this coupling is the emergence of chiral magnetic waves (CMWs). We confirm numerically that linear CMWs and MHD waves are not interacting. Our simulations suggest that the chemical potential has only a minor effect on the non-linear evolution of the chiral dynamo.
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| 15. |
- Schober, J., et al.
(författare)
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Energetics of turbulence generated by chiral MHD dynamos
- 2019
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Ingår i: Geophysical and Astrophysical Fluid Dynamics. - : Informa UK Limited. - 0309-1929 .- 1029-0419. ; 113:1-2, s. 107-130
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 16. |
- Singh, Nishant K., et al.
(författare)
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f-mode strengthening from a localised bipolar subsurface magnetic field
- 2019
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Ingår i: Geophysical and Astrophysical Fluid Dynamics. - : TAYLOR & FRANCIS LTD. - 0309-1929 .- 1029-0419.
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Tidskriftsartikel (refereegranskat)abstract
- 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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| 17. |
- Yoshizawa, A., et al.
(författare)
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A Reynolds-averaged turbulence modelling approach to the maintenance of the Venus superrotation
- 2013
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Ingår i: Geophysical and Astrophysical Fluid Dynamics. - : Informa UK Limited. - 0309-1929 .- 1029-0419. ; 107:6, s. 614-639
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Tidskriftsartikel (refereegranskat)abstract
- A maintenance mechanism of an approximately linear velocity profile of the Venus zonal flow or superrotation is explored, with the aid of a Reynolds-averaged turbulence modelling approach. The basic framework is similar to that of Gierasch (Meridional circulation and maintenance of the Venus atmospheric rotation. J. Atmos. Sci. 1975, 32, 1038-1044) in the sense that the mechanism is examined under a given meridional circulation. The profile mimicking the observations of the flow is initially assumed, and its maintenance mechanism in the presence of turbulence effects is investigated from a viewpoint of the suppression of energy cascade. In the present work, the turbulent viscosity is regarded as an indicator of the intensity of the cascade. A novelty of this formalism is the use of the isotropic turbulent viscosity based on a non-local time scale linked to a large-scale flow structure. The mechanism is first discussed qualitatively. On the basis of these discussions, the two-dimensional numerical simulation of the proposed model is performed, with an initially assumed superrotation, and the fast zonal flow is shown to be maintained, compared with the turbulent viscosity lacking the non-local time scale. The relationship of the present model with the current general circulation model simulation is discussed in light of a crucial role of the vertical viscosity.
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| 18. |
- Olsson, Sara, et al.
(författare)
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Experiences of transition to adulthood and transfer to adult care in young adults with type 1 diabetes : a qualitative study
- 2023
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Ingår i: Journal of Advanced Nursing. - 0309-2402 .- 1365-2648. ; 79:12, s. 4621-4634
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Tidskriftsartikel (refereegranskat)abstract
- Aim: To explore young adults' experiences of living with type 1 diabetes in the transition to adulthood, including experiences of the transfer from paediatric to adult care.Design: A qualitative approach was used.Method: Ten young adults, six women and four men, aged 19–29 years, participated. Participants were recruited at their regular diabetes clinic from spring 2021 to spring 2022. Semi-structured interviews were transcribed and analysed using qualitative content analysis.Findings: Dreaming of being nurtured towards self-reliance was the overarching theme. Personal experiences of the transition to adulthood, including the transfer from paediatric to adult care, were described in terms of struggling to find balance in daily life, dealing with feelings of being different, being gradually supported to achieve independence, and wishing to be approached as a unique person in healthcare.Conclusion: In healthcare, it is important to emphasize not only diabetes-related factors but also emotional and psychosocial aspects of life connected to the transition to adulthood, including the transfer to adult care. The young adults wished to be seen as unique persons in healthcare during their emerging adulthood and should therefore be supported to achieve self-reliance through personal preparations for new challenges and for the consequences of transitioning to adulthood. Specialist nurses can provide appropriate knowledge and leadership.Implications for the Profession: These findings can guide nurse specialists in support for emerging adults to achieve self-reliance and indicate the importance of person-centred care when experiencing transition and transfer.Reporting Method: The study adhered to EQUATOR guidelines, and the COREQ checklist for qualitative studies was used as the reporting method.
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