3,924 research outputs found
Complex Networks
Introduction to the Special Issue on Complex Networks, Artificial Life
journal.Comment: 7 pages, in pres
Modeling Transport Mode Decisions Using Hierarchical Binary Spatial Regression Models with Cluster Effects
This work is motivated by a mobility study conducted in the city of Munich, Germany. The variable of interest is a binary response, which indicates whether public transport has been utilized or not. One of the central questions is to identify areas of low/high utilization of public transport after adjusting for explanatory factors such as trip, individual and household attributes. The goal is to develop flexible statistical models for a binary response with covariate, spatial and cluster effects. One approach for modeling spatial effects are Markov Random Fields (MRF). A modification of a class of MRF models with proper joint distributions introduced by Pettitt et al. (2002) is developed. This modification has the desirable property to contain the intrinsic MRF in the limit and still allows for efficient spatial parameter updates in Markov Chain Monte Carlo (MCMC) algorithms. In addition to spatial effects, cluster effects are taken into consideration. Group and individual approaches for modeling these effects are suggested. The first one models heterogeneity between clusters, while the second one models heterogeneity within clusters. A naive approach to include individual cluster effects results in an unidentifiable model. It is shown how an appropriate reparametrization gives identifiable parameters. This provides a new approach for modeling heterogeneity within clusters. For hierarchical spatial binary regression models with individual cluster effects two MCMC algorithms for parameter estimation are developed. The first one is based on a direct evaluation of the likelihood. The second one is based on the representation of binary responses with Gaussian latent variables through a threshold mechanism, which is particularly useful for probit models. Simulation results show a satisfactory behavior of the MCMC algorithms developed. Finally the proposed model classes are applied to the mobility study and results are interpreted
An algebraic multigrid method for mixed discretizations of the Navier-Stokes equations
Algebraic multigrid (AMG) preconditioners are considered for discretized
systems of partial differential equations (PDEs) where unknowns associated with
different physical quantities are not necessarily co-located at mesh points.
Specifically, we investigate a mixed finite element discretization of
the incompressible Navier-Stokes equations where the number of velocity nodes
is much greater than the number of pressure nodes. Consequently, some velocity
degrees-of-freedom (dofs) are defined at spatial locations where there are no
corresponding pressure dofs. Thus, AMG approaches leveraging this co-located
structure are not applicable. This paper instead proposes an automatic AMG
coarsening that mimics certain pressure/velocity dof relationships of the
discretization. The main idea is to first automatically define coarse
pressures in a somewhat standard AMG fashion and then to carefully (but
automatically) choose coarse velocity unknowns so that the spatial location
relationship between pressure and velocity dofs resembles that on the finest
grid. To define coefficients within the inter-grid transfers, an energy
minimization AMG (EMIN-AMG) is utilized. EMIN-AMG is not tied to specific
coarsening schemes and grid transfer sparsity patterns, and so it is applicable
to the proposed coarsening. Numerical results highlighting solver performance
are given on Stokes and incompressible Navier-Stokes problems.Comment: Submitted to a journa
Prospects of utilization mining methane on the basis of gas hydrate technologies
The peculiarities of the innovative approach to the utilization of coal mine methane, which is extracted by significant volumes due to the technological process of underground development of coal-gas fields, are revealed. An innovative energy-saving technology for producing gas hydrates with the maximum possible
accumulation of alternative gas volume on the basis of methane degassing systems of coal mines in the Western Donbass and transporting them to the infrastructure of the region to reduce the share of imported gas and pollution
KINETIC FEATURES OF THE DISSOCIATION PROCESS OF GAS HYDRATE DEPOSITS
The research is aimed at revealing the theoretical features of the kinetics of the process of
dissociation of natural gas hydrates, the accounting of which is useful in designing technologies for
their development. The process of decomposition of gas hydrates is considered as a heterogeneous
process, proceeding at the interface of the phases and contains three consecutive stages. The
mathematically grounded speed with which molecules fluctuate around fixed positions and the velocity
of their translational motion in the system “gas – rock environment”. The mathematical expression of
the constant coefficient of molecular diffusion is made. It has been established that when gas is
extracted from gas hydrates, external and internal diffusion zones, as well as diffusion inhibition, are
caused, which causes an increase in the decay time of gas hydrate to gas and water. The character of
the dependence of the change in the rate of dissociation during its various periods is revealed. It was
found that the speed of the dissociation process in these conditions varies with the temperature under
the exponential la
Optimizing Associative Information Transfer within Content-addressable Memory
Original article can be found at: http://www.oldcitypublishing.com/IJUC/IJUC.htmlPeer reviewe
Numerical simulation of spacecraft charging phenomena
A numerical simulation program is being constructed having the following features: (1) infinite circular cylindrical geometry with angle-dependence, (2) inclusion of incident particles, photoelectrons, secondary electrons, backscattered electrons, any gun emissions, and any internal current pathways including surface conductive layers, (3) quasistatic time-dependent iteration, in which sheath potential changes during particle transit times are ignored, (4) use of approximate, locally-dependent space charge density expressions in solving Poisson's equation for sheath potentials, with use of numerical orbit-following to determine surface currents, and (5) incident particle velocity distributions isotropic or beam-like, or some superposition of these. Rationales for each of these features are discussed
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