4,492 research outputs found
Sparse spectral-tau method for the three-dimensional helically reduced wave equation on two-center domains
We describe a multidomain spectral-tau method for solving the
three-dimensional helically reduced wave equation on the type of two-center
domain that arises when modeling compact binary objects in astrophysical
applications. A global two-center domain may arise as the union of Cartesian
blocks, cylindrical shells, and inner and outer spherical shells. For each such
subdomain, our key objective is to realize certain (differential and
multiplication) physical-space operators as matrices acting on the
corresponding set of modal coefficients. We achieve sparse banded realizations
through the integration "preconditioning" of Coutsias, Hagstrom, Hesthaven, and
Torres. Since ours is the first three-dimensional multidomain implementation of
the technique, we focus on the issue of convergence for the global solver, here
the alternating Schwarz method accelerated by GMRES. Our methods may prove
relevant for numerical solution of other mixed-type or elliptic problems, and
in particular for the generation of initial data in general relativity.Comment: 37 pages, 3 figures, 12 table
Self-adapting structuring and representation of space
The objective of this report is to propose a syntactic formalism for space representation. Beside the well known advantages of hierarchical data structure, the underlying approach has the additional strength of self-adapting to a spatial structure at hand. The formalism is called puzzletree because its generation results in a number of blocks which in a certain order -- like a puzzle - reconstruct the original space. The strength of the approach does not lie only in providing a compact representation of space (e.g. high compression), but also in attaining an ideal basis for further knowledge-based modeling and recognition of objects. The approach may be applied to any higher-dimensioned space (e.g. images, volumes). The report concentrates on the principles of puzzletrees by explaining the underlying heuristic for their generation with respect to 2D spaces, i.e. images, but also schemes their application to volume data. Furthermore, the paper outlines the use of puzzletrees to facilitate higher-level operations like image segmentation or object recognition. Finally, results are shown and a comparison to conventional region quadtrees is done
Extending a serial 3D two-phase CFD code to parallel execution over MPI by using the PETSc library for domain decomposition
To leverage the last two decades' transition in High-Performance Computing
(HPC) towards clusters of compute nodes bound together with fast interconnects,
a modern scalable CFD code must be able to efficiently distribute work amongst
several nodes using the Message Passing Interface (MPI). MPI can enable very
large simulations running on very large clusters, but it is necessary that the
bulk of the CFD code be written with MPI in mind, an obstacle to parallelizing
an existing serial code.
In this work we present the results of extending an existing two-phase 3D
Navier-Stokes solver, which was completely serial, to a parallel execution
model using MPI. The 3D Navier-Stokes equations for two immiscible
incompressible fluids are solved by the continuum surface force method, while
the location of the interface is determined by the level-set method.
We employ the Portable Extensible Toolkit for Scientific Computing (PETSc)
for domain decomposition (DD) in a framework where only a fraction of the code
needs to be altered. We study the strong and weak scaling of the resulting
code. Cases are studied that are relevant to the fundamental understanding of
oil/water separation in electrocoalescers.Comment: 8 pages, 6 figures, final version for to the CFD 2014 conferenc
Surveillance centric coding
PhDThe research work presented in this thesis focuses on the development of techniques
specific to surveillance videos for efficient video compression with higher processing
speed. The Scalable Video Coding (SVC) techniques are explored to achieve higher
compression efficiency. The framework of SVC is modified to support Surveillance
Centric Coding (SCC). Motion estimation techniques specific to surveillance videos
are proposed in order to speed up the compression process of the SCC.
The main contributions of the research work presented in this thesis are divided into
two groups (i) Efficient Compression and (ii) Efficient Motion Estimation. The
paradigm of Surveillance Centric Coding (SCC) is introduced, in which coding aims
to achieve bit-rate optimisation and adaptation of surveillance videos for storing and
transmission purposes. In the proposed approach the SCC encoder communicates
with the Video Content Analysis (VCA) module that detects events of interest in
video captured by the CCTV. Bit-rate optimisation and adaptation are achieved by
exploiting the scalability properties of the employed codec. Time segments
containing events relevant to surveillance application are encoded using high spatiotemporal
resolution and quality while the irrelevant portions from the surveillance
standpoint are encoded at low spatio-temporal resolution and / or quality. Thanks to
the scalability of the resulting compressed bit-stream, additional bit-rate adaptation is
possible; for instance for the transmission purposes. Experimental evaluation showed
that significant reduction in bit-rate can be achieved by the proposed approach
without loss of information relevant to surveillance applications.
In addition to more optimal compression strategy, novel approaches to performing
efficient motion estimation specific to surveillance videos are proposed and
implemented with experimental results. A real-time background subtractor is used to
detect the presence of any motion activity in the sequence. Different approaches for
selective motion estimation, GOP based, Frame based and Block based, are
implemented. In the former, motion estimation is performed for the whole group of
pictures (GOP) only when a moving object is detected for any frame of the GOP.
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While for the Frame based approach; each frame is tested for the motion activity and
consequently for selective motion estimation. The selective motion estimation
approach is further explored at a lower level as Block based selective motion
estimation. Experimental evaluation showed that significant reduction in
computational complexity can be achieved by applying the proposed strategy. In
addition to selective motion estimation, a tracker based motion estimation and fast
full search using multiple reference frames has been proposed for the surveillance
videos.
Extensive testing on different surveillance videos shows benefits of
application of proposed approaches to achieve the goals of the SCC
Compact binary evolutions with the Z4c formulation
Numerical relativity simulations of compact binaries with the Z4c and BSSNOK
formulations are compared. The Z4c formulation is advantageous in every case
considered. In simulations of non-vacuum spacetimes the constraint violations
due to truncation errors are between one and three orders of magnitude lower in
the Z4c evolutions. Improvements are also found in the accuracy of the computed
gravitational radiation. For equal-mass irrotational binary neutron star
evolutions we find that the absolute errors in phase and amplitude of the
waveforms can be up to a factor of four smaller. The quality of the Z4c
numerical data is also demonstrated by a remarkably accurate computation of the
ADM mass from surface integrals. For equal-mass non-spinning binary puncture
black hole evolutions we find that the absolute errors in phase and amplitude
of the waveforms can be up to a factor of two smaller. In the same evolutions
we find that away from the punctures the Hamiltonian constraint violation is
reduced by between one and two orders of magnitude. Furthermore, the utility of
gravitational radiation controlling, constraint preserving boundary conditions
for the Z4c formulation is demonstrated. The evolution of spacetimes containing
a single compact object confirm earlier results in spherical symmetry. The
boundary conditions avoid spurious and non-convergent effects present in high
resolution runs with either formulation with a more naive boundary treatment.
We conclude that Z4c is preferable to BSSNOK for the numerical solution of the
3+1 Einstein equations with the puncture gauge
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