114,660 research outputs found
Simple free-surface detection in two and three-dimensional SPH solver
A simple free-surface particle detection method for two and three-dimensional
SPH simulation has been implemented. The method uses sphere representation for
the SPH particle. The fluid domain is covered by overlapping spheres. A sphere
whose surface is not fully covered considered as boundary. To test particle
boundary status, we used a sum of normalized relative position vectors from
neighbouring particles to the test particle. By checking the existence of un-
covered sphere surface by this vector sum, boundary status of the test particle
can be determined. This boundary detection method can be easily embedded in the
SPH solver algorithm.Comment: 10 pages, 11 figures, Selected Paper from the International Symposium
on Computational Science 201
Probing turbulent superstructures in Rayleigh-B\'{e}nard convection by Lagrangian trajectory clusters
We analyze large-scale patterns in three-dimensional turbulent convection in
a horizontally extended square convection cell by Lagrangian particle
trajectories calculated in direct numerical simulations. A simulation run at a
Prandtl number Pr , a Rayleigh number Ra , and an aspect ratio
is therefore considered. These large-scale structures, which are
denoted as turbulent superstructures of convection, are detected by the
spectrum of the graph Laplacian matrix. Our investigation, which follows
Hadjighasem {\it et al.}, Phys. Rev. E {\bf 93}, 063107 (2016), builds a
weighted and undirected graph from the trajectory points of Lagrangian
particles. Weights at the edges of the graph are determined by a mean dynamical
distance between different particle trajectories. It is demonstrated that the
resulting trajectory clusters, which are obtained by a subsequent -means
clustering, coincide with the superstructures in the Eulerian frame of
reference. Furthermore, the characteristic times and lengths
of the superstructures in the Lagrangian frame of reference agree
very well with their Eulerian counterparts, and ,
respectively. This trajectory-based clustering is found to work for times
. Longer time periods require a
change of the analysis method to a density-based trajectory clustering by means
of time-averaged Lagrangian pseudo-trajectories, which is applied in this
context for the first time. A small coherent subset of the pseudo-trajectories
is obtained in this way consisting of those Lagrangian particles that are
trapped for long times in the core of the superstructure circulation rolls and
are thus not subject to ongoing turbulent dispersion.Comment: 12 pages, 7 downsized figures, to appear in Phys. Rev. Fluid
Edge Detecting New Physics the Voronoi Way
We point out that interesting features in high energy physics data can be
determined from properties of Voronoi tessellations of the relevant phase
space. For illustration, we focus on the detection of kinematic "edges" in two
dimensions, which may signal physics beyond the standard model. After deriving
some useful geometric results for Voronoi tessellations on perfect grids, we
propose several algorithms for tagging the Voronoi cells in the vicinity of
kinematic edges in real data. We show that the efficiency is improved by the
addition of a few Voronoi relaxation steps via Lloyd's method. By preserving
the maximum spatial resolution of the data, Voronoi methods can be a valuable
addition to the data analysis toolkit at the LHC.Comment: 6 pages, 7 figure
Identifying Phase Space Boundaries with Voronoi Tessellations
Determining the masses of new physics particles appearing in decay chains is
an important and longstanding problem in high energy phenomenology. Recently it
has been shown that these mass measurements can be improved by utilizing the
boundary of the allowed region in the fully differentiable phase space in its
full dimensionality. Here we show that the practical challenge of identifying
this boundary can be solved using techniques based on the geometric properties
of the cells resulting from Voronoi tessellations of the relevant data. The
robust detection of such phase space boundaries in the data could also be used
to corroborate a new physics discovery based on a cut-and-count analysis.Comment: 48 pages, 23 figures, Journal-submitted versio
REBOUND: An open-source multi-purpose N-body code for collisional dynamics
REBOUND is a new multi-purpose N-body code which is freely available under an
open-source license. It was designed for collisional dynamics such as planetary
rings but can also solve the classical N-body problem. It is highly modular and
can be customized easily to work on a wide variety of different problems in
astrophysics and beyond.
REBOUND comes with three symplectic integrators: leap-frog, the symplectic
epicycle integrator (SEI) and a Wisdom-Holman mapping (WH). It supports open,
periodic and shearing-sheet boundary conditions. REBOUND can use a Barnes-Hut
tree to calculate both self-gravity and collisions. These modules are fully
parallelized with MPI as well as OpenMP. The former makes use of a static
domain decomposition and a distributed essential tree. Two new collision
detection modules based on a plane-sweep algorithm are also implemented. The
performance of the plane-sweep algorithm is superior to a tree code for
simulations in which one dimension is much longer than the other two and in
simulations which are quasi-two dimensional with less than one million
particles.
In this work, we discuss the different algorithms implemented in REBOUND, the
philosophy behind the code's structure as well as implementation specific
details of the different modules. We present results of accuracy and scaling
tests which show that the code can run efficiently on both desktop machines and
large computing clusters.Comment: 10 pages, 9 figures, accepted by A&A, source code available at
https://github.com/hannorein/reboun
An adaptive hierarchical domain decomposition method for parallel contact dynamics simulations of granular materials
A fully parallel version of the contact dynamics (CD) method is presented in
this paper. For large enough systems, 100% efficiency has been demonstrated for
up to 256 processors using a hierarchical domain decomposition with dynamic
load balancing. The iterative scheme to calculate the contact forces is left
domain-wise sequential, with data exchange after each iteration step, which
ensures its stability. The number of additional iterations required for
convergence by the partially parallel updates at the domain boundaries becomes
negligible with increasing number of particles, which allows for an effective
parallelization. Compared to the sequential implementation, we found no
influence of the parallelization on simulation results.Comment: 19 pages, 15 figures, published in Journal of Computational Physics
(2011
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