scholarly journals Time-dependent Turbulence in Stars

2010 ◽  
Vol 6 (S271) ◽  
pp. 205-212
Author(s):  
W. David Arnett ◽  
Casey Meakin
Keyword(s):  
Stellar Evolution ◽  
Three Dimensional ◽  
Red Giants ◽  
Global Instability ◽  
Lorenz Model ◽  
Hydrodynamic Simulations ◽  
Nuclear Burning ◽  
Mixing Length Theory ◽  
Convective Layer ◽  
Random Fluctuations

AbstractThree-dimensional (3D) hydrodynamic simulations of shell oxygen burning by Meakin & Arnett (2007b) exhibit bursty, recurrent fluctuations in turbulent kinetic energy. These are shown to be due to a global instability in the convective region, which has been suppressed in simulations of stellar evolution which use mixing-length theory (MLT). Quantitatively similar behavior occurs in the model of a convective roll (cell) of Lorenz (1963), which is known to have a strange attractor that gives rise to random fluctuations in time. An extension of the Lorenz model, which includes Kolmogorov damping and nuclear burning, is shown to exhibit bursty, recurrent fluctuations like those seen in the 3D simulations. A simple model of a convective layer (composed of multiple Lorenz cells) gives luminosity fluctuations which are suggestive of irregular variables (red giants and supergiants, see Schwarzschild (1975). Details and additional discussion may be found in Arnett & Meakin (2011).Apparent inconsistencies between Arnett, Meakin, & Young (2009) and Nordlund, Stein, & Asplund (2009) on the nature of convective driving have been resolved, and are discussed.

scholarly journals Stellar convection with nuclear burning

2006 ◽  
Vol 2 (S239) ◽  
pp. 247-257 ◽  
Author(s):  
David Arnett ◽  
Casey Meakin ◽  
Patrick A. Young
Keyword(s):  
Stellar Evolution ◽  
Wave Equations ◽  
Three Dimensional ◽  
3D Simulations ◽  
Simple Theoretical Model ◽  
Additional Detail ◽  
Velocity Scale ◽  
Nuclear Burning ◽  
Mixing Length Theory ◽  
Zone Growth

AbstractCareful choice of of method, problem, and zoning has allowed us to do three-dimensional (3D) simulations of thermally relaxed, nearly adiabatic convection (with nuclear burning). The simulations are run long enough so that a robust statistical state is found. We find that 2D simulations are biased relative to 3D simulations: 2D shows larger velocities and less mixing than their 3D counterparts. Detailed theoretical analysis of these numerical experiments allows us to begin to build a simple theoretical model of turbulent convection in stars, which may be used in 1D calculations of stellar evolution. Implications for stellar evolution, will be discussed. Oxygen shell burning simulations in 3D, and multishell burning of C, Ne, O, and Si in 2D will be presented, as will aspherical distortion in supernovae progenitors (Meakin and Arnett, 2006a). Contact will be made with convective driving of waves, convective zone growth by entrainment, the velocity scale and the geometric parameters in mixing length theory, and the solar Ne abundance problem. Explicit comparisons of compressible and anelastic methods at modest Mach numbers (M ≈ 0.01 to 0.1), as well as solutions of the nonradial wave equations, are presented here. Additional detail is presented in the poster by Meakin.

scholarly journals Posters also presented at the Symposium

2016 ◽  
Vol 12 (S329) ◽  
pp. 455-464
Keyword(s):  
Stellar Evolution ◽  
Scaling Laws ◽  
Convection Zone ◽  
Massive Stars ◽  
Convective Boundary ◽  
Hydrodynamic Simulations ◽  
Fundamental Properties ◽  
Mass Entrainment ◽  
Nuclear Burning ◽  
Advanced Stages

