39 research outputs found
Ocean turbulence, III : new GISS vertical mixing scheme
Author Posting. © The Author(s), 2010. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Ocean Modelling 34 (2010): 70-91, doi:10.1016/j.ocemod.2010.04.006.We have found a new way to express the solutions of the RSM (Reynolds Stress
Model) equations that allows us to present the turbulent diffusivities for heat, salt and
momentum in a way that is considerably simpler and thus easier to implement than in
previous work. The RSM provides the dimensionless mixing efficiencies Γα (α stands for
heat, salt and momentum). However, to compute the diffusivities, one needs additional
information, specifically, the dissipation ε. Since a dynamic equation for the latter that
includes the physical processes relevant to the ocean is still not available, one must resort
to different sources of information outside the RSM to obtain a complete Mixing Scheme
usable in OGCMs.
As for the RSM results, we show that the Γα’s are functions of both Ri and Rρ
(Richardson number and density ratio representing double diffusion, DD); the Γα are
different for heat, salt and momentum; in the case of heat, the traditional value Γh = 0.2
is valid only in the presence of strong shear (when DD is inoperative) while when shear
subsides, NATRE data show that Γh can be three times as large, a result that we
reproduce. The salt Γs is given in terms of Γh. The momentum Γm has thus far been
guessed with different prescriptions while the RSM provides a well defined expression
for Γm (Ri, Rρ). Having tested Γh, we then test the momentum Γm by showing that the
turbulent Prandtl number Γm/Γh vs. Ri reproduces the available data quite well.
As for the dissipation ε, we use different representations, one for the mixed layer
(ML), one for the thermocline and one for the ocean’s bottom. For the ML, we adopt a
procedure analogous to the one successfully used in PB (planetary boundary layer)
studies; for the thermocline, we employ an expression for the variable εN-2 from studies
of the internal gravity waves spectra which includes a latitude dependence; for the ocean
bottom, we adopt the enhanced bottom diffusivity expression used by previous authors
but with a state of the art internal tidal energy formulation and replace the fixed Γα = 0.2
with the RSM result that brings into the problem the Ri,Rρ dependence of the Γα; the
unresolved bottom drag, which has thus far been either ignored or modeled with heuristic
relations, is modeled using a formalism we previously developed and tested in PBL
studies.
We carried out several tests without an OGCM. Prandtl and flux Richardson
numbers vs. Ri. The RSM model reproduces both types of data satisfactorily. DD and
Mixing efficiency Γh (Ri, Rρ). The RSM model reproduces well the NATRE data.
Bimodal ε-distribution. NATRE data show that ε (Ri1), which our model
reproduces. Heat to salt flux ratio. In the Ri>>1 regime, the RSM predictions reproduce
the data satisfactorily. NATRE mass diffusivity. The z-profile of the mass diffusivity
reproduces well the measurements at NATRE. The local form of the mixing scheme is
algebraic with one cubic equation to solve
CLES, Code Liegeois d'Evolution Stellaire
Cles is an evolution code recently developed to produce stellar models
meeting the specific requirements of studies in asteroseismology. It offers the
users a lot of choices in the input physics they want in their models and its
versatility allows them to tailor the code to their needs and implement easily
new features. We describe the features implemented in the current version of
the code and the techniques used to solve the equations of stellar structure
and evolution. A brief account is given of the use of the program and of a
solar calibration realized with it.Comment: Comments: 8 pages, Astrophys. Space Sci. CoRoT-ESTA Volume, in the
pres
Reheating and turbulence
We show that the ''turbulent'' particle spectra found in numerical
simulations of the behavior of matter fields during reheating admit a simple
interpretation in terms of hydrodynamic models of the reheating period. We
predict a particle number spectrum with for Comment: 10 pages, one figure included in tex
On two-dimensionalization of three-dimensional turbulence in shell models
Applying a modified version of the Gledzer-Ohkitani-Yamada (GOY) shell model,
the signatures of so-called two-dimensionalization effect of three-dimensional
incompressible, homogeneous, isotropic fully developed unforced turbulence have
been studied and reproduced. Within the framework of shell models we have
obtained the following results: (i) progressive steepening of the energy
spectrum with increased strength of the rotation, and, (ii) depletion in the
energy flux of the forward forward cascade, sometimes leading to an inverse
cascade. The presence of extended self-similarity and self-similar PDFs for
