21,602 research outputs found
Self-organizing map based adaptive sampling
We propose a new adaptive sampling method that uses Self-Organizing Maps (SOM). In SOM, densely sampled regions in the input space is represented by a larger area on the map than that of sparsely sampled regions. We use this property to progressively tune-in on the interesting region of the design space. The method does not rely on parameterized distribution, and can sample from multi-modal and non-convex distributions. In this paper, we minimize several mathematical test functions. We also show its performance in inequality-constrained objective satisfaction problem, in which the objective is to seek diversity in solutions satisfying certain upper-bound constraint in the minimized objective. A new merit function and a measure of space-filling quality were proposed for this purpose
Efficient Implementations of Molecular Dynamics Simulations for Lennard-Jones Systems
Efficient implementations of the classical molecular dynamics (MD) method for
Lennard-Jones particle systems are considered. Not only general algorithms but
also techniques that are efficient for some specific CPU architectures are also
explained. A simple spatial-decomposition-based strategy is adopted for
parallelization. By utilizing the developed code, benchmark simulations are
performed on a HITACHI SR16000/J2 system consisting of IBM POWER6 processors
which are 4.7 GHz at the National Institute for Fusion Science (NIFS) and an
SGI Altix ICE 8400EX system consisting of Intel Xeon processors which are 2.93
GHz at the Institute for Solid State Physics (ISSP), the University of Tokyo.
The parallelization efficiency of the largest run, consisting of 4.1 billion
particles with 8192 MPI processes, is about 73% relative to that of the
smallest run with 128 MPI processes at NIFS, and it is about 66% relative to
that of the smallest run with 4 MPI processes at ISSP. The factors causing the
parallel overhead are investigated. It is found that fluctuations of the
execution time of each process degrade the parallel efficiency. These
fluctuations may be due to the interference of the operating system, which is
known as OS Jitter.Comment: 33 pages, 19 figures, add references and figures are revise
Coherent control of photocurrent in a strongly scattering photoelectrochemical system
A fundamental issue that limits the efficiency of many photoelectrochemical
systems is that the photon absorption length is typically much longer than the
electron diffusion length. Various photon management schemes have been
developed to enhance light absorption; one simple approach is to use randomly
scattering media to enable broadband and wide-angle enhancement. However, such
systems are often opaque, making it difficult to probe photo-induced processes.
Here we use wave interference effects to modify the spatial distribution of
light inside a highly-scattering dye-sensitized solar cell to control photon
absorption in a space-dependent manner. By shaping the incident wavefront of a
laser beam, we enhance or suppress photocurrent by increasing or decreasing
light concentration on the front side of the mesoporous photoanode where the
collection efficiency of photoelectrons is maximal. Enhanced light absorption
is achieved by reducing reflection through the open boundary of the photoanode
via destructive interference, leading to a factor of two increase in
photocurrent. This approach opens the door to probing and manipulating
photoelectrochemical processes in specific regions inside nominally opaque
media.Comment: 21 pages, 4 figures, in submission. The first two authors contributed
equally to this paper, and should be regarded as co-first author
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