15 research outputs found
Using Numerical Relativity to Explore Strong Gravity and Develop Force-Free Electrodynamics Simulation Software with Best-Practice Development
In this dissertation, we explore the effects of extremely strong gravitational and electro- dynamic fields using the techniques of numerical relativity. We use the existing black hole simulation software in the Einstein Toolkit to compute the spin-up of two initially nonspin- ning black holes as they pass by each other in space. The angular momentum is imparted by the tidal interaction between the two black holes, in a parallel to Earth’s tides, as described by classical mechanics, which also transfer angular momentum between the Earth’s rotation and the Moon’s orbit. The largest observed dimensionless spin observed was 0.20 with an initial boost of 0.78c, and we conclude that higher spin-ups may be possible with larger initial boosts. We also use NRPy+ to generate C code for new simulations. The first two are written as thorns, or modules, for the Einstein Toolkit (ETK). The first is a simple scalar wave solver, which has proven useful as an introduction to NRPy+ for new users who wish to write their own ETK thorns. The second is an open-source alternative to the Kranc-generated WeylScal4, which calculates the Weyl scalars that are needed to extract gravitational wave information from a simulation. We also port GiRaFFE to NRPy+. While this was originally intended to be another ETK thorn, as the project progressed, it ultimately became a stan- dalone simulation. The individual modules of this code also have their own unit tests as an additional validation step. These unit tests also use continuous integration to ensure that bugs are not unknowingly introduced into the code. In the future, this code will be modified to be able to use arbitrary coordinate systems
Physical chemistry of the interactions between multicomponent solvents and biomass
Extraction of cellulose and lignin from plant biomass remains a major issue for enabling more economic and green production of lignocellulosic renewable fuels and byproducts. Although the use of multicomponent solvents has provided remarkable results in wood fractionation processes most of the currently used methods rely on empirically elaborated protocols. Understanding the physicochemical mechanisms of biomass breakdown and its interactions with solvent medium during fractionation will lead to more efficient use of biomass. This defined the focus in this thesis work on a systematic and detailed description of the interactions between ligoncellulose components with binary water-organic mixtures of ethanol and acetonitrile. Our results and their analysis were obtained predominantly with molecular dynamics (MD) simulations, and supported by additional studies of quantum chemical (Density Functional Theory) and mixed quantum mechanical (QM) and classical MD scheme (QM/MM). With these tools we first established a non-linear behavior of the mixed solvent structures, thermodynamic properties and dynamic hardness, as a measure for their global reactivity. The analysis of the average numbers of HBs with the liquid composition shows that alcohol molecules tend to substitute water molecules, allowing compensating for the loss of H-bonds in the water solvent domains. The role of organic component in water solvent mixtures on the conformational changes induced in the main wood components (cellulose, lignin and hemicellulose) is highlighted and their dependence on distinct solvent compositions is unveiled for each organic solvent component and its content in water. This dependence is explained by preferential solute-solvent interatomic interactions as a function of solvent compositions. Subsequently, the evolution of interaction forces in lignin-cellulose and lignin-xylan complexes are also found to have solvent-dependent profiles. All this supports the general conclusion about specific solvent actions on lignocellulose compounds being the driving factors in the observed macroscopic non-linear behavior in wood swelling in mixed water-organics
XXI Workshop de Investigadores en Ciencias de la Computación - WICC 2019: libro de actas
Trabajos presentados en el XXI Workshop de Investigadores en Ciencias de la Computación (WICC), celebrado en la provincia de San Juan los dÃas 25 y 26 de abril 2019, organizado por la Red de Universidades con Carreras en Informática (RedUNCI) y la Facultad de Ciencias Exactas, FÃsicas y Naturales de la Universidad Nacional de San Juan.Red de Universidades con Carreras en Informátic
XXI Workshop de Investigadores en Ciencias de la Computación - WICC 2019: libro de actas
Trabajos presentados en el XXI Workshop de Investigadores en Ciencias de la Computación (WICC), celebrado en la provincia de San Juan los dÃas 25 y 26 de abril 2019, organizado por la Red de Universidades con Carreras en Informática (RedUNCI) y la Facultad de Ciencias Exactas, FÃsicas y Naturales de la Universidad Nacional de San Juan.Red de Universidades con Carreras en Informátic
XX Workshop de Investigadores en Ciencias de la Computación - WICC 2018 : Libro de actas
Actas del XX Workshop de Investigadores en Ciencias de la Computación (WICC 2018), realizado en Facultad de Ciencias Exactas y Naturales y Agrimensura de la Universidad Nacional del Nordeste, los dìas 26 y 27 de abril de 2018.Red de Universidades con Carreras en Informática (RedUNCI
Algorithmic skeletons for exact combinatorial search at scale
Exact combinatorial search is essential to a wide range of application areas including constraint optimisation, graph matching, and computer algebra. Solutions to combinatorial problems are found by systematically exploring a search space, either to enumerate solutions, determine if a specific solution exists, or to find an optimal solution. Combinatorial searches are computationally hard both in theory and practice, and efficiently exploring the huge number of combinations is a real challenge, often addressed using approximate search algorithms. Alternatively, exact search can be parallelised to reduce execution time. However, parallel search is challenging due to both highly irregular search trees and sensitivity to search order, leading to anomalies that can cause unexpected speedups and slowdowns. As core counts continue to grow, parallel search becomes increasingly useful for improving the performance of existing searches, and allowing larger instances to be solved.
