16,890 research outputs found
Numerical Modelling for Process Investigation of a Single Coal Particle Combustion and Gasification
Combustion and Gasification are commercial
processes of coal utilization, and therefore continuous
improvement is needed for these applications. The difference
between these processes is the reaction mechanism, in the case
of combustion the reaction products are CO2 and H2O, whereas
in the case of gasification the products are CO, H2 and CH4. In
order to investigate these processes further, a single coal particle
model has been developed. The definition of the chemical
reactions for each process is key for model development. The
developed numerical model simulation uses CFD
(Computational Fluid Dynamic) techniques with an Eddy Break
Up (EBU) model and a kinetics parameter for controlling the
process reaction. The combustion model has been validated and
extended to model the gasification process by inclusion of an
additional chemical reaction. Finally, it is shown that the single
coal particle model could describe single coal particle
combustion and gasification. From the result, the difference
between single coal particle combustion and gasification can
clearly be seen. This simulation model can be considered for
further investigation of coal combustion and gasification
application processes
Growing Attributed Networks through Local Processes
This paper proposes an attributed network growth model. Despite the knowledge
that individuals use limited resources to form connections to similar others,
we lack an understanding of how local and resource-constrained mechanisms
explain the emergence of rich structural properties found in real-world
networks. We make three contributions. First, we propose a parsimonious and
accurate model of attributed network growth that jointly explains the emergence
of in-degree distributions, local clustering, clustering-degree relationship
and attribute mixing patterns. Second, our model is based on biased random
walks and uses local processes to form edges without recourse to global network
information. Third, we account for multiple sociological phenomena: bounded
rationality, structural constraints, triadic closure, attribute homophily, and
preferential attachment. Our experiments indicate that the proposed Attributed
Random Walk (ARW) model accurately preserves network structure and attribute
mixing patterns of six real-world networks; it improves upon the performance of
eight state-of-the-art models by a statistically significant margin of 2.5-10x.Comment: 11 pages, 13 figure
A Fast Integrated Planning and Control Framework for Autonomous Driving via Imitation Learning
For safe and efficient planning and control in autonomous driving, we need a
driving policy which can achieve desirable driving quality in long-term horizon
with guaranteed safety and feasibility. Optimization-based approaches, such as
Model Predictive Control (MPC), can provide such optimal policies, but their
computational complexity is generally unacceptable for real-time
implementation. To address this problem, we propose a fast integrated planning
and control framework that combines learning- and optimization-based approaches
in a two-layer hierarchical structure. The first layer, defined as the "policy
layer", is established by a neural network which learns the long-term optimal
driving policy generated by MPC. The second layer, called the "execution
layer", is a short-term optimization-based controller that tracks the reference
trajecotries given by the "policy layer" with guaranteed short-term safety and
feasibility. Moreover, with efficient and highly-representative features, a
small-size neural network is sufficient in the "policy layer" to handle many
complicated driving scenarios. This renders online imitation learning with
Dataset Aggregation (DAgger) so that the performance of the "policy layer" can
be improved rapidly and continuously online. Several exampled driving scenarios
are demonstrated to verify the effectiveness and efficiency of the proposed
framework
Institutions, Inequality and Growth: A review of theory and evidence on the institutional determinants of growth and inequality
The difference in the development experiences between the most developed countries and the least developed countries of today is vast. Luxembourg’s per capita income is 200 times larger than Liberia’s. Even within the developing world, growth is very unequal. East Asia and parts of Latin America are growing at impressive rates, while many other countries - especially in Sub-Saharan Africa - struggle with sluggish and volatile growth. This study discusses the theoretical challenge posed in identifying the mechanisms that link institutions and equitable economic growth at various levels of aggregation. The relationship between governance modes and institutions on the one hand, and economic growth and development on the other hand, may take very different forms. This relates to the question of whether a single and unique combination of institutions and governance modes is optimal for (equitable) growth, or whether different governance modes and institutions may lead to good or equitable growth performance in different locations and historical contexts.development administration; growth policy; institutional framework;
Social networks: evolving graphs with memory dependent edges
The plethora, and mass take up, of digital communication tech-
nologies has resulted in a wealth of interest in social network data
collection and analysis in recent years. Within many such networks
the interactions are transient: thus those networks evolve over time.
In this paper we introduce a class of models for such networks using
evolving graphs with memory dependent edges, which may appear and
disappear according to their recent history. We consider time discrete
and time continuous variants of the model. We consider the long
term asymptotic behaviour as a function of parameters controlling
the memory dependence. In particular we show that such networks
may continue evolving forever, or else may quench and become static
(containing immortal and/or extinct edges). This depends on the ex-
istence or otherwise of certain infinite products and series involving
age dependent model parameters. To test these ideas we show how
model parameters may be calibrated based on limited samples of time
dependent data, and we apply these concepts to three real networks:
summary data on mobile phone use from a developing region; online
social-business network data from China; and disaggregated mobile
phone communications data from a reality mining experiment in the
US. In each case we show that there is evidence for memory dependent
dynamics, such as that embodied within the class of models proposed
here
Information is not a Virus, and Other Consequences of Human Cognitive Limits
The many decisions people make about what to pay attention to online shape
the spread of information in online social networks. Due to the constraints of
available time and cognitive resources, the ease of discovery strongly impacts
how people allocate their attention to social media content. As a consequence,
the position of information in an individual's social feed, as well as explicit
social signals about its popularity, determine whether it will be seen, and the
likelihood that it will be shared with followers. Accounting for these
cognitive limits simplifies mechanics of information diffusion in online social
networks and explains puzzling empirical observations: (i) information
generally fails to spread in social media and (ii) highly connected people are
less likely to re-share information. Studies of information diffusion on
different social media platforms reviewed here suggest that the interplay
between human cognitive limits and network structure differentiates the spread
of information from other social contagions, such as the spread of a virus
through a population.Comment: accepted for publication in Future Interne
The transmission of foreign financial crises to South Africa: a firm-level study
The process of financial integration has increased the exposure of South African financial markets to foreign financial crises. This paper contributes to the understanding of crisis transmission by evaluating several hypotheses that claim to explain how financial crises are transmitted to South African financial markets. The study proceeds from a firm-level perspective, which it argues overcomes the potential loss of information when using aggregate economic data. Consequently, the different transmission hypotheses are evaluated for the East Asian, Russian and Argentinean crises using firm-level daily stock return data from the JSE Securities Exchange. A multivariate regression model, supplemented by sensitivity tests, forms the core of the empirical methodology.financial contagion; crisis; South Africa; financial linkages
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