2,393 research outputs found
Asymptotic Steady State Solution to a Bow Shock with an Infinite Mach Number
The problem of a cold gas flowing past a stationary object is considered. It
is shown that at large distances from the obstacle the shock front forms a
parabolic solid of revolution. The interior of the shock front is obtained by
solution of the hydrodynamic equations in parabolic coordinates. The results
are verified with a hydrodynamic simulation. The drag force and expected
spectra are calculated for such shock, both in case of an optically thin and
thick media. Finally, relations to astrophysical bow shocks and other analytic
works on oblique shocks are discussed
Binary versus non-binary information in real time series: empirical results and maximum-entropy matrix models
The dynamics of complex systems, from financial markets to the brain, can be
monitored in terms of multiple time series of activity of the constituent
units, such as stocks or neurons respectively. While the main focus of time
series analysis is on the magnitude of temporal increments, a significant piece
of information is encoded into the binary projection (i.e. the sign) of such
increments. In this paper we provide further evidence of this by showing strong
nonlinear relations between binary and non-binary properties of financial time
series. These relations are a novel quantification of the fact that extreme
price increments occur more often when most stocks move in the same direction.
We then introduce an information-theoretic approach to the analysis of the
binary signature of single and multiple time series. Through the definition of
maximum-entropy ensembles of binary matrices and their mapping to spin models
in statistical physics, we quantify the information encoded into the simplest
binary properties of real time series and identify the most informative
property given a set of measurements. Our formalism is able to accurately
replicate, and mathematically characterize, the observed binary/non-binary
relations. We also obtain a phase diagram allowing us to identify, based only
on the instantaneous aggregate return of a set of multiple time series, a
regime where the so-called `market mode' has an optimal interpretation in terms
of collective (endogenous) effects, a regime where it is parsimoniously
explained by pure noise, and a regime where it can be regarded as a combination
of endogenous and exogenous factors. Our approach allows us to connect spin
models, simple stochastic processes, and ensembles of time series inferred from
partial information
Atmospheric Mass Loss During Planet Formation: The Importance of Planetesimal Impacts
We quantify the atmospheric mass loss during planet formation by examining
the contributions to atmospheric loss from both giant impacts and planetesimal
accretion. Giant impacts cause global motion of the ground. Using analytic
self-similar solutions and full numerical integrations we find (for isothermal
atmospheres with adiabatic index () that the local atmospheric mass
loss fraction for ground velocities is given by
, where is the escape velocity
from the target. Yet, the global atmospheric mass loss is a weaker function of
the impactor velocity and mass and given by (isothermal atmosphere) and
(adiabatic atmosphere), where . Atmospheric mass loss
due to planetesimal impacts proceeds in two different regimes: 1) Large enough
impactors (25~km for the current Earth),
are able to eject all the atmosphere above the tangent plane of the impact
site, which is of the whole atmosphere, where , and are
the atmospheric scale height, radius of the target, and its atmospheric density
at the ground. 2) Smaller impactors, but above (1~km for
the current Earth) are only able to eject a fraction of the atmospheric mass
above the tangent plane. We find that the most efficient impactors (per unit
impactor mass) for atmospheric loss are planetesimals just above that lower
limit and that the current atmosphere of the Earth could have resulted from an
equilibrium between atmospheric erosion and volatile delivery to the atmosphere
from planetesimals. We conclude that planetesimal impacts are likely to have
played a major role in atmospheric mass loss over the formation history of the
terrestrial planets. (Abridged)Comment: Submitted to Icarus, 39 pages, 16 figure
Rich: Open Source Hydrodynamic Simulation on a Moving Voronoi Mesh
We present here RICH, a state of the art 2D hydrodynamic code based on
Godunov's method, on an unstructured moving mesh (the acronym stands for Racah
Institute Computational Hydrodynamics). This code is largely based on the code
AREPO. It differs from AREPO in the interpolation and time advancement scheme
as well as a novel parallelization scheme based on Voronoi tessellation. Using
our code we study the pros and cons of a moving mesh (in comparison to a static
mesh). We also compare its accuracy to other codes. Specifically, we show that
our implementation of external sources and time advancement scheme is more
accurate and robust than AREPO's, when the mesh is allowed to move. We
performed a parameter study of the cell rounding mechanism (Llyod iterations)
and it effects. We find that in most cases a moving mesh gives better results
than a static mesh, but it is not universally true. In the case where matter
moves in one way, and a sound wave is traveling in the other way (such that
relative to the grid the wave is not moving) a static mesh gives better results
than a moving mesh. Moreover, we show that Voronoi based moving mesh schemes
suffer from an error, that is resolution independent, due to inconsistencies
between the flux calculation and change in the area of a cell. Our code is
publicly available as open source and designed in an object oriented, user
friendly way that facilitates incorporation of new algorithms and physical
processes
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