1,721,037 research outputs found
Injection and combustion modelling in hydrogen internal combustion engines
No full textThis thesis focuses on the numerical analysis of the injection, mixing, and combustion
in direct injection internal combustion engines fueled by hydrogen.
Most likely, the current transition towards a sustainable mobility will be achieved only
through a combination of different technologies. For specific applications, such as heavyduty
road transport, hydrogen-powered internal combustion engines—particularly those
based on direct injection — represent a competitive powertrain solution in terms of cost
and reliability compared to more innovative alternatives like fuel cells.
Although internal combustion engines have been in use for over a century, the unique
chemical and physical properties of hydrogen necessitate a renewed analysis of the fundamental
processes governing engine operation, such as injection, fuel-air mixing, and
combustion.
For this reason, the first step of this thesis is the characterization of the fluid dynamics
of high-pressure hydrogen direct injection into the combustion chamber. The study analyzes
the fluid-dynamic structures characterizing these flows, with particular attention
to the numerical setup required to accurately resolve them. With respect to the state of
art, the analysis focuses on the effect of the typical combustion chamber conditions on
the jet phenomenology. More specifically, an analysis is proposed by varying the cylinder
pressure over time to determine the possible presence of dynamic phenomena that
could alter the flow configuration compared to what is observed under constant pressure
ratios.
As a second step, focus has been posed on the influence of the injector exit section geometry,
to determine whether specific configurations are more suitable for use in internal
combustion engines. Although the injector outlet sections are primarily circular or annular,
new geometries (triangular, star-shaped, rectangular, and elliptical) have been
studied to understand their effect on the mixing between the jet and the surrounding
environment. While some of these geometric configurations have already been analyzed
in the aerospace field, their applicability in the automotive sector has been tested in
this thesis work. Significant differences have been observed in the shock and expansion
system characterizing these flows, for which a detailed explanation has been provided.
Subsequently, the analysis has been extended to the design space of key variables affecting
the mixing process, such as the injection pressure, timing, and nozzle geometry.
The goal has been to extend the engine operability under increasingly lean mixtures
compared to the current state of the art. More specifically, focus has been placed on
mixtures featured by an equivalence ratio of 0.25, which help to reduce wall heat losses
while limiting nitrogen oxide emissions. It has been demonstrated that the mixture
stratification under these conditions is the key to increasing the combustion process
speed. In particular, multi-injection strategies have been tested, showing the potential
to optimize the combustion speed and NOx emissions.
Finally, in an effort to improve internal combustion engine modeling, a procedure has
been developed to account for the effects of thermodiffusive instabilities in hydrogen
flames within the framework of the G-equation combustion model. Preliminary results
show the potential to effectively capture pressure trends with a single preliminary calibration
process regarding the interaction between the flame front and the in-cylinder
turbulence
DEVELOPMENT OF AN INTEGRATED SYSTEM BIOLOGY MODEL FOR PREDICTING MIXTURES OF CHEMICALS ACTING ON THE SAME PATHWAY
Get PDFExposure of the embryo to environmental chemicals (pesticides, air and water pollutants) can result in congenital malformations or developmental defects such as oro-facial cleftings. Unfortunately, the human embryo is not usually exposed to a single substance, but to many substances simultaneously. Despite the efforts in elucidating mechanism of action (MoA) of substances that perturb the normal embryonic development, only a small part of involved pathways have been understood to date (Giavini and Menegola, 2004). This is why, evaluating the toxicity of mixtures of multiple chemicals is one of the major objectives of today’s toxicology despite the effect of exposure to a mixture is still difficult to understand. To arrive in the future to the creation of a realistic overall picture of human exposure to mixtures, the development of integrated approaches between in vitro and in silico techniques and computational systems biology, able to predict the effects of mixtures starting from the concentrations of their individual components, will be essential. Recent studies suggest that the similarity of molecular initiating events (MIEs) is not an essential requirement to induce additive effects, because mixtures composed of chemicals with different MIE can exhibit mixture effects too, probably acting on the same biological pathway and contributing to the same adverse outcome (EFSA, 2013). This is in recognition of emerging evidence that biological effects can be similar, although the molecular details of toxicological mechanisms may profoundly differ in many respects (Kortenkamp, 2007). Considering the previously reported data, this could be the case of embryonic co-exposure to fluconazole (FLUCO) and ethanol (Eth). Both, in fact, lead to the same adverse outcome (AO), craniofacial malformations, both after in utero and in vitro exposure. The two molecules are able to induce cranio-facial defects (in embryos visible as cranio-facial abnormalities), probably acting on the same Adverse Outcome Pathway (AOP) altering, with different MIEs, the biological event cascade regulated by the morphogen retinoic acid (RA).
