4,345 research outputs found
Numerical solution of a non-linear conservation law applicable to the interior dynamics of partially molten planets
The energy balance of a partially molten rocky planet can be expressed as a
non-linear diffusion equation using mixing length theory to quantify heat
transport by both convection and mixing of the melt and solid phases. In this
formulation the effective or eddy diffusivity depends on the entropy gradient,
, as well as entropy. First we present a simplified
model with semi-analytical solutions, highlighting the large dynamic range of
, around 12 orders of magnitude, for physically-relevant
parameters. It also elucidates the thermal structure of a magma ocean during
the earliest stage of crystal formation. This motivates the development of a
simple, stable numerical scheme able to capture the large dynamic range of
and provide a flexible and robust method for
time-integrating the energy equation.
We then consider a full model including energy fluxes associated with
convection, mixing, gravitational separation, and conduction that all depend on
the thermophysical properties of the melt and solid phases. This model is
discretised and evolved by applying the finite volume method (FVM), allowing
for extended precision calculations and using as the
solution variable. The FVM is well-suited to this problem since it is naturally
energy conserving, flexible, and intuitive to incorporate arbitrary non-linear
fluxes that rely on lookup data. Special attention is given to the numerically
challenging scenario in which crystals first form in the centre of a magma
ocean.
Our computational framework is immediately applicable to modelling high melt
fraction phenomena in Earth and planetary science research. Furthermore, it
provides a template for solving similar non-linear diffusion equations arising
in other disciplines, particularly for non-linear functional forms of the
diffusion coefficient
Reversal and amplification of zonal flows by boundary enforced thermal wind
Zonal flows in rapidly-rotating celestial objects such as the Sun, gas or ice
giants form in a variety of surface patterns and amplitudes. Whereas the
differential rotation on the Sun, Jupiter and Saturn features a super-rotating
equatorial region, the ice giants, Neptune and Uranus harbour an equatorial jet
slower than the planetary rotation. Global numerical models covering the
optically thick, deep-reaching and rapidly rotating convective envelopes of gas
giants reproduce successfully the prograde jet at the equator. In such models,
convective columns shaped by the dominant Coriolis force typically exhibit a
consistent prograde tilt. Hence angular momentum is pumped away from the
rotation axis via Reynolds stresses. Those models are found to be strongly
geostrophic, hence a modulation of the zonal flow structure along the axis of
rotation, e.g. introduced by persistent latitudinal temperature gradients,
seems of minor importance. Within our study we stimulate these thermal
gradients and the resulting ageostrophic flows by applying an axisymmetric and
equatorially symmetric outer boundary heat flux anomaly () with
variable amplitude and sign. Such a forcing pattern mimics the thermal effect
of intense solar or stellar irradiation. Our results suggest that the
ageostrophic flows are linearly amplified with the forcing amplitude
leading to a more pronounced dimple of the equatorial jet (alike Jupiter). The
geostrophic flow contributions, however, are suppressed for weak , but
inverted and re-amplified once exceeds a critical value. The inverse
geostrophic differential rotation is consistently maintained by now also
inversely tilted columns and reminiscent of zonal flow profiles observed for
the ice giants. Analysis of the main force balance and parameter studies
further foster these results
Tracing cosmic evolution with clusters of galaxies
The most successful cosmological models to date envision structure formation
as a hierarchical process in which gravity is constantly drawing lumps of
matter together to form increasingly larger structures. Clusters of galaxies
currently sit atop this hierarchy as the largest objects that have had time to
collapse under the influence of their own gravity. Thus, their appearance on
the cosmic scene is also relatively recent. Two features of clusters make them
uniquely useful tracers of cosmic evolution. First, clusters are the biggest
things whose masses we can reliably measure because they are the largest
objects to have undergone gravitational relaxation and entered into virial
equilibrium. Mass measurements of nearby clusters can therefore be used to
determine the amount of structure in the universe on scales of 10^14 to 10^15
solar masses, and comparisons of the present-day cluster mass distribution with
the mass distribution at earlier times can be used to measure the rate of
structure formation, placing important constraints on cosmological models.
