342,694 research outputs found
Methods for Large Scale Hydraulic Fracture Monitoring
In this paper we propose computationally efficient and robust methods for
estimating the moment tensor and location of micro-seismic event(s) for large
search volumes. Our contribution is two-fold. First, we propose a novel
joint-complexity measure, namely the sum of nuclear norms which while imposing
sparsity on the number of fractures (locations) over a large spatial volume,
also captures the rank-1 nature of the induced wavefield pattern. This
wavefield pattern is modeled as the outer-product of the source signature with
the amplitude pattern across the receivers from a seismic source. A rank-1
factorization of the estimated wavefield pattern at each location can therefore
be used to estimate the seismic moment tensor using the knowledge of the array
geometry. In contrast to existing work this approach allows us to drop any
other assumption on the source signature. Second, we exploit the recently
proposed first-order incremental projection algorithms for a fast and efficient
implementation of the resulting optimization problem and develop a hybrid
stochastic & deterministic algorithm which results in significant computational
savings.Comment: arXiv admin note: text overlap with arXiv:1305.006
Online Multivariate Changepoint Detection: Leveraging Links With Computational Geometry
The increasing volume of data streams poses significant computational
challenges for detecting changepoints online. Likelihood-based methods are
effective, but their straightforward implementation becomes impractical online.
We develop two online algorithms that exactly calculate the likelihood ratio
test for a single changepoint in p-dimensional data streams by leveraging
fascinating connections with computational geometry. Our first algorithm is
straightforward and empirically quasi-linear. The second is more complex but
provably quasi-linear: for data points.
Through simulations, we illustrate, that they are fast and allow us to process
millions of points within a matter of minutes up to .Comment: 31 pages,15 figure
Two-step percolation in aggregating systems
The two-step percolation behavior in aggregating systems was studied both
experimentally and by means of Monte Carlo (MC) simulations. In experimental
studies, the electrical conductivity, , of colloidal suspension of
multiwalled carbon nanotubes (CNTs) in decane was measured. The suspension was
submitted to mechanical de-liquoring in a planar filtration-compression
conductometric cell. During de-liquoring, the distance between the measuring
electrodes continuously decreased and the CNT volume fraction
continuously increased (from up to % v/v). The two
percolation thresholds at and can reflect the interpenetration of loose CNT aggregates and
percolation across the compact conducting aggregates, respectively. The MC
computational model accounted for the core-shell structure of conducting
particles or their aggregates, the tendency of a particle for aggregation, the
formation of solvation shells, and the elongated geometry of the conductometric
cell. The MC studies revealed two smoothed percolation transitions in
dependencies that correspond to the percolation through the
shells and cores, respectively. The data demonstrated a noticeable impact of
particle aggregation on anisotropy in electrical conductivity
measured along different directions in the conductometric cell.Comment: 10 pages, 6 figure
Development of High-Order P\u3cem\u3e\u3csub\u3eN\u3c/sub\u3e\u3c/em\u3e Models for Radiative Heat Transfer in Special Geometries and Boundary Conditions
The high-order spherical harmonics () method for 2-D Cartesian domains is extracted from the 3-D formulation. The number of equations and intensity coefficients reduces to (N+1)2/4 in the 2-D Cartesian formulation compared with N(N+1)/2 for the general 3-D formulation. The Marshak boundary conditions are extended to solve problems with nonblack and mixed diffuse-specular surfaces. Additional boundary conditions for specified radiative wall flux, for symmetry/specular reflection boundaries have also been developed. The mathematical details of the formulations and their implementation in the OpenFOAM finite volume based CFD software platform are presented. The accuracy and computational cost of the 2-D Cartesian are compared with that of the 3-D solver and a Photon Monte Carlo solver for a square enclosure, as well as a 45° wedge geometry with variable radiative properties. The new boundary conditions have been applied for both test cases, and the boundary condition for mixed diffuse-specular surfaces is further illustrated by numerical examples of a rectangular geometry enclosed by walls with different surface characteristics
A Moving Boundary Flux Stabilization Method for Cartesian Cut-Cell Grids using Directional Operator Splitting
An explicit moving boundary method for the numerical solution of
time-dependent hyperbolic conservation laws on grids produced by the
intersection of complex geometries with a regular Cartesian grid is presented.
As it employs directional operator splitting, implementation of the scheme is
rather straightforward. Extending the method for static walls from Klein et
al., Phil. Trans. Roy. Soc., A367, no. 1907, 4559-4575 (2009), the scheme
calculates fluxes needed for a conservative update of the near-wall cut-cells
as linear combinations of standard fluxes from a one-dimensional extended
stencil. Here the standard fluxes are those obtained without regard to the
small sub-cell problem, and the linear combination weights involve detailed
information regarding the cut-cell geometry. This linear combination of
standard fluxes stabilizes the updates such that the time-step yielding
marginal stability for arbitrarily small cut-cells is of the same order as that
for regular cells. Moreover, it renders the approach compatible with a wide
range of existing numerical flux-approximation methods. The scheme is extended
here to time dependent rigid boundaries by reformulating the linear combination
weights of the stabilizing flux stencil to account for the time dependence of
cut-cell volume and interface area fractions. The two-dimensional tests
discussed include advection in a channel oriented at an oblique angle to the
Cartesian computational mesh, cylinders with circular and triangular
cross-section passing through a stationary shock wave, a piston moving through
an open-ended shock tube, and the flow around an oscillating NACA 0012 aerofoil
profile.Comment: 30 pages, 27 figures, 3 table
Mechanistic and pathological study of the genesis, growth, and rupture of abdominal aortic aneurysms
Postprint (published version
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