A high-level approach to parallel search allows non-expert users to benefit from increasing core counts. Algorithmic Skeletons provide reusable implementations of common parallelism patterns that are parameterised with user code which determines the specific computation, e.g. a particular search. We define a set of skeletons for exact search, requiring the user to provide in the minimal case a single class that specifies how the search tree is generated and a parameter that specifies the type of search required. The five are: Sequential search; three general-purpose parallel search methods: Depth-Bounded, Stack-Stealing, and Budget; and a specific parallel search method, Ordered, that guarantees replicable performance. We implement and evaluate the skeletons in a new C++ parallel search framework, YewPar. YewPar provides both high-level skeletons and low-level search specific schedulers and utilities to deal with the irregularity
of search and knowledge exchange between workers. YewPar is based on the HPX library for distributed task-parallelism potentially allowing search to execute on multi-cores, clusters, cloud, and high performance computing systems.
Underpinning the skeleton design is a novel formal model, MT^3 , a parallel operational semantics that describes multi-threaded tree traversals, allowing reasoning about parallel search, e.g. describing common parallel search phenomena such as performance anomalies.
YewPar is evaluated using seven different search applications (and over 25 specific instances): Maximum Clique, k-Clique, Subgraph Isomorphism, Travelling Salesperson, Binary Knapsack, Enumerating Numerical Semigroups, and the Unbalanced Tree Search Benchmark. The search instances are evaluated at multiple scales from 1 to 255 workers, on a 17 host, 272 core Beowulf cluster. The overheads of the skeletons are low, with a mean 6.1% slowdown compared to hand-coded sequential implementation. Crucially, for all search applications YewPar reduces search times by an order of magnitude, i.e hours/minutes to minutes/seconds, and we commonly see greater than 60% (average) parallel efficiency speedups for up to 255 workers. Comparing skeleton performance reveals that no one skeleton is best for all searches, highlighting a benefit of a skeleton approach that allows multiple parallelisations to be explored with minimal refactoring.
The Ordered skeleton avoids slowdown anomalies where, due to search knowledge being order dependent, a parallel search takes longer than a sequential search. Analysis of Ordered shows that, while being 41% slower on average (73% worse-case) than Depth-Bounded, in nearly all cases it maintains the following replicable performance properties: 1) parallel executions are no slower than one worker sequential executions 2) runtimes do not increase as workers are added, and 3) variance between repeated runs is low. In particular, where Ordered maintains a relative standard deviation (RSD) of less than 15%, Depth-Bounded suffers from an RSD greater than 50%, showing the importance of carefully controlling search orders for repeatability
XX Workshop de Investigadores en Ciencias de la Computación - WICC 2018 : Libro de actas
Actas del XX Workshop de Investigadores en Ciencias de la Computación (WICC 2018), realizado en Facultad de Ciencias Exactas y Naturales y Agrimensura de la Universidad Nacional del Nordeste, los dìas 26 y 27 de abril de 2018.Red de Universidades con Carreras en Informática (RedUNCI
Numerical relativity simulations of tilted black hole-torus systems
Las fusiones de objetos compactos se encuentran entre los eventos más interesantes
de la astrofÃsica relativista, siendo, en particular, el principal objetivo de la
astronomÃa de ondas gravitatorias. En esta tesis investigamos
los posibles estados finales de la fusión de sistemas binarios formados por
agujero negro-estrella de
neutrones o por dos estrellas de neutrones: discos gruesos (o toros) de
acrecimiento alrededor de agujeros negros en rotación tipo Kerr.
