158,672 research outputs found
Galacticus: A Semi-Analytic Model of Galaxy Formation
We describe a new, free and open source semi-analytic model of galaxy
formation, Galacticus. The Galacticus model was designed to be highly modular
to facilitate expansion and the exploration of alternative descriptions of key
physical ingredients. We detail the Galacticus engine for evolving galaxies
through a merging hierarchy of dark matter halos and give details of the
specific implementations of physics currently available in Galacticus. Finally,
we show results from an example model that is in reasonably good agreement with
several observational datasets. We use this model to explore numerical
convergence and to demonstrate the types of information which can be extracted
from Galacticus.Comment: 35 pages, submitted to New Astronom
Contact Line Instability and Pattern Selection in Thermally Driven Liquid Films
Liquids spreading over a solid substrate under the action of various forces
are known to exhibit a long wavelength contact line instability. We use an
example of thermally driven spreading on a horizontal surface to study how the
stability of the flow can be altered, or patterns selected, using feedback
control. We show that thermal perturbations of certain spatial structure
imposed behind the contact line and proportional to the deviation of the
contact line from its mean position can completely suppress the instability.
Due to the presence of mean flow and a spatially nonuniform nature of spreading
liquid films the dynamics of disturbances is governed by a nonnormal evolution
operator, opening up a possibility of transient amplification and nonlinear
instabilities. We show that in the case of thermal driving the nonnormality can
be significant, especially for small wavenumber disturbances, and trace the
origin of transient amplification to a close alignment of a large group of
eigenfunctions of the evolution operator. However, for values of noise likely
to occur in experiments we find that the transient amplification is not
sufficiently strong to either change the predictions of the linear stability
analysis or invalidate the proposed control approach.Comment: 13 pages, 14 figure
Voltage Stabilization in Microgrids via Quadratic Droop Control
We consider the problem of voltage stability and reactive power balancing in
islanded small-scale electrical networks outfitted with DC/AC inverters
("microgrids"). A droop-like voltage feedback controller is proposed which is
quadratic in the local voltage magnitude, allowing for the application of
circuit-theoretic analysis techniques to the closed-loop system. The operating
points of the closed-loop microgrid are in exact correspondence with the
solutions of a reduced power flow equation, and we provide explicit solutions
and small-signal stability analyses under several static and dynamic load
models. Controller optimality is characterized as follows: we show a one-to-one
correspondence between the high-voltage equilibrium of the microgrid under
quadratic droop control, and the solution of an optimization problem which
minimizes a trade-off between reactive power dissipation and voltage
deviations. Power sharing performance of the controller is characterized as a
function of the controller gains, network topology, and parameters. Perhaps
surprisingly, proportional sharing of the total load between inverters is
achieved in the low-gain limit, independent of the circuit topology or
reactances. All results hold for arbitrary grid topologies, with arbitrary
numbers of inverters and loads. Numerical results confirm the robustness of the
controller to unmodeled dynamics.Comment: 14 pages, 8 figure
A Bayesian approach to the semi-analytic model of galaxy formation: methodology
We believe that a wide range of physical processes conspire to shape the
observed galaxy population but we remain unsure of their detailed interactions.
The semi-analytic model (SAM) of galaxy formation uses multi-dimensional
parameterisations of the physical processes of galaxy formation and provides a
tool to constrain these underlying physical interactions. Because of the high
dimensionality, the parametric problem of galaxy formation may be profitably
tackled with a Bayesian-inference based approach, which allows one to constrain
theory with data in a statistically rigorous way. In this paper we develop a
SAM in the framework of Bayesian inference. We show that, with a parallel
implementation of an advanced Markov-Chain Monte-Carlo algorithm, it is now
possible to rigorously sample the posterior distribution of the
high-dimensional parameter space of typical SAMs. As an example, we
characterise galaxy formation in the current CDM cosmology using the
stellar mass function of galaxies as an observational constraint. We find that
the posterior probability distribution is both topologically complex and
degenerate in some important model parameters, suggesting that thorough
explorations of the parameter space are needed to understand the models. We
also demonstrate that because of the model degeneracy, adopting a narrow prior
strongly restricts the model. Therefore, the inferences based on SAMs are
conditional to the model adopted. Using synthetic data to mimic systematic
errors in the stellar mass function, we demonstrate that an accurate
observational error model is essential to meaningful inference.Comment: revised version to match published article published in MNRA
Space Structures: Issues in Dynamics and Control
A selective technical overview is presented on the vibration and control of large space structures, the analysis, design, and construction of which will require major technical contributions from the civil/structural, mechanical, and extended engineering communities. The immediacy of the U.S. space station makes the particular emphasis placed on large space structures and their control appropriate. The space station is but one part of the space program, and includes the lunar base, which the space station is to service. This paper attempts to summarize some of the key technical issues and hence provide a starting point for further involvement. The first half of this paper provides an introduction and overview of large space structures and their dynamics; the latter half discusses structural control, including control‐system design and nonlinearities. A crucial aspect of the large space structures problem is that dynamics and control must be considered simultaneously; the problems cannot be addressed individually and coupled as an afterthought
The role of fluid pressure in induced vs. triggered seismicity. Insights from rock deformation experiments on carbonates
Fluid overpressure is one of the primary mechanisms for tectonic fault slip, because fluids lubricate
the fault and fluid pressure reduces the effective normal stress that holds the fault in place. However,
current models of earthquake nucleation, based on rate- and state- friction laws, imply that stable
sliding is favoured by the increase of pore fluid pressure. Despite this controversy, currently, there are
only a few studies on the role of fluid pressure under controlled, laboratory conditions. Here, we use
laboratory experiments, to show that the rate- and state- friction parameters do change with increasing
fluid pressure. We tested carbonate gouges from sub hydrostatic to near lithostatic fluid pressure
conditions, and show that the friction rate parameter (a−b) evolves from velocity strengthening
to velocity neutral behaviour. Furthermore, the critical slip distance, Dc, decreases from about 90 to
10μm. Our data suggest that fluid overpressure plays an important role in controlling the mode of fault
slip. Since fault rheology and fault stability parameters change with fluid pressure, we suggest that a
comprehensive characterization of these parameters is fundamental for better assessing the role of
fluid pressure in natural and human induced earthquakes
Ongoing transients in carbonate compensation
Uptake of anthropogenic CO2 is acidifying the oceans. Over the next 2000 years, this will modify the dissolution and preservation of sedimentary carbonate. By coupling new formulas for the positions of the calcite saturation horizon, zsat, the compensation depth, zcc, and the snowline, zsnow, to a biogeochemical model of the oceanic carbonate system, we evaluate how these horizons will change with ongoing ocean acidification. Our model is an extended Havardton-Bear-type box model, which includes novel kinetic descriptions for carbonate dissolution above, between, and below these critical depths. In the preindustrial ocean, zsat and zcc are at 3939 and 4750 m, respectively. When forced with the IS92a CO2 emission scenario, the model forecasts (1) that zsat will rise rapidly (“runaway” conditions) so that all deep water becomes undersaturated, (2) that zcc will also rise and over 1000 years will pass before it will be stabilized by the dissolution of previously deposited CaCO3, and (3) that zsnow will respond slowly to acidification, rising by ∼1150 m during a 2000 year timeframe. A further simplified model that equates the compensation and saturation depths produces quantitatively different results. Finally, additional feedbacks due to acidification on calcification and increased atmospheric CO2 on organic matter productivity strongly affect the positions of the compensation horizons and their dynamics.
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