43,098 research outputs found
Investigation and modeling of space shuttle main engine shutdown transient chugging
The space shuttle main engines experience a low frequency pressure pulsation in both the fuel and oxidizer preburners during the shutdown transient. This pressure pulsation, called chugging, has been linked to undesirable bearing loads and possible damage to the spark ignitor supply piping for the fuel preburner. The problem is briefly described and a model is proposed that includes: (1) a transient stirred tank reactor model for the combustion chamber, (2) a resistance capacitance model for the supply piping and (3) purge gas/liquid oxygen interface tracking
Coherent spin control by electrical manipulation of the magnetic anisotropy
High-spin paramagnetic manganese defects in polar piezoelectric zinc oxide
exhibit a simple almost axial anisotropy and phase coherence times of the order
of a millisecond at low temperatures. The anisotropy energy is tunable using an
externally applied electric field. This can be used to control electrically the
phase of spin superpositions and to drive spin transitions with resonant
microwave electric fields
Training Restricted Boltzmann Machines on Word Observations
The restricted Boltzmann machine (RBM) is a flexible tool for modeling
complex data, however there have been significant computational difficulties in
using RBMs to model high-dimensional multinomial observations. In natural
language processing applications, words are naturally modeled by K-ary discrete
distributions, where K is determined by the vocabulary size and can easily be
in the hundreds of thousands. The conventional approach to training RBMs on
word observations is limited because it requires sampling the states of K-way
softmax visible units during block Gibbs updates, an operation that takes time
linear in K. In this work, we address this issue by employing a more general
class of Markov chain Monte Carlo operators on the visible units, yielding
updates with computational complexity independent of K. We demonstrate the
success of our approach by training RBMs on hundreds of millions of word
n-grams using larger vocabularies than previously feasible and using the
learned features to improve performance on chunking and sentiment
classification tasks, achieving state-of-the-art results on the latter
Direct Use of Low Enthalpy Deep Geothermal Resources in the East African Rift Valley
Geothermal energy is already harnessed across East Africa to provide hundreds of megawatts of electricity, with significant plans for future expansion towards generation at the gigawatt scale. This power generation utilizes the high steam temperatures (typically more than 200 °C) that are available in several locations in Kenya, Ethiopia and elsewhere. The presence of these high enthalpy resources has deflected attention from the often attractive low and medium enthalpy resources present across a more extensive portion of the region. Geothermally heated water at cooler temperatures (less than 90 °C) could be widely produced by drilling shallower and cheaper boreholes than those required for power production. This low enthalpy resource could be widely exploitable throughout the Rift Valley, offering a low carbon, sustainable, reliable and commercially competitive source of heating, drying and cooling (via absorption chillers) to local farmers and growers, and for low temperature commercial and industrial uses. Applications of this type would displace expensive fossil fuels, reducing costs and carbon emissions as well as improving the region’s energy and food security. The power input for pump systems can be accommodated by relatively small generators, so direct heat projects could be beneficial to consumers in areas with no grid access
Gravity on a Little Warped Space
We investigate the consistent inclusion of 4D Einstein gravity on a truncated
slice of AdS_5 whose bulk-gravity and UV scales are much less than the 4D
Planck scale, M_* << M_{Pl}. Such "Little Warped Spaces" have found
phenomenological utility and can be motivated by string realizations of the
Randall-Sundrum framework. Using the interval approach to brane-world gravity,
we show that the inclusion of a large UV-localized Einstein-Hilbert term allows
one to consistently incorporate 4D Einstein gravity into the low-energy theory.
We detail the spectrum of Kaluza-Klein metric fluctuations and, in particular,
examine the coupling of the little radion to matter. Furthermore, we show that
Goldberger-Wise stabilization can be successfully implemented on such spaces.
Our results demonstrate that realistic low-energy effective theories can be
constructed on these spaces, and have relevance for existing models in the
literature.Comment: 1+24 page
Instability of nonminimally coupled scalar fields in the spacetime of slowly rotating compact objects
Nonminimally coupled free scalar fields may be unstable in the spacetime of
compact objects. Such instability can be triggered by classical seeds or, more
simply, by quantum fluctuations giving rise to the so-called {\em vacuum
awakening effect}. Here, we investigate how the parameter space which
characterizes the instability is affected when the object gains some rotation.
For this purpose, we focus on the stability analysis of nonminimally coupled
scalar fields in the spacetime of slowly spinning matter shells.Comment: 11 pages, 6 figure
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