scholarly journals Turbulence in Stellar Interiors

1993 ◽  
Vol 137 ◽  
pp. 236-245 ◽  
Author(s):  
Jean-Paul Zahn
Keyword(s):  
Numerical Simulations ◽  
Recent Work ◽  
Thermal Convection ◽  
Convective Instability ◽  
Turbulent Convection ◽  
Stable Region ◽  
The Sun ◽  
Acoustic Sounding ◽  
Convective Core ◽  
Stellar Interiors

AbstractThis review focusses on the most recent work which has been achieved concerning turbulence in stellar interiors. Among all possible causes for such turbulence, the most powerful is certainly the convective instability in unstably stratified regions, but little was known until now beyond the fact that thermal convection is capable of establishing an almost adiabatic stratification. The latest numerical simulations (Stein & Nordlund 1989; Cataneo et al. 1991) show that turbulent convection is highly intermittent, with strong downdrafts carrying most of the energy flux.These plumes penetrate in the stable region below, which they render nearly adiabatic over some distance (Zahn 1991); the penetration depth should be measured soon through acoustic sounding of the Sun (Berthomieu et al. 1992). Such plumes are likely to exist also in a convective core, and to penetrate into the radiative region above (Roxburgh 1989).

scholarly journals Stochastic Excitation in Solar-Type Stars

2002 ◽  
Vol 185 ◽  
pp. 447-455 ◽  
Author(s):  
G. Houdek
Keyword(s):  
Numerical Simulations ◽  
Theoretical Models ◽  
Basic Mechanism ◽  
Convincing Evidence ◽  
Turbulent Convection ◽  
The Sun ◽  
Stochastic Excitation ◽  
Current Belief ◽  
Solar Type

AbstractThe most convincing evidence to date of solar-type oscillations in other stars comes from recent observations of β Hydri (Bedding et al., 2001) and α Cen A (Bouchy & Carrier, 2001). It is the current belief that the convection dynamics in the outer layers of sun-like stars is the source for driving the intrinsically stable modes to the observed amplitudes. Comparing such observations with theoretical models will help us improve our understanding of the interaction between convection and pulsation.In this contribution I review the mechanisms responsible for mode damping in stars with convective envelopes, and the basic mechanism of stochastic driving by turbulent convection. The application of a stochastic excitation formalism to the Sun is discussed and compared with recent measurements and numerical simulations. Amplitude predictions for models of Procyon, α Cen A and β Hydri are compared with observations.

scholarly journals Coherent structures in granulation convection and their importance for higher order closure models

2006 ◽  
Vol 2 (S239) ◽  
pp. 74-76 ◽  
Author(s):  
F. Kupka ◽  
F. J. Robinson
Keyword(s):  
Numerical Simulations ◽  
Coherent Structures ◽  
Normal Approximation ◽  
Substantial Improvement ◽  
Higher Order ◽  
Turbulent Convection ◽  
The Sun ◽  
Order Of Magnitude ◽  
Non Local ◽  
Stress Models

AbstractWe use numerical simulations of granulation in the Sun and a K dwarf to study the effects of coherent structures on higher order moments. The latter need to be calculated in non-local Reynolds stress models of turbulent convection. Models that explicitly account for the asymmetry between up- and downflows as well as hot and cold drafts provide a substantial improvement over traditional ones, such as the quasi-normal approximation, which is only able to provide order of magnitude estimates for this type of flow.

scholarly journals Convective Overshooting: Physical Properties and Seismic Evidence

2002 ◽  
Vol 185 ◽  
pp. 58-69 ◽  
Author(s):  
J.-P. Zahn
Keyword(s):  
High Resolution ◽  
Physical Properties ◽  
Numerical Simulations ◽  
Temperature Profile ◽  
Thermal Diffusion ◽  
Temperature Stratification ◽  
Space Filling ◽  
Acoustic Sounding ◽  
Stable Domain ◽  
Stellar Interiors

AbstractWe review briefly the different prescriptions which have been proposed to predict the extent of convective penetration (or overshoot) in stellar interiors, and we confront them with the results of numerical simulations and with helioseismic data. It appears that the penetrative motions are structured in plumes, and that thermal diffusion plays an important role in controlling the temperature stratification in the stable domain. The most recent high-resolution simulations suggest that these plumes are less space-filling than thought before, and that they are therefore less efficient in establishing an adiabatic temperature profile. This property is compatible with the solar profiles obtained through acoustic sounding.

