155 research outputs found
Optimal Inspection and Maintenance Planning for Deteriorating Structural Components through Dynamic Bayesian Networks and Markov Decision Processes
Civil and maritime engineering systems, among others, from bridges to
offshore platforms and wind turbines, must be efficiently managed as they are
exposed to deterioration mechanisms throughout their operational life, such as
fatigue or corrosion. Identifying optimal inspection and maintenance policies
demands the solution of a complex sequential decision-making problem under
uncertainty, with the main objective of efficiently controlling the risk
associated with structural failures. Addressing this complexity, risk-based
inspection planning methodologies, supported often by dynamic Bayesian
networks, evaluate a set of pre-defined heuristic decision rules to reasonably
simplify the decision problem. However, the resulting policies may be
compromised by the limited space considered in the definition of the decision
rules. Avoiding this limitation, Partially Observable Markov Decision Processes
(POMDPs) provide a principled mathematical methodology for stochastic optimal
control under uncertain action outcomes and observations, in which the optimal
actions are prescribed as a function of the entire, dynamically updated, state
probability distribution. In this paper, we combine dynamic Bayesian networks
with POMDPs in a joint framework for optimal inspection and maintenance
planning, and we provide the formulation for developing both infinite and
finite horizon POMDPs in a structural reliability context. The proposed
methodology is implemented and tested for the case of a structural component
subject to fatigue deterioration, demonstrating the capability of
state-of-the-art point-based POMDP solvers for solving the underlying planning
optimization problem. Within the numerical experiments, POMDP and
heuristic-based policies are thoroughly compared, and results showcase that
POMDPs achieve substantially lower costs as compared to their counterparts,
even for traditional problem settings
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Characterization of string cavitation in large-scale Diesel nozzles with tapered holes
The cavitation structures formed inside enlarged transparent replicas of tapered Diesel valve covered orifice nozzles have been characterized using high speed imaging visualization. Cavitation images obtained at fixed needle lift and flow rate conditions have revealed that although the conical shape of the converging tapered holes suppresses the formation of geometric cavitation, forming at the entry to the cylindrical injection hole, string cavitation has been found to prevail, particularly at low needle lifts. Computational fluid dynamics simulations have shown that cavitation strings appear in areas where large-scale vortices develop. The vortical structures are mainly formed upstream of the injection holes due to the nonuniform flow distribution and persist also inside them. Cavitation strings have been frequently observed to link adjacent holes while inspection of identical real-size injectors has revealed cavitation erosion sites in the area of string cavitation development. Image postprocessing has allowed estimation of their frequency of appearance, lifetime, and size along the injection hole length, as function of cavitation and Reynolds numbers and needle lif
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On the formation of string cavitation inside fuel injectors
The formation of vortex or ‘string’ cavitation has been visualised in the flow upstream of the injection hole inlet of an automotive-sized optical diesel fuel injector nozzle operating at pressures up to 2,000 bar. Three different nozzle geometries and three-dimensional flow simulations have been employed to describe how, for two adjacent nozzle holes, their relative positions influenced the formation and hole-to-hole interaction of the observed string cavitation vortices. Each hole was shown to contain two counter-rotating vortices: the first extending upstream on axis with the nozzle hole into the nozzle sac volume and the second forming a single ‘bridging’ string linked to the adjacent hole. Steady-state and transient fuel injection conditions were shown to produce significantly different nozzle-flow characteristics with regard to the formation and interaction of these vortices in the geometries tested, with good agreement between the experimental and simulation results being achieved. The study further confirms that the visualised vortices do not cavitate themselves but act as carriers of gas-phase components within the injector flow
Orthorhombicity mixing of s- and d- gap components in without involving the chains
Momentum decoupling develops when forward scattering dominates the pairing
interaction and implies tendency for decorrelation between the physical
behavior in the various regions of the Fermi surface. In this regime it is
possible to obtain anisotropic s- or d-wave superconductivity even with
isotropic pairing scattering. We show that in the momentum decoupling regime
the distortion of the planes is enough to explain the experimental
reports for s- mixing in the dominantly d-wave gap of . In the
case of spin fluctuations mediated pairing instead, a large part of the
condensate must be located in the chains in order to understand the
experiments.Comment: LATEX file and 3 Postscript figure
Noncollinear magnetic ordering in small Chromium Clusters
We investigate noncollinear effects in antiferromagnetically coupled clusters
using the general, rotationally invariant form of local spin-density theory.
The coupling to the electronic degrees of freedom is treated with relativistic
non-local pseudopotentials and the ionic structure is optimized by Monte-Carlo
techniques. We find that small chromium clusters (N \le 13) strongly favor
noncollinear configurations of their local magnetic moments due to frustration.
This effect is associated with a significantly lower total magnetization of the
noncollinear ground states, ameliorating the disagreement between Stern-Gerlach
measurements and previous collinear calculations for Cr_{12} and Cr_{13}. Our
results further suggest that the trend to noncollinear configurations might be
a feature common to most antiferromagnetic clusters.Comment: 9 pages, RevTeX plus .eps/.ps figure
Social representations and community attitudes towards spring breakers
Social representations theory has been adopted for explaining tourism impacts and local attitudes. However, its usefulness in segmenting local population in terms of their attitudes towards specific types of tourists has not been tested. This study identifies the attitudes of local people towards spring break, a North American young tourist phenomenon in the context of the Mexican beach resort of Acapulco. Although residents perceive an increase in alcohol consumption, drug use, noise and litter during the spring break season, they largely recognise economic benefits and are thus generally supportive for the phenomenon. Based on these attitudes, three clusters were identified: spring break supporters (identified by their high appreciation of spring break benefits), ambivalents (who are uncertain about both benefits and costs) and realistics (characterised by recognising both benefits and costs). The main contribution of this study lies in the confirmation of the usefulness of social representations theory in explaining residents’ attitudes towards a very specific type of tourists whose hedonist behaviours are a common characteristic
Aspects of sustainability in the destination branding process: a bottom-up approach
The concept of sustainability is based on the premise that the inhabitants of a destination should be involved in the way that destination is managed and promoted. At the same time, the literature of place branding emphasizes the important role of local stakeholders in the creation of a true and reliable place brand. In fact, the process of developing a destination brand begins with the aim of shaping the identity of a destination; what the destination stands for. The sustainable dimensions of the destination branding process are explored, while focus groups and structured questionnaires are used to evaluate the usefulness of projection techniques in the process of building a brand identity. It seems that the use of the personification technique could work as an effective destination positioning exercise and as an alternative proposal to the outdated clichés used in tourism promotion
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