Second, clusters are essentially ``closed boxes'' that retain all their gaseous
matter, despite the enormous energy input associated with supernovae and active
galactic nuclei, because the gravitational potential wells of clusters are so
deep. The baryonic component of clusters therefore contains a wealth of
information about the processes associated with galaxy formation, including the
efficiency with which baryons are converted into stars and the effects of the
resulting feedback processes on galaxy formation. This article reviews our
theoretical understanding of both the dark-matter component and the baryonic
component of clusters. (Abridged)Comment: 54 pages, 15 figures, Rev. Mod. Phys. (in press
Curvature pressure in a cosmology with a tired-light redshift
A hypothesis of curvature pressure is used to derive a static and stable
cosmology with a tired-light redshift. The idea is that the high energy
particles in the inter-galactic medium do not travel along geodesics because of
the strong electrostatic forces. The result is a reaction back on the medium
that is seen as an additional pressure. Combined with the explanation of the
Hubble redshift as a gravitational interaction results in a static and stable
cosmology. The predicted Hubble constant is 60.2 km/s/Mpc, the predicted
background microwave temperature is 3 degrees and quasar luminosity functions
and angular size distributions are shown to be consistent with the model. Since
most observations that imply dark matter rely on redshift data it is argued
that there is no dark matter. Observations of quasar absorption lines,
supernovae light curves and the Butcher-Oemler effect are discussed. The
curvature pressure is important for stellar structure and may explain the solar
neutrino deficiency.Comment: 27 pages, no figures. This is a rewritten version of astro-ph/9803009
Accepted by Australian J. Phys. Changes to title, typo's and updated
reference
Mass transport by buoyant bubbles in galaxy clusters
We investigate the effect of three important processes by which AGN-blown
bubbles transport material: drift, wake transport and entrainment. The first of
these, drift, occurs because a buoyant bubble pushes aside the adjacent
material, giving rise to a net upward displacement of the fluid behind the
bubble. For a spherical bubble, the mass of upwardly displaced material is
roughly equal to half the mass displaced by the bubble, and should be ~
10^{7-9} solar masses depending on the local ICM and bubble parameters. We show
that in classical cool core clusters, the upward displacement by drift may be a
key process in explaining the presence of filaments behind bubbles. A bubble
also carries a parcel of material in a region at its rear, known as the wake.
The mass of the wake is comparable to the drift mass and increases the average
density of the bubble, trapping it closer to the cluster centre and reducing
the amount of heating it can do during its ascent. Moreover, material dropping
out of the wake will also contribute to the trailing filaments. Mass transport
by the bubble wake can effectively prevent the build-up of cool material in the
central galaxy, even if AGN heating does not balance ICM cooling. Finally, we
consider entrainment, the process by which ambient material is incorporated
into the bubble. AbridgedComment: Accepted for publication in MNRAS. 17 pages, 4 figures, 2 tables.
Formatted for letter paper and adjusted author affiliations
Stellar Models with Magnetism and Rotation: Mixing Length Theories and Convection Simulations
Some low-mass stars appear to have larger radii than predicted by standard 1D structure models; prior work has suggested that inefficient convective heat transport, due to rotation and/or magnetism, may ultimately be responsible. In this thesis, we explore this possibility using a combination of 1D stellar models, 2D and 3D simulations, and analytical theory. First, we examine this issue using 1D stellar models constructed using the Modules for Experiments in Stellar Astrophysics (MESA) code. We begin by considering standard models that do not explicitly include rotational/magnetic effects, with convective inhibition modelled by decreasing a depth-independent mixing length theory (MLT) parameter αMLT. We provide formulae linking changes in αMLT to changes in the interior specific entropy, and hence to the stellar radius. Next, we modify the MLT formulation in MESA to mimic explicitly the influence of rotation and magnetism, using formulations suggested by Stevenson (1979) and MacDonald and Mullan (2014) respectively. We find rapid rotation in these models has a negligible impact on stellar structure, primarily because a starâs adiabat, and hence its radius, is predominantly affected by layers near the surface; convection is rapid and largely uninfluenced by rotation there. Magnetic fields, if they influenced convective transport in the manner described by MacDonald and Mullan (2014), could lead to more noticeable radius inflation. Finally, we show that these non-standard effects on stellar structure can be fabricated using a depth-dependent αMLT: a non-magnetic, non-rotating model can be produced that is virtually indistinguishable from one that explicitly parameterises rotation and/or magnetism using the two formulations above. We provide formulae linking the radially-variable αMLT to these putative MLT reformulations.
We make further comparisons between MLT and simulations of convection, to establish how heat transport and stellar structure are influenced by rotation and magnetism, by looking at the entropy content of 2D local and 3D global convective calculations. Using 2D âbox in a starâ simulations, created using the convection code Dedalus, we investigate changes in bulk properties of the specific entropy for increasingly stratified domains. We observe regions stable against convection near the bottom boundary, resulting in the specific entropy in the bulk of the domain exceeding the bottom boundary value: this could be a result of physical effects, such as increased amounts of viscous dissipation for more supercritical, highly stratified cases, but may also be influenced by the artificial boundary conditions imposed by these local simulations. We then turn to 3D global simulations, created using the convection code Rayleigh, and investigate these same properties as a function of rotation rate. We find the average of the shell-averaged specific entropy gradient in the middle third of the domain to scale with rotation rate in a similar fashion to the scaling law derived via MLT arguments in Barker et al. (2014), i.e., |âšds/drâ©| â Ω^4/5.This research has been supported by the European Research Council, from the European Unionâs Horizon 2020 research and innovation programme, under grant agreement No. 337705 (CHASM), and by a Consolidated Grant from the UK STFC (ST/J001627/1)
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