longitudinal velocity differences are also presented for the rotating 3D
turbulence case
Stellar evolution and modelling stars
In this chapter I give an overall description of the structure and evolution
of stars of different masses, and review the main ingredients included in
state-of-the-art calculations aiming at reproducing observational features. I
give particular emphasis to processes where large uncertainties still exist as
they have strong impact on stellar properties derived from large compilations
of tracks and isochrones, and are therefore of fundamental importance in many
fields of astrophysics.Comment: Lecture presented at the IVth Azores International Advanced School in
Space Sciences on "Asteroseismology and Exoplanets: Listening to the Stars
and Searching for New Worlds" (arXiv:1709.00645), which took place in Horta,
Azores Islands, Portugal in July 201
YREC: The Yale Rotating Stellar Evolution Code
The stellar evolution code YREC is outlined with emphasis on its applications
to helio- and asteroseismology. The procedure for calculating calibrated solar
and stellar models is described. Other features of the code such as a non-local
treatment of convective core overshoot, and the implementation of a
parametrized description of turbulence in stellar models, are considered in
some detail. The code has been extensively used for other astrophysical
applications, some of which are briefly mentioned at the end of the paper.Comment: 10 pages, 2 figures, ApSS accepte
Physics of rotation in stellar models
In these lecture notes, we present the equations presently used in stellar
interior models in order to compute the effects of axial rotation. We discuss
the hypotheses made. We suggest that the effects of rotation might play a key
role at low metallicity.Comment: 32 pages, 7 figures, lectures, CNRS school, will be published by
Springe
ASTEC -- the Aarhus STellar Evolution Code
The Aarhus code is the result of a long development, starting in 1974, and
still ongoing. A novel feature is the integration of the computation of
adiabatic oscillations for specified models as part of the code. It offers
substantial flexibility in terms of microphysics and has been carefully tested
for the computation of solar models. However, considerable development is still
required in the treatment of nuclear reactions, diffusion and convective
mixing.Comment: Astrophys. Space Sci, in the pres
Presupernova Structure of Massive Stars
Issues concerning the structure and evolution of core collapse progenitor
stars are discussed with an emphasis on interior evolution. We describe a
program designed to investigate the transport and mixing processes associated
with stellar turbulence, arguably the greatest source of uncertainty in
progenitor structure, besides mass loss, at the time of core collapse. An
effort to use precision observations of stellar parameters to constrain
theoretical modeling is also described.Comment: Proceedings for invited talk at High Energy Density Laboratory
Astrophysics conference, Caltech, March 2010. Special issue of Astrophysics
and Space Science, submitted for peer review: 7 pages, 3 figure
Asteroseismology of red giants & galactic archaeology
Red-giant stars are low- to intermediate-mass (~M)
stars that have exhausted hydrogen in the core. These extended, cool and hence
red stars are key targets for stellar evolution studies as well as galactic
studies for several reasons: a) many stars go through a red-giant phase; b) red
giants are intrinsically bright; c) large stellar internal structure changes as
well as changes in surface chemical abundances take place over relatively short
time; d) red-giant stars exhibit global intrinsic oscillations. Due to their
large number and intrinsic brightness it is possible to observe many of these
stars up to large distances. Furthermore, the global intrinsic oscillations
provide a means to discern red-giant stars in the pre-helium core burning from
the ones in the helium core burning phase and provide an estimate of stellar
ages, a key ingredient for galactic studies. In this lecture I will first
discuss some physical phenomena that play a role in red-giant stars and several
phases of red-giant evolution. Then, I will provide some details about
asteroseismology -- the study of the internal structure of stars through their
intrinsic oscillations -- of red-giant stars. I will conclude by discussing
galactic archaeology -- the study of the formation and evolution of the Milky
Way by reconstructing its past from its current constituents -- and the role
red-giant stars can play in that.Comment: Lecture presented at the IVth Azores International Advanced School in
Space Sciences on "Asteroseismology and Exoplanets: Listening to the Stars
and Searching for New Worlds" (arXiv:1709.00645), which took place in Horta,
Azores Islands, Portugal in July 201