Estos sistemas agujero negro-toro se cree que
constituyen el motor central de los eventos más luminosos del Universo: los llamados
estallidos de rayos gamma. Nuestro conocimiento sobre la evolución y la estabilidad de
estos sistemas depende crucialmente de la realización de simulaciones numéricas
precisas de su dinámica, en el contexto de la relatividad numérica, es decir,
tanto el espacio-tiempo como la materia.
El principal trabajo de esta tesis es llevar a cabo tales simulaciones en relatividad numérica
de sistemas agujero negro-toro. Para ello utilizamos modelos de equilibrio de discos
auto-gravitantes alrededor de agujeros negros como datos iniciales, los cuales representan
modelos idealizados (aunque apropiados) de los discos post-fusión reales obtenidos de manera
auto-consistente en simulaciones de fusión de binarias compactas en relatividad numérica.
Tradicionalmente, la simulación numérica de estos sistemas se ha realizado bajo la
hipótesis de que el espÃn del agujero negro y el momento angular del disco están
alineados. Sin embargo, existen razones para creer que muchos de estos sistemas
deberÃan estar inclinados, lo cual significa que el plano ecuatorial del disco ya
no es paralelo al del agujero negro. Ciertamente, sistemas agujero
negro-toro inclinados se han obtenido auto-consistentemente en simulaciones de relatividad
numérica de la fusión de un agujero negro y una estrella de neutrones con los momentos
angulares no alineados.
En tales sistemas inclinados, la dinámica del sistema agujero negro-toro cambia
completamente con respecto al caso alineado, debido a los efectos de relatividad
general que surgen de la estructura del espacio tiempo de la métrica de Kerr.
Especialmente, debido a la precesión diferencial de Lense-Thirring, el disco puede l
legar a estar trenzado y combado. Para capturar la respuesta dinámica
correcta de estos efectos en la evolución del disco es necesario evolucionar los sistemas
mediante simulaciones hidrodinámicas en relatividad general. El trabajo pionero en
este campo fue llevado a cabo por Fragile y colaboradores, quienes
realizaron las primeras simulaciones relativistas de sistemas agujero negro-toro
inclinados en la aproximación fluido de prueba, es decir, asumiendo que el espacio-tiempo
se mantiene inalterado en el tiempo e ignorando la auto-gravedad del disco. Basándonos en
este influyente trabajo, hemos realizado en esta tesis el primer estudio numérico sistemático
de sistemas agujero negro-toro inclinados en un espacio-tiempo totalmente evolucionado.
Nuestro trabajo confirma la mayorÃa de las caracterÃsticas morfológicas del disco
descritas en las simulaciones con fluidos prueba de Fragile y colaboradores.
Además, nuestros resultados muestran precesión y nutación significativas en el agujero negro como
consecuencia de la evolución del disco inclinado para todas las proporciones entre las
masas del disco y del agujero negro consideradas en esta tesis. La dirección del espÃn
del agujero negro se mide utilizando un método estándar en relatividad numérica que,
como se demuestra en la tesis, puede ser derivado a partir del pseudotensor de Weinberg.
Hemos visto que la precesión del agujero negro surge del par de fuerzas que el disco ejerce
cuando se inicia su movimiento de precesión debida al par de fuerzas tipo Lense-Thirring
del agujero negro central.
Por otro lado, simulaciones previas han mostrado que modelos iniciales que involucran
agujeros negros sin rotación y sistemas alineados son propensos a desarrollar la
inestabilidad global hidrodinámica no axisimétrica de Papaloizou-Pringle. En este
trabajo estudiamos estos modelos en el caso inclinado
para estimar el efecto que la inclinación del agujero negro tiene sobre el desarrollo
de tal inestabilidad. Nuestros resultados muestran que la inestabilidad de
Papaloizou-Pringle es una caracterÃstica muy robusta de nuestro sistema fÃsico puesto que,
de hecho, se desarrolla para todas las magnitudes de espÃn y ángulos de inclinación
iniciales investigados.
Lo contrario también es cierto, es decir, un modelo inicialmente estable permanece
estable para todo el espacio de parámetros considerados en nuestro estudio.
En esta tesis investigamos en detalle el mecanismo de la
inestabilidad y mostramos que ésta se manifiesta en la forma de una onda de densidad espiral,
con una velocidad de patrón constante, que viaja a través del disco que gira con
rotación diferencial. Esta onda de densidad facilita el transporte de momento angular
hacia afuera desde las regiones internas del toro, donde tiene momento angular negativo
con respeto al fluido, acoplándose al fluido mediante disipación una vez su amplitud
se vuelve no-lineal y desarrollando choques moderados.