I am reporting on our team's progress in investigating fundamental properties of convective shells in the deep stellar interior during advanced stages of stellar evolution. We have performed a series of 3D hydrodynamic simulations of convection in conditions similar to those in the O-shell burning phase of massive stars. We focus on characterizing the convective boundary and the mixing of material across this boundary. Results from 7683 and 15363 grids are encouragingly similar (typically within 20%). Several global quantities, including the rate of mass entrainment at the convective boundary and the driving luminosity, are related by scaling laws. We investigate the effect of several of our assumptions, including the treatment of the nuclear burning driving the convection or that of neutrino cooling. The burning of the entrained material from above the convection zone could have important implications for pre-supernova nucleosynthesis.

scholarly journals The i-process yields of rapidly accreting white dwarfs from multicycle He-shell flash stellar evolution models with mixing parametrizations from 3D hydrodynamics simulations

2019 ◽  
Vol 488 (3) ◽  
pp. 4258-4270 ◽  
Author(s):  
Pavel A Denissenkov ◽  
Falk Herwig ◽  
Paul Woodward ◽  
Robert Andrassy ◽  
Marco Pignatari ◽  
...  
Keyword(s):  
Stellar Evolution ◽  
White Dwarfs ◽  
Binary Systems ◽  
Scaling Laws ◽  
Ingestion Rate ◽  
Three Dimensional ◽  
Close Binary ◽  
Convective Boundary ◽  
Retention Efficiency ◽  
Hydrodynamic Simulations

ABSTRACT We have modelled the multicycle evolution of rapidly accreting CO white dwarfs (RAWDs) with stable H burning intermittent with strong He-shell flashes on their surfaces for 0.7 ≤ MRAWD/M⊙ ≤ 0.75 and [Fe/H] ranging from 0 to −2.6. We have also computed the i-process nucleosynthesis yields for these models. The i process occurs when convection driven by the He-shell flash ingests protons from the accreted H-rich surface layer, which results in maximum neutron densities Nn, max ≈ 1013–1015 cm−3. The H-ingestion rate and the convective boundary mixing (CBM) parameter ftop adopted in the one-dimensional nucleosynthesis and stellar evolution models are constrained through three-dimensional (3D) hydrodynamic simulations. The mass ingestion rate and, for the first time, the scaling laws for the CBM parameter ftop have been determined from 3D hydrodynamic simulations. We confirm our previous result that the high-metallicity RAWDs have a low mass retention efficiency ($\eta \lesssim 10{{\ \rm per\ cent}}$). A new result is that RAWDs with [Fe/H] $\lesssim -2$ have $\eta \gtrsim 20{{\ \rm per\ cent}}$; therefore, their masses may reach the Chandrasekhar limit and they may eventually explode as SNeIa. This result and the good fits of the i-process yields from the metal-poor RAWDs to the observed chemical composition of the CEMP-r/s stars suggest that some of the present-day CEMP-r/s stars could be former distant members of triple systems, orbiting close binary systems with RAWDs that may have later exploded as SNeIa.

A Stochastic Hierarchical Population System: Excitement, Extinction and Transition to Chaos

2021 ◽  
Vol 31 (14) ◽  
Author(s):  
Irina Bashkirtseva ◽  
Tatyana Perevalova ◽  
Lev Ryashko
Keyword(s):  
Mathematical Modeling ◽  
Food Chain ◽  
Analytical Method ◽  
Three Dimensional ◽  
Biological Parameters ◽  
Top Predator ◽  
Population System ◽  
Modeling And Analysis ◽  
Transition To Chaos ◽  
Random Fluctuations

A problem of the mathematical modeling and analysis of noise-induced transformations of complex oscillatory regimes in hierarchical population systems is considered. As a key example, we use a three-dimensional food chain dynamical model of the interacting prey, predator, and top predator. We perform a comparative study of the impacts of random fluctuations on three key biological parameters of prey growth, predator mortality, and the top predator growth. A detailed investigation of the stochastic excitement, noise-induced transition from order to chaos, and various scenarios of extinction is carried out. Constructive abilities of the semi-analytical method of confidence domains in the analysis of the noise-induced extinction are demonstrated.