The Sun as a Magnetic Star

1976 ◽  
Vol 32 ◽  
pp. 457-463
Author(s):  
John M. Wilcox ◽  
Leif Svalgaard
Keyword(s):  
Recent Work ◽  
Rotation Axis ◽  
Polar Field ◽  
Magnetic Star ◽  
Rotational Axis ◽  
Field Variation ◽  
The Sun ◽  
Arbitrary Angle ◽  
Solar Magnetism ◽  
Oblique Rotator

SummaryThe sun as a magnetic star is described on the basis of recent work on solar magnetism. Observations at an arbitrary angle to the rotation axis would show a 22-year polar field variation and a 25-day equatorial sector variation. The sector variation would be similar to an oblique rotator with an angle of 90° between the magnetic and rotational axis.

scholarly journals Energetics of magnetic transients in a solar active region plage

2019 ◽  
Vol 623 ◽  
pp. A176 ◽  
Author(s):  
L. P. Chitta ◽  
A. R. C. Sukarmadji ◽  
L. Rouppe van der Voort ◽  
H. Peter
Keyword(s):  
Magnetic Field ◽  
Active Region ◽  
Magnetic Flux ◽  
Numerical Simulations ◽  
Extreme Ultraviolet ◽  
Spatial Scales ◽  
Coronal Loops ◽  
The Sun ◽  
Flux Emergence ◽  
Transient Events

Context. Densely packed coronal loops are rooted in photospheric plages in the vicinity of active regions on the Sun. The photospheric magnetic features underlying these plage areas are patches of mostly unidirectional magnetic field extending several arcsec on the solar surface. Aims. We aim to explore the transient nature of the magnetic field, its mixed-polarity characteristics, and the associated energetics in the active region plage using high spatial resolution observations and numerical simulations. Methods. We used photospheric Fe I 6173 Å spectropolarimetric observations of a decaying active region obtained from the Swedish 1-m Solar Telescope (SST). These data were inverted to retrieve the photospheric magnetic field underlying the plage as identified in the extreme-ultraviolet emission maps obtained from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). To obtain better insight into the evolution of extended unidirectional magnetic field patches on the Sun, we performed 3D radiation magnetohydrodynamic simulations of magnetoconvection using the MURaM code. Results. The observations show transient magnetic flux emergence and cancellation events within the extended predominantly unipolar patch on timescales of a few 100 s and on spatial scales comparable to granules. These transient events occur at the footpoints of active region plage loops. In one case the coronal response at the footpoints of these loops is clearly associated with the underlying transient. The numerical simulations also reveal similar magnetic flux emergence and cancellation events that extend to even smaller spatial and temporal scales. Individual simulated transient events transfer an energy flux in excess of 1 MW m−2 through the photosphere. Conclusions. We suggest that the magnetic transients could play an important role in the energetics of active region plage. Both in observations and simulations, the opposite-polarity magnetic field brought up by transient flux emergence cancels with the surrounding plage field. Magnetic reconnection associated with such transient events likely conduits magnetic energy to power the overlying chromosphere and coronal loops.

Turbulent convection driven by surface cooling in shallow water

2002 ◽  
Vol 464 ◽  
pp. 81-111 ◽  
Author(s):  
OLEG ZIKANOV ◽  
DONALD N. SLINN ◽  
MANHAR R. DHANAK
Keyword(s):  
Thermal Convection ◽  
Large Scale ◽  
Fourier Method ◽  
Vertical Direction ◽  
Finite Depth ◽  
Turbulent Convection ◽  
Bottom Surface ◽  
Computational Domain ◽  
Surface Cooling ◽  
Adverse Weather

We present the results of large-eddy simulations (LES) of turbulent thermal convection generated by surface cooling in a finite-depth stably stratified horizontal layer with an isothermal bottom surface. The flow is a simplified model of turbulent convection occurring in the warm shallow ocean during adverse weather events. Simulations are performed in a 6 × 6 × 1 aspect ratio computational domain using the pseudo-spectral Fourier method in the horizontal plane and finite-difference discretization on a high-resolution clustered grid in the vertical direction. A moderate value of the Reynolds number and two different values of the Richardson number corresponding to a weak initial stratification are considered. A version of the dynamic model is applied as a subgrid-scale (SGS) closure. Its performance is evaluated based on comparison with the results of direct numerical simulations (DNS) and simulations using the Smagorinsky model. Comprehensive study of the spatial structure and statistical properties of the developed turbulent state shows some similarity to Rayleigh–Bénard convection and other types of turbulent thermal convection in horizontal layers, but also reveals distinctive features such as the dominance of a large-scale pattern of descending plumes and strong turbulent fluctuations near the surface.

scholarly journals Numerical simulations of thermal convection on a hemisphere

2018 ◽  
Vol 3 (4) ◽  
Author(s):  
C.-H. Bruneau ◽  
P. Fischer ◽  
Y.-L. Xiong ◽  
H. Kellay ◽  
Keyword(s):  
Numerical Simulations ◽  
Thermal Convection
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