62,897 research outputs found
solveME: fast and reliable solution of nonlinear ME models.
BackgroundGenome-scale models of metabolism and macromolecular expression (ME) significantly expand the scope and predictive capabilities of constraint-based modeling. ME models present considerable computational challenges: they are much (>30 times) larger than corresponding metabolic reconstructions (M models), are multiscale, and growth maximization is a nonlinear programming (NLP) problem, mainly due to macromolecule dilution constraints.ResultsHere, we address these computational challenges. We develop a fast and numerically reliable solution method for growth maximization in ME models using a quad-precision NLP solver (Quad MINOS). Our method was up to 45 % faster than binary search for six significant digits in growth rate. We also develop a fast, quad-precision flux variability analysis that is accelerated (up to 60× speedup) via solver warm-starts. Finally, we employ the tools developed to investigate growth-coupled succinate overproduction, accounting for proteome constraints.ConclusionsJust as genome-scale metabolic reconstructions have become an invaluable tool for computational and systems biologists, we anticipate that these fast and numerically reliable ME solution methods will accelerate the wide-spread adoption of ME models for researchers in these fields
Limits on Fundamental Limits to Computation
An indispensable part of our lives, computing has also become essential to
industries and governments. Steady improvements in computer hardware have been
supported by periodic doubling of transistor densities in integrated circuits
over the last fifty years. Such Moore scaling now requires increasingly heroic
efforts, stimulating research in alternative hardware and stirring controversy.
To help evaluate emerging technologies and enrich our understanding of
integrated-circuit scaling, we review fundamental limits to computation: in
manufacturing, energy, physical space, design and verification effort, and
algorithms. To outline what is achievable in principle and in practice, we
recall how some limits were circumvented, compare loose and tight limits. We
also point out that engineering difficulties encountered by emerging
technologies may indicate yet-unknown limits.Comment: 15 pages, 4 figures, 1 tabl
Investigation of FACTS devices to improve power quality in distribution networks
Flexible AC transmission system (FACTS) technologies are power electronic solutions
that improve power transmission through enhanced power transfer volume and stability,
and resolve quality and reliability issues in distribution networks carrying sensitive
equipment and non-linear loads. The use of FACTS in distribution systems is still in
its infancy. Voltages and power ratings in distribution networks are at a level where
realistic FACTS devices can be deployed. Efficient power converters and therefore loss
minimisation are crucial prerequisites for deployment of FACTS devices.
This thesis investigates high power semiconductor device losses in detail. Analytical
closed form equations are developed for conduction loss in power devices as a function
of device ratings and operating conditions. These formulae have been shown to predict
losses very accurately, in line with manufacturer data. The developed formulae enable
circuit designers to quickly estimate circuit losses and determine the sensitivity of those
losses to device voltage and current ratings, and thus select the optimal semiconductor
device for a specific application.
It is shown that in the case of majority carrier devices (such as power MOSFETs), the
conduction power loss (at rated current) increases linearly in relation to the varying rated
current (at constant blocking voltage), but is a square root of the variable blocking voltage
when rated current is fixed. For minority carrier devices (such as a pin diode or IGBT),
a similar relationship is observed for varying current, however where the blocking voltage
is altered, power losses are derived as a square root with an offset (from the origin).
Finally, this thesis conducts a power loss-oriented evaluation of cascade type multilevel
converters suited to reactive power compensation in 11kV and 33kV systems. The cascade
cell converter is constructed from a series arrangement of cell modules. Two prospective
structures of cascade type converters were compared as a case study: the traditional type
which uses equal-sized cells in its chain, and a second with a ternary relationship between
its dc-link voltages. Modelling (at 81 and 27 levels) was carried out under steady state
conditions, with simplified models based on the switching function and using standard
circuit simulators. A detailed survey of non punch through (NPT) and punch through
(PT) IGBTs was completed for the purpose of designing the two cascaded converters.
Results show that conduction losses are dominant in both types of converters in NPT
and PT IGBTs for 11kV and 33kV systems. The equal-sized converter is only likely to
be useful in one case (27-levels in the 33kV system). The ternary-sequence converter
produces lower losses in all other cases, and this is especially noticeable for the 81-level
converter operating in an 11kV network
Speeding up neighborhood search in local Gaussian process prediction
Recent implementations of local approximate Gaussian process models have
pushed computational boundaries for non-linear, non-parametric prediction
problems, particularly when deployed as emulators for computer experiments.
Their flavor of spatially independent computation accommodates massive
parallelization, meaning that they can handle designs two or more orders of
magnitude larger than previously. However, accomplishing that feat can still
require massive supercomputing resources. Here we aim to ease that burden. We
study how predictive variance is reduced as local designs are built up for
prediction. We then observe how the exhaustive and discrete nature of an
important search subroutine involved in building such local designs may be
overly conservative. Rather, we suggest that searching the space radially,
i.e., continuously along rays emanating from the predictive location of
interest, is a far thriftier alternative. Our empirical work demonstrates that
ray-based search yields predictors with accuracy comparable to exhaustive
search, but in a fraction of the time - bringing a supercomputer implementation
back onto the desktop.Comment: 24 pages, 5 figures, 4 table
Effects of Hyperbolic Rotation in Minkowski Space on the Modeling of Plasma Accelerators in a Lorentz Boosted Frame
Laser driven plasma accelerators promise much shorter particle accelerators
but their development requires detailed simulations that challenge or exceed
current capabilities. We report the first direct simulations of stages up to 1
TeV from simulations using a Lorentz boosted calculation frame resulting in a
million times speedup, thanks to a frame boost as high as gamma=1300. Effects
of the hyperbolic rotation in Minkowski space resulting from the frame boost on
the laser propagation in the plasma is shown to be key in the mitigation of a
numerical instability that was limiting previous attempts
Desynchronization: Synthesis of asynchronous circuits from synchronous specifications
Asynchronous implementation techniques, which measure logic delays at run time and activate registers accordingly, are inherently more robust than their synchronous counterparts, which estimate worst-case delays at design time, and constrain the clock cycle accordingly. De-synchronization is a new paradigm to automate the design of asynchronous circuits from synchronous specifications, thus permitting widespread adoption of asynchronicity, without requiring special design skills or tools. In this paper, we first of all study different protocols for de-synchronization and formally prove their correctness, using techniques originally developed for distributed deployment of synchronous language specifications. We also provide a taxonomy of existing protocols for asynchronous latch controllers, covering in particular the four-phase handshake protocols devised in the literature for micro-pipelines. We then propose a new controller which exhibits provably maximal concurrency, and analyze the performance of desynchronized circuits with respect to the original synchronous optimized implementation. We finally prove the feasibility and effectiveness of our approach, by showing its application to a set of real designs, including a complete implementation of the DLX microprocessor architectur
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