Quasi-Periodic Orbits in the Five-Dimensional Nondissipative Lorenz Model: The Role of the Extended Nonlinear Feedback Loop

2018 ◽  
Vol 28 (06) ◽  
pp. 1850072 ◽  
Author(s):  
Sara Faghih-Naini ◽  
Bo-Wen Shen
Keyword(s):  
Periodic Solution ◽  
Feedback Loop ◽  
Three Dimensional ◽  
Nonlinear Feedback ◽  
Lorenz Model ◽  
Restoring Force ◽  
Oscillatory Solutions ◽  
Nonlinear Temperature

A recent study suggested that the nonlinear feedback loop (NFL) of the three-dimensional nondissipative Lorenz model (3D-NLM) serves as a nonlinear restoring force by producing nonlinear oscillatory solutions as well as linear periodic solutions near a nontrivial critical point. This study discusses the role of the extension of the NFL in producing quasi-periodic trajectories using a five-dimensional nondissipative Lorenz model (5D-NLM). An analytical solution to the locally linear 5D-NLM is first obtained to illustrate the association of the extended NFL and two incommensurate frequencies whose ratio is irrational, yielding a quasi-periodic solution. The quasi-periodic solution trajectory moves endlessly on a torus but never intersects itself. While the NFL of the 3D-NLM consists of a pair of downscaling and upscaling processes, the extended NFL within the 5D-NLM additionally introduces two new pairs of downscaling and upscaling processes that are enabled by two high wavenumber modes. One pair of downscaling and upscaling processes provides a two-way interaction between the original (primary) Fourier modes of the 3D-NLM and the newly-added (secondary) Fourier modes of the 5D-NLM. The other pair of downscaling and upscaling processes involves interactions amongst the secondary modes. By comparing the numerical simulations using one- and two-way interactions, we illustrate that the two-way interaction is crucial for producing the quasi-periodic solution. A follow-up study using a 7D nondissipative LM shows that a further extension of NFL, which may appear throughout the spatial mode-mode interactions rooted in the nonlinear temperature advection, is capable of producing one more incommensurate frequency.

scholarly journals Peculiar Red Giants in External Galaxies

1989 ◽  
Vol 106 ◽  
pp. 35-50 ◽  
Author(s):  
Harvey B. Richer
Keyword(s):  
Star Formation ◽  
Stellar Evolution ◽  
Hubble Constant ◽  
Red Giants ◽  
Stellar Content ◽  
Late Type ◽  
External Galaxies ◽  
Distance Indicators ◽  
Luminosity Evolution ◽  
Local Value

AbstractStudy of the late-type stellar content in external galaxies provides numerous clues for the theory of stellar evolution, for star-formation scenarios in galaxies, and for proper models of the luminosity evolution of galaxies which are then used in cosmological studies. In addition, these late-type stars can be used as distance indicators themselves and yield a local value of the Hubble constant consistent with recent Cepheid determinations.

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.

scholarly journals Multidimensional Hydrodynamic and Hydrostatic Stellar Models

2004 ◽  
Vol 215 ◽  
pp. 378-387
Author(s):  
Robert G. Deupree
Keyword(s):  
Stellar Evolution ◽  
Rotation Axis ◽  
Finite Difference Methods ◽  
Stellar Model ◽  
Current Status ◽  
Hydrodynamic Models ◽  
Intermediate Mass ◽  
Hydrodynamic Simulations ◽  
Axis Rotation ◽  
2D And 3D

Results for multidimensional stellar model simulations of both 2D and 3D hydrodynamic models and 2D stellar evolution sequences are presented. Simulations of the highly superadiabatic region of the solar convective region provide a good example of the current status and limitations of explicit 3D finite difference methods in stellar problems. Such simulations cannot be used for stellar cores, where the motion is expected to be well subsonic. The results of some 2D fully implcit hydrodynamic simulations of convective cores and shells are given for models with and without rotation, and their effects examined through fully 2D stellar evolution sequences. One effect of moderate to rapid rotation in convective cores is to alter the convective flow pattern so that convective eddies tend to line up parallel to the rotation axis. Rotation also appears to modestly reduce the amount of convective core overshooting, at least for intermediate mass models.

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