scholarly journals Application of convection simulations to oscillation excitation and local helioseismology

2006 ◽  
Vol 2 (S239) ◽  
pp. 331-342
Author(s):  
Robert F. Stein ◽  
David Benson ◽  
Dali Georgobiani ◽  
Åke Nordlund
Keyword(s):  
Computational Domain ◽  
Reynolds Stresses ◽  
Surface Shear ◽  
Continuous Increase ◽  
Solar Convection ◽  
Surface Time ◽  
Propagation Pattern ◽  
Uniform Background ◽  
Time Distance ◽  
Surface Convection

AbstractExcitation of f- and p- modes by Reynolds stresses and entropy fluctuations is reviewed. Approximations made to allow semi-analytic analysis are discussed. The spectrum of solar convection is presented and shown to NOT be separable into independent spatial and temporal factors. An appropriate fitting formula is presented.Time-distance local helioseismology has been analyzed using numerical simulations. One approach is simulation of solar surface convection on supergranule scales (48 Mm wide by 20 Mm deep). A surface shear layer develops. There is a continuous increase in the horizontal scale of the convective motions with increasing depth. Some small granular scale downflows at the surface are swept sideways by diverging larger scale upflows from below to merge into stronger downdrafts in these larger downflow boundaries. Elsewhere, some granular downflows have to beat their way against the upflows from below are halted. These simulations have a rich spectrum of f- and p- modes that turn within the computational domain. Cross-correlations between each surface location and each location below the surface reveals the wave propagation pattern from the surface. Waves at the surface that propagate into the interior, spread horizontally and are refracted back toward the surface. Time-distance diagrams have been constructed and inverted to determine the subsurface flows, which can then be compared with the average flows in the simulation.A second approach calculates the propagation of linearized waves through a fixed, but non-uniform, background state. Some examples of such analysis are presented.

scholarly journals Rotational Effects on Reynolds Stresses in the Solar Convection Zone

1991 ◽  
Vol 130 ◽  
pp. 98-100
Author(s):  
P. Pulkkinen ◽  
I. Tuominen ◽  
A. Brandenburg ◽  
Å. Nordlund ◽  
R.F. Stein
Keyword(s):  
Reynolds Stress ◽  
Convection Zone ◽  
Rotation Axis ◽  
Three Dimensional ◽  
External Parameter ◽  
Reynolds Stresses ◽  
Solar Convection Zone ◽  
Hydrodynamic Simulations ◽  
Solar Convection ◽  
Good Agreement

AbstractThree-dimensional hydrodynamic simulations are carried out in a rectangular box. The angle between gravity and rotation axis is kept as an external parameter in order to study the latitude-dependence of convection. Special attention is given to the horizontal Reynolds stress and the ∧-effect (Rüdiger, 1989). The results of the simulations are compared with observations and theory and a good agreement is found.

Challenges of magnetism in the turbulent Sun

2006 ◽  
Vol 2 (S239) ◽  
pp. 488-493
Author(s):  
Allan Sacha Brun ◽  
Mark S. Miesch ◽  
Juri Toomre
Keyword(s):  
Magnetic Fields ◽  
Differential Rotation ◽  
Convection Zone ◽  
Three Dimensional ◽  
Reynolds Stresses ◽  
Turbulent Layer ◽  
Solar Convection ◽  
Mean Fields ◽  
Global Modelling ◽  
Parallel Supercomputers

AbstractThree-dimensional global modelling of turbulent convection coupled to rotation and magnetism within the Sun are revealing processes relevant to many stars. We study spherical shells of compressible convection spanning many density scale heights using the MHD version of the anelastic spherical harmonic (ASH) code on massively parallel supercomputers. The simulations reveal that strong magnetic fields can be realized in the bulk of the solar convection zone while still attaining differential rotation profiles that make good contact with helioseismic findings. We find that the Maxwell and Reynolds stresses present in such a turbulent layer play an important role in redistributing angular momentum, with the latter maintaining the differential rotation, aided by baroclinic forcing at the base of the convection zone which is consistent with a tachocline there. The dynamo processes generate strong non-axisymmetric and intermittent fields and weak mean (axisymmetric) fields, but do not possess a regular cyclic magnetism. The explicit inclusion of penetrative convection into the tachocline below is modifying such behavior, serving to build strong toroidal magnetic fields there that may yield more prominent mean fields that have the potential of erupting upward.

scholarly journals Onward from solar convection to dynamos in cores of massive stars

2010 ◽  
Vol 6 (S271) ◽  
pp. 347-352
Author(s):  
Juri Toomre
Keyword(s):  
Convection Zone ◽  
Massive Stars ◽  
Dynamo Theory ◽  
Dynamo Action ◽  
Surface Shear ◽  
Near Surface ◽  
Solar Convection ◽  
Core Convection ◽  
Central Regions ◽  
Ultimate Fate

AbstractWe reflect upon a few of the research challenges in stellar convection and dynamo theory that are likely to be addressed in the next five or so years. These deal firstly with the Sun and continuing study of the two boundary layers at the top and bottom of its convection zone, namely the tachocline and the near-surface shear layer, both of which are likely to have significant roles in how the solar dynamo may be operating. Another direction concerns studying core convection and dynamo action within the central regions of more massive A, B and O-type stars, for the magnetism may have a key role in controlling the winds from these stars, thus influencing their ultimate fate. Such studies of the interior dynamics of massive stars are becoming tractable with recent advances in codes and supercomputers, and should also be pursued with some vigor.