Nuestras simulaciones tridimensionales muestran la presencia de oscilaciones
cuasi-periódicas en la tasa de acreción instantánea,
con frecuencias en un rango compatible con las observadas en binarias de rayos
X de baja masa con la componente compacta siendo o bien una estrella de neutrones
o bien un agujero negro. La razón de frecuencias entre el pico de baja frecuencia
dominante y el primer armónico es o_1/f~1.9, una razón no reproducible
cuando se modelan las oscilaciones cuasi-periódicas como modos p-de oscilación de
toros axisimétricos.Mergers of compact objects are among the most interesting events of relativistic astrophysics,
being, in particular, the prime target for gravitational wave astronomy.
In this thesis, we investigate the possible end-states of black hole-neutron star
and binary neutron star mergers: thick accretion tori around spinning Kerr
black holes. These black hole-torus systems are believed to be the engines
of the brightest events in the Universe, so-called gamma-ray bursts. Our
understanding of the evolution and stability of these systems relies crucially
on accurate numerical relativity simulations of their dynamics, evolving both
spacetime and matter fields. The main work in this thesis are
such numerical relativity simulations of black hole--torus systems.
We use equilibrium models of self-gravitating discs around black holes as
initial data, which represent idealised (yet relevant) models of
the actual post-merger discs found in self-consistent numerical relativity merger
simulations.
Traditionally, numerical simulations of these systems have been carried out under the
assumption that the black hole spin and disc angular momentum are aligned.
There are arguments, however, that many black hole-torus systems should be tilted
instead, which means that the equatorial plane of the disc is no longer parallel to
that of the black hole. These tilted black hole-torus systems form indeed
self-consistently in numerical relativity simulations of misaligned black
hole-neutron star mergers.
In these tilted systems, the dynamics of the torus and black hole change completely
as compared to the untilted case due to general relativistic effects arising from the
structure of the Kerr spacetime. Specifically, due to differential Lense-Thirring precession,
the disc may become twisted and warped.
To capture the correct dynamical response of these effects on the disc evolution,
it is therefore necessary to evolve the systems in general relativistic hydrodynamics
simulations.
Pioneering work in this field has been carried out by Fragile and collaborators, who performed
the first relativistic simulations of tilted black hole-torus systems
in the test-fluid approximation, which means that the spacetime
was held fixed in time and the self-gravity of the disc ignored.
Building on this seminal work, we have performed in this thesis the first systematic study of
tilted black hole-torus systems with a fully evolved spacetime. We observe
many of the disc morphological features described in the test-fluid simulations of
Fragile and collaborators. Additionally, we observe significant black hole precession
and nutation as a result of the tilted disc evolution for all disc-to-black hole
mass ratios considered in this thesis. The black hole spin direction is measured
using a standard method in numerical relativity, which, as we show in this thesis,
can be derived from Weinberg's pseudotensor. The black hole precession is seen to arise
from the torque the disc exerts as it starts to precess due to
the Lense-Thirring torque of the central black hole.
When considering a non-rotating black hole, some of our initial models were known
to be prone to develop the global, non-axisymmetric hydrodynamic Papaloizou-Pringle
instability. We study these models in the tilted case in order
to gauge the effect the black hole tilt has on the development of the
instability. Our results show that the instability is a very robust feature of this
physical system in the sense that it indeed develops for all initial black hole spin magnitudes
and tilt angles we investigate. The contrary is also true, namely that a stable initial
model remains so for the entire parameter space of the study.
We investigate the precise mechanism of the instability and show that it manifests itself in
a spiral density wave of constant pattern speed traveling through the differentially
rotating disc. The density wave facilitates the outward transport of angular
momentum from the inner region of the torus, where it has negative angular momentum
with respect to the fluid, as it couples to the fluid via dissipation when its
amplitude becomes non-linear and mild shocks develop.
Our three-dimensional simulations show the presence of quasi-periodic
oscillations in the instantaneous accretion rate, with frequencies
in a range compatible with those observed in low mass X-ray binaries with either
a black hole or a neutron star component. The frequency ratio of the dominant
low frequency peak and the first overtone is o_1/f~1.9, a frequency ratio
not attainable when modelling the quasi-periodic
oscillations as p-mode oscillations in axisymmetric tori