10,786 research outputs found
Ammonia removal in anaerobic digestion by biogas stripping: an evaluation of process alternatives using a first order rate model based on experimental findings
The feasibility of biogas stripping to remove ammonia in the anaerobic digestion of source segregated food waste was investigated. It was found in batch experiments that ammonia could be removed from digestate and that the removal followed 1st order kinetics with respect to total ammonia nitrogen concentration. Increasing temperature, biogas flow rate and initial pH all increased removal rates. Using kinetic data gathered in these experiments allowed the integration of ammonia stripping with an anaerobic digestion plant to be modelled for different configurations. Four scenarios were identified: post digestion, in situ, side-stream and pre-digestion ammonia removal relating to where in the process the ammonia stripping was performed. The modelling showed that in situ ammonia removal may be best able to reduce in-digester ammonia concentrations over a wide range of organic loading rates whereas pre-digestion showed most promise in terms of application due to the flexibility to control each part of the process separately. Further experimental work is required into these scenarios to confirm their viability
Combined State and Parameter Estimation for a Static Model of the Maypole (Hoop-Column) Antenna Suface
Parameter and state estimation techniques are discussed for an elliptic system arising in a developmental model for the antenna surface of the Maypole Hoop/Column antenna. A computational algorithm based on spline approximations for the state and elastic parameters is given and numerical results obtained using this algorithm are summarized
Ion beam sputter etching and deposition of fluoropolymers
Fluoropolymer etching and deposition techniques including thermal evaporation, RF sputtering, plasma polymerization, and ion beam sputtering are reviewed. Etching and deposition mechanism and material characteristics are discussed. Ion beam sputter etch rates for polytetrafluoroethylene (PTFE) were determined as a function of ion energy, current density and ion beam power density. Peel strengths were measured for epoxy bonds to various ion beam sputtered fluoropolymers. Coefficients of static and dynamic friction were measured for fluoropolymers deposited from ion bombarded PTFE
Quantum Moduli Spaces of String Theories
Generically, string models with supersymmetry are not expected to have
moduli beyond perturbation theory; stringy non-perturbative effects as well as
low energy field-theoretic phenomena such as gluino condensation will lift any
flat directions. In this note, we describe models where some subspace of the
moduli space survives non-perturbatively. Discrete symmetries forbid any
inherently stringy effects, and dynamical considerations control the
field-theoretic effects. The surviving subspace is a space of high symmetry;
the system is attracted to this subspace by a potential which we compute.
Models of this type may be useful for considerations of duality and raise
troubling cosmological questions about string theory. Our considerations also
suggest a mechanism for fixing the expectation value of the dilaton.Comment: 26 pages; uses harvmac. Footnote re fixing dilaton adde
Supersymmetry Changing Bubbles in String Theory
We give examples of string compactifications to 4d Minkowski space with
different amounts of supersymmetry that can be connected by spherical domain
walls. The tension of these domain walls is tunably lower than the 4d Planck
scale. The ``stringy'' description of these walls is known in terms of certain
configurations of wrapped Dirichlet and NS branes. This construction allows us
to connect a variety of vacua with 4d N=4,3,2,1 supersymmetry.Comment: 11 pages, harvmac, no figures, reference added, minor correction
Spontaneous Flavor and Parity Breaking with Wilson Fermions
We discuss the phase diagram of Wilson fermions in the -- plane for
two-flavor QCD. We argue that, as originally suggested by Aoki, there is a
phase in which flavor and parity are spontaneously broken. Recent numerical
results on the spectrum of the overlap Hamiltonian have been interpreted as
evidence against Aoki's conjecture. We show that they are in fact consistent
with the presence of a flavor-parity broken ``Aoki phase''. We also show how,
as the continuum limit is approached, one can study the lattice theory using
the continuum chiral Lagrangian supplemented by additional terms proportional
to powers of the lattice spacing. We find that there are two possible phase
structures at non-zero lattice spacing: (1) there is an Aoki phase of width
with two massless Goldstone pions; (2) there is no
symmetry breaking, and all three pions have an equal non-vanishing mass of
order . Present numerical evidence suggests that the former option is
realized for Wilson fermions. Our analysis then predicts the form of the pion
masses and the flavor-parity breaking condensate within the Aoki phase. Our
analysis also applies for non-perturbatively improved Wilson fermions.Comment: 22 pages, LaTeX, 5 figures (added several references and a comment
Comments on information loss and remnants
The information loss and remnant proposals for resolving the black hole
information paradox are reconsidered. It is argued that in typical cases
information loss implies energy loss, and thus can be thought of in terms of
coupling to a spectrum of ``fictitious'' remnants. This suggests proposals for
information loss that do not imply planckian energy fluctuations in the low
energy world. However, if consistency of gravity prevents energy
non-conservation, these remnants must then be considered to be real. In either
case, the catastrophe corresponding to infinite pair production remains a
potential problem. Using Reissner-Nordstrom black holes as a paradigm for a
theory of remnants, it is argued that couplings in such a theory may give
finite production despite an infinite spectrum. Evidence for this is found in
analyzing the instanton for Schwinger production; fluctuations from the
infinite number of states lead to a divergent stress tensor, spoiling the
instanton calculation. Therefore naive arguements for infinite production fail.Comment: 30 pages (harvmac l mode) UCSBTH-93-35 (minor reference and typo
corrections
Remarks on the Racetrack Scheme
There are only a small number of ideas for stabilizing the moduli of string
theory. One of the most appealing of these is the racetrack mechanism, in which
a delicate interplay between two strongly interacting gauge groups fixes the
value of the coupling constant. In this note, we explore this scenario. We find
that quite generally, some number of discrete tunings are required in order
that the mechanism yield a small gauge coupling. Even then, there is no sense
in which a weak coupling approximation is valid. On the other hand, certain
holomorphic quantities can be computed, so such a scheme is in principle
predictive. Searching for models which realize this mechanism is thus of great
interest. We also remark on cosmology in these schemes.Comment: 20 pp, latex, discussion of calculability modifie
A Class of Bandit Problems Yielding Myopic Optimal Strategies
We consider the class of bandit problems in which each of the n ≧ 2 independent arms generates rewards according to one of the same two reward distributions, and discounting is geometric over an infinite horizon. We show that the dynamic allocation index of Gittins and Jones (1974) in this context is strictly increasing in the probability that an arm is the better of the two distributions. It follows as an immediate consequence that myopic strategies are the uniquely optimal strategies in this class of bandit problems, regardless of the value of the discount parameter or the shape of the reward distributions. Some implications of this result for bandits with Bernoulli reward distributions are given
Switching Costs and the Gittins Index
The Theorem of Gittins and Jones (1974) is, perhaps, the single most powerful result
in the literature on Bandit problems. This result establishes that in independent-armed
Bandit problems with geometric discounting over an infinite horizon, all optimal strategies
may be obtained by solving a family of simple optimal stopping problems that
associate with each arm an index known as the dynamic allocation index or, more
popularly, as the Gittins index. Importantly, the Gittins index of an arm depends solely
on the characteristics of that arm and the rate of discounting, and is otherwise
completely independent of the problem under consideration. These features simplify
significantly the task of characterizing optimal strategies in this class of problems
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