Modeling the Transport of Fuel Mixture Perturbations and Entropy Waves in the Linearized Framework

2020 ◽  
Author(s):  
Thomas Ludwig Kaiser ◽  
Kilian Oberleithner
Keyword(s):  
Turbulent Flows ◽  
Channel Flow ◽  
Turbulent Mixing ◽  
Equivalence Ratio ◽  
Stokes Equations ◽  
Turbulent Channel Flow ◽  
Passive Scalar ◽  
Computational Domain ◽  
Reynolds Stresses ◽  
Mean Flow

Abstract In this paper a new method is introduced to model the transport of entropy waves and equivalence ratio fluctuations in turbulent flows. The model is based on the Navier-Stokes equations and includes a transport equation for a passive scalar, which may stand for entropy or equivalence ratio fluctuations. The equations are linearized around the mean turbulent fields, which serve as the input to the model in addition to a turbulent eddy viscosity, which accounts for turbulent diffusion of the perturbations. Based on these inputs, the framework is able to predict the linear response of the flow velocity and passive scalar to harmonic perturbations that are imposed at the boundaries of the computational domain. These in this study are fluctuations in the passive scalar and/or velocities at the inlet of a channel flow. The code is first validated against analytic results, showing very good agreement. Then the method is applied to predict the convection, mean flow dispersion and turbulent mixing of passive scalar fluctuations in a turbulent channel flow, which has been studied in previous work with Direct Numerical Simulations (DNS). Results show that our code reproduces the dynamics of coherent passive scalar transport in the DNS with very high accuracy and low numerical costs, when the DNS mean flow and Reynolds stresses are provided. Furthermore, we demonstrate that turbulent mixing has a significant effect on the transport of the passive scalar fluctuations. Finally, we apply the method to explain experimental observations of transport of equivalence ratio fluctuations in the mixing duct of a model burner.

An inverse energy cascade in two-dimensional low Reynolds number bubbly flows

1996 ◽  
Vol 314 ◽  
pp. 315-330 ◽  
Author(s):  
Asghar Esmaeeli ◽  
Gréatar Tryggvason
Keyword(s):  
Reynolds Number ◽  
Bubbly Flows ◽  
Flow Structures ◽  
Computational Domain ◽  
Two Dimensional ◽  
Continuous Increase ◽  
Bubble Deformation ◽  
Inverse Energy Cascade ◽  
Average Rise ◽  
Work Done

Two direct numerical simulations of several buoyant bubbles in a two-dimensional periodic domain are presented. The average rise Reynolds number of the bubbles is close to 2, and surface tension is high, resulting in small bubble deformation. The void fraction is relatively high, and the bubbles interact strongly. Simulations of the motion of both 144 and 324 bubbles show the formation of flow structures much larger than the bubble size, and a continuous increase in the energy of the low-wavenumber velocity modes. Plots of the energy spectrum show a range of wavenumbers with an approximately -5/3 slope. This suggests that a part of the work done by the buoyant bubbles is not dissipated, but instead increases the energy of flow structures much larger than the bubbles. This phenomenon, which is also seen in numerical simulation of forced two-dimensional turbulence, prevents the appearance of a statistically steady-state motion that is independent of the size of the computational domain.

scholarly journals Validation of Time‐Distance Helioseismology by Use of Realistic Simulations of Solar Convection

2007 ◽  
Vol 659 (1) ◽  
pp. 848-857 ◽  
Author(s):  
Junwei Zhao ◽  
Dali Georgobiani ◽  
Alexander G. Kosovichev ◽  
David Benson ◽  
Robert F. Stein ◽  
...  
Keyword(s):  
Solar Convection ◽  
Time Distance

scholarly journals Simulations of solar magnetic dynamo action in the convection zone and tachocline

2006 ◽  
Vol 2 (S239) ◽  
pp. 510-512
Author(s):  
Matthew K. Browning ◽  
Mark S. Miesch ◽  
Allan Sacha Brun ◽  
Juri Toomre
Keyword(s):  
Convection Zone ◽  
Stable Region ◽  
Hydrodynamic Drag ◽  
Computational Domain ◽  
Simulated Evolution ◽  
Dynamo Action ◽  
Nonlinear Simulation ◽  
Solar Convection ◽  
Thermal Perturbations ◽  
Drag Term

AbstractWe present results from a global 3-D nonlinear simulation of magnetic dynamo action achieved by solar convection in a penetrative geometry. We include within the spherical computational domain both the bulk of the convection zone and a portion of the underlying stable layer. A tachocline of rotational shear is realized below the convection zone, where we have imposed both a hydrodynamic drag term and small thermal perturbations consistent with thermal wind balance. Thus we are capturing many of the dynamical elements thought to be essential in the operation of the global solar dynamo, including differential rotation arising from convection, magnetic pumping, and the stretching and amplification of toroidal fields within the tachocline. In the stable region, the simulation reveals that strong axisymmetric toroidal magnetic fields (about 3000 G in strength) are realized, in contrast to the mostly fluctuating fields that predominate in the convection zone. The toroidal fields in the stable region exhibit a striking antisymmetric parity akin to that observed in sunspots, with fields in the northern hemisphere largely of the opposite sign to those in the southern hemisphere. These deep toroidal fields are accompanied by mostly dipolar mean poloidal fields, whose polarity has retained the same sense over multiple years of simulated evolution.

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