40,785 research outputs found
VADER: A Flexible, Robust, Open-Source Code for Simulating Viscous Thin Accretion Disks
The evolution of thin axisymmetric viscous accretion disks is a classic
problem in astrophysics. While models based on this simplified geometry provide
only approximations to the true processes of instability-driven mass and
angular momentum transport, their simplicity makes them invaluable tools for
both semi-analytic modeling and simulations of long-term evolution where two-
or three-dimensional calculations are too computationally costly. Despite the
utility of these models, the only publicly-available frameworks for simulating
them are rather specialized and non-general. Here we describe a highly
flexible, general numerical method for simulating viscous thin disks with
arbitrary rotation curves, viscosities, boundary conditions, grid spacings,
equations of state, and rates of gain or loss of mass (e.g., through winds) and
energy (e.g., through radiation). Our method is based on a conservative,
finite-volume, second-order accurate discretization of the equations, which we
solve using an unconditionally-stable implicit scheme. We implement Anderson
acceleration to speed convergence of the scheme, and show that this leads to
factor of speed gains over non-accelerated methods in realistic
problems, though the amount of speedup is highly problem-dependent. We have
implemented our method in the new code Viscous Accretion Disk Evolution
Resource (VADER), which is freely available for download from
https://bitbucket.org/krumholz/vader/ under the terms of the GNU General Public
License.Comment: 58 pages, 13 figures, accepted to Astronomy & Computing; this version
includes more discussion, but no other changes; code is available for
download from https://bitbucket.org/krumholz/vader
Acceleration of Coarse Grain Molecular Dynamics on GPU Architectures
Coarse grain (CG) molecular models have been proposed to simulate complex sys- tems with lower computational overheads and longer timescales with respect to atom- istic level models. However, their acceleration on parallel architectures such as Graphic Processing Units (GPU) presents original challenges that must be carefully evaluated. The objective of this work is to characterize the impact of CG model features on parallel simulation performance. To achieve this, we implemented a GPU-accelerated version of a CG molecular dynamics simulator, to which we applied specic optimizations for CG models, such as dedicated data structures to handle dierent bead type interac- tions, obtaining a maximum speed-up of 14 on the NVIDIA GTX480 GPU with Fermi architecture. We provide a complete characterization and evaluation of algorithmic and simulated system features of CG models impacting the achievable speed-up and accuracy of results, using three dierent GPU architectures as case studie
Thermal recoil force, telemetry, and the Pioneer anomaly
Precision navigation of spacecraft requires accurate knowledge of small
forces, including the recoil force due to anisotropies of thermal radiation
emitted by spacecraft systems. We develop a formalism to derive the thermal
recoil force from the basic principles of radiative heat exchange and
energy-momentum conservation. The thermal power emitted by the spacecraft can
be computed from engineering data obtained from flight telemetry, which yields
a practical approach to incorporate the thermal recoil force into precision
spacecraft navigation. Alternatively, orbit determination can be used to
estimate the contribution of the thermal recoil force. We apply this approach
to the Pioneer anomaly using a simulated Pioneer 10 Doppler data set.Comment: 10 pages, 3 figures. Published versio
The Classical Stellar Atmosphere Problem
We introduce the classical stellar atmosphere problem and describe in detail
its numerical solution. The problem consists of the solution of the radiation
transfer equations under the constraints of hydrostatic, radiative and
statistical equilibrium (non-LTE). We outline the basic idea of the Accelerated
Lambda Iteration (ALI) technique and statistical methods which finally allow
the construction of non-LTE model atmospheres considering the influence of
millions of metal absorption lines. Some applications of the new models are
presented.Comment: accepted for publication in The Journal of Computational and Applied
Mathematics, Computational Astrophysics, eds. H. Riffert, K. Werne
The Pioneer Anomaly
Radio-metric Doppler tracking data received from the Pioneer 10 and 11
spacecraft from heliocentric distances of 20-70 AU has consistently indicated
the presence of a small, anomalous, blue-shifted frequency drift uniformly
changing with a rate of ~6 x 10^{-9} Hz/s. Ultimately, the drift was
interpreted as a constant sunward deceleration of each particular spacecraft at
the level of a_P = (8.74 +/- 1.33) x 10^{-10} m/s^2. This apparent violation of
the Newton's gravitational inverse-square law has become known as the Pioneer
anomaly; the nature of this anomaly remains unexplained. In this review, we
summarize the current knowledge of the physical properties of the anomaly and
the conditions that led to its detection and characterization. We review
various mechanisms proposed to explain the anomaly and discuss the current
state of efforts to determine its nature. A comprehensive new investigation of
the anomalous behavior of the two Pioneers has begun recently. The new efforts
rely on the much-extended set of radio-metric Doppler data for both spacecraft
in conjunction with the newly available complete record of their telemetry
files and a large archive of original project documentation. As the new study
is yet to report its findings, this review provides the necessary background
for the new results to appear in the near future. In particular, we provide a
significant amount of information on the design, operations and behavior of the
two Pioneers during their entire missions, including descriptions of various
data formats and techniques used for their navigation and radio-science data
analysis. As most of this information was recovered relatively recently, it was
not used in the previous studies of the Pioneer anomaly, but it is critical for
the new investigation.Comment: 165 pages, 40 figures, 16 tables; accepted for publication in Living
Reviews in Relativit
Pioneer Anomaly: Evaluating Newly Recovered Data
The Pioneer 10/11 spacecraft yielded the most precise navigation in deep
space to date. However, their radio-metric tracking data received from the
distances between 20--70 astronomical units from the Sun consistently indicated
the presence of a small, anomalous, Doppler frequency drift. The drift is a
blue frequency shift that can be interpreted as a sunward acceleration of a_P =
(8.74 +/- 1.33) x 10^(-10) m/s^2 for each particular spacecraft. This signal
has become known as the Pioneer anomaly; the nature of this anomaly remains
unexplained.
New Pioneer 10 and 11 radio-metric Doppler data recently became available.
The much extended set of Pioneer Doppler data is the primary source for new
upcoming investigation of the anomaly. We also have almost entire records of
flight telemetry files received from the the Pioneers. Together with original
project documentation and newly developed software tools, this additional
information is now used to reconstruct the engineering history of both
spacecraft. To that extent, a thermal model of the Pioneer vehicles is being
developed to study possible contribution of thermal recoil force acting on the
two spacecraft. In addition, to improve the accuracy of orbital reconstruction,
we developed a new approach that uses actual flight telemetry data during
trajectory analysis of radio-metric Doppler files. The ultimate goal of these
efforts is to investigate possible contributions of the thermal recoil force to
the detected anomalous acceleration.Comment: 12 pages, 15 figures, invited talk at the "III Mexican Meeting on
Mathematical and Experimental Physics," Mexico City, Mexico, 10-14 September
200
- …