1,720,991 research outputs found
CFD modelling of breakwaters embedding wave energy converters
Get PDFIn recent years the raising supply and environmental problems related to traditional fossil fuel exploitation for energy production have pushed the research on renewable sources. Among these, sea waves have a high potential, but still poorly used. Different technologies have been developed in order to harness wave energy, and the Oscillating Water Column (OWC) devices are the most accredited for an actual diffusion. This work is focused on the performance analysis of particular OWC-type devices, namely the REWECs (REsonant sea
Wave Energy Converter). Two different versions have been considered, the REWEC1 and the REWEC3, constituting a submerged and an emerged breakwater, respectively. The most interesting aspect of REWECs is the possibility to operate them under resonant conditions with that sea state which is the one that gives the highest yearly energy contribution. Both REWEC1 and REWEC3 dynamic behavior can be approximated by
means of a mass-spring-damper system. According to this approximation, a criterion for evaluating the oscillating natural frequency of the REWECs has been derived. A CFD model has been developed in order to verify the resonance behaviour of a scaled REWEC1 device. The water-air interaction has been taken into account by means of the Volume Of Fluid (VOF) model implemented in the commercial code Ansys Fluent. Both air and water fow felds have been assumed to be unsteady. The CFD model has been validated against
both analytical and experimental results. Simulation results showed a good agreement with both measurements and predictions, particularly when the Standard k-ω turbulence model is implemented inside the REWEC.
The CFD model has been applied also to the simulation of both conventional and REWEC1 submerged breakwater placed into a two-dimensional wave fume, obtaining a novel method for the valuation of the REWEC1 absorption coefcient. In fact, when considering the interaction of waves with a conventional submerged breakwater, the incident wave energy is shared into: a refected fractiona a dissipated fraction, due to the wave breaking and the friction losses on ita and a transmitted fraction to the shore. If the
submerged breakwater embeds a REWEC1, a further reduction of the transmitted energy can be achieved due to device absorption, improving the coast protection performance of the structure. Actually, a scaled REWEC1 breakwater without PTO-system (Power Take-Off) has been considered. Then, the simulation results showed only a small difference between the transmission coefcients of the two breakwaters, allowing however to calculate
the absorption coefcient of the REWEC1 due to the water losses inside the device. Finally, a full-scale REWEC3 breakwater has been simulated by means of CFD, taking into account the characteristics of the air turbine adopted as power take-off device. The breakwater replicated the one installed in the Civitavecchia harbour (Italy). A typical sea state has been reproduced by means of regular waves generated by a piston-type wave-maker. As in the case of the scaled REWEC1, CFD unsteady simulation have been carried out in a two dimensional (2D) reference frame, reducing the huge need of computational resources. In order to simulate the oscillating fow across the air turbine, a new method has been also proposed here. The zone of the domain corresponding to the duct where the air turbine is embedded has been modelled as a porous zone. Analytical equations able to model the exchange of mass and energy across the air duct and the turbine have been
implemented. The simulation results are in good agreement with the ones registered during the operation of the Civitavecchia plant. A further outcome of the work has been a deep insight into the fuid-dynamic behaviour of the water fow inside the REWEC3, highlighting the design improvements needed to reduce friction losses
Adjoint methods and Bayesian data assimilation for modeling the thermoacoustic behaviour of premixed flames
Get PDFThe aviation industry heavily relies on combustion for long-haul air travel due to essential
power-to-weight and energy-to-weight ratios. To address environmental concerns, research
focuses on reducing carbon dioxide (CO2) and nitrous oxide (NOx) emissions through
lean premixed combustion, sustainable fuels, and hydrogen fuel. Lean combustion, while
promising for lowering NOx emissions at reduced temperatures, poses challenges related
to thermoacoustic oscillations, necessitating ongoing research.
Thermoacoustic instability occurs when flame perturbations synchronize with pressure
oscillations, converting heat to work over an oscillation cycle—a fundamental thermoacoustic
mechanism described by the Rayleigh criterion. These instabilities can severely
affect performance, cause component damage, and even lead to catastrophic failures.
Effective modelling of combustion instability remains challenging due to the extreme
sensitivity of the thermoacoustic mechanism to minor system changes. Successful development
requires comprehensive testing and modifications, emphasizing the need for
quantitatively accurate models and adjoint methods to efficiently identify and implement
necessary changes for mitigating combustion instability in engine designs.
The literature offers numerous thermoacoustic models, making model selection challenging.
An alternative approach involves Bayesian inference, leveraging extensive experimental
data and prior beliefs to identify the most suitable model. This approach benefits from the
sensitivity of measurable features in thermoacoustics to model parameters. This process
results in quantitatively-accurate models that are interpretable and can be extrapolated.
Moreover, the assimilation process is expedited when the model is differentiable, allowing
efficient optimisation using gradient-descent methods based on the flame position gradient
with respect to the model parameters.
This research aims to enhance understanding of thermoacoustic instability by developing
differentiable computational tools, allowing differentiation with respect to parameters and initial conditions. The study comprises four main research topics: firstly, an
examination of triggering and non-normality in thermoacoustics, delving into minimal
triggering thresholds in the Rijke tube while considering the influence of time delay
on perturbations; secondly, the creation of a differentiable model for laminar premixed
flames, with a focus on evaluating how influential parameters affect flame transfer
function and studying the effects of flame base oscillations; thirdly, the application of
Bayesian data assimilation to assimilate experimental flame images of acoustically forced
laminar premixed conical flames into a quantitatively accurate reduced-order model to
identify the most probable model parameters; and finally, an exploration of the impact
of hydrogen enrichment on thermoacoustic instabilities in laminar conical premixed
methane/air flames, shedding light on hydrogen fraction as a modifiable parameter to
control thermoacoustic instabilities
Marche alla ribalta. Il product placement cinematografico in Italia e la sua gestione manageriale
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Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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