The specific aim of this PhD project was to investigate this hypothesis through the development of an in silico model, useful to simulate and predict the effects on embryo development after co-exposure to substances with independent MoAs but acting on the same biological pathway and potentially contributing to the same adverse outcome (cranio-facial malformations). The in silico model was based on experimental data and validated by in vitro experiments. For this purpose, the project was divided into three parts.
In the first part, we evaluated the effects of the molecules on post-implantation rat embryos cultures, using the in vitro technique WEC (Whole Embryo Culture). Embryos were cultured in presence of increasing concentration of RA (0.025-0.0375-0.05-0.125-0.25 μM), to increasing concentrations of Eth (17-42.5-85-127 mM), to increasing concentrations of FLUCO (62.5-125-250-500 μM). Specific and concentration related abnormalities at the level of the branchial arches (reductions or fusions) were observed after exposure to single Eth or FLUCO and were comparable to those elicited by RA. These results suggest a common AO for Eth, FLUCO and RA. Embryos were then co-exposed to binary mixtures of FLUCO and Eth. To better characterize the contribution of each component to the observed effects, the “fixed + moving” approach was applied: embryos were exposed to the no effect concentration (NOAEL) of one chemical (“fixed”) and increasing concentrations of the other chemical (“moving”). A significant enhancement of teratogenic effects was observed after co-exposure to FLUCO and Eth in comparison to the single exposure. The mixture between the two NOAELs was effective too, inducing almost 40% of branchial arch abnormalities. These data suggest the presence of a cumulative effect in mixture, probably due to the capability of both molecules to perturb RA endogenous concentrations in specific tissues (the precursors of cranio-facial skeletal tissues, originated in the embryonic hindbrain). This theory could be explained considering the ability of Eth to induce alcohol-dehydrogenases (including ADH7, the embryonic enzyme involved in RA synthesis) and the inhibitory effects of FLUCO on cytochromes p450 (including CYP26, involved in embryonic RA degradation).
In the second part of the project, we evaluated these hypothetical mechanisms inducing the Eth-FLUCO mixture effects through the development of an in silico tool, able to simulate both the formation of the physiological RA gradient in the rat embryo hindbrain and its perturbation after exposure to FLUCO, Eth and to their binary mixtures. The obtained system biology model, developed using an integrated approach combining mathematical modelling, molecular docking and in vitro experiments, seems to be reasonably predictive for the mixture’s effects, confirming the accuracy of the hypothesized pathogenic pathway. Experimental data and model predictions, in fact, showed a promising agreement. The model, in spite of its limitations, could have many potential mechanistic or predictive applications for the study of risk assessment. However, since the model is based on experimental data obtained in mammals, the last part of the project was aimed to evaluate alternative animal models.
In the third part of the project, the evaluation of the effects after co-exposure to FLUCO and Eth were performed using the ascidian Ciona intestinalis embryo model as a new alternative teratological screening test (AET, Ascidian Embryo Teratogenicity assay). An ascidian species was selected because Ascidiacea represent the sister group of Vertebrates. For this purpose, C. intestinalis embryos were exposed to Eth alone (1.7-8.5-17-42.5-85 mM) to FLUCO alone (7.8-15.75-31.5-250 μM), or co-exposed to binary mixtures of FLUCO and Eth until the larval stage, applying the fix + moving protocol. Specific and concentration related abnormalities at the level of the anterior structures were elicited by Eth or FLUCO, and were comparable to those described in literature after RA exposure. A significant enhancement of the general teratogenicity was observed after co-exposure to FLUCO and Eth in comparison to the single exposure, suggesting the presence of a mixture effect induced by FLUCO and Eth also in this model. These results, similar to those observed in the WEC model, encourage the use of AET as a complementary alternative method for embryotoxicity studies on mixtures. The possibility to translate data obtained in this model in our in silico model is still to evaluate.
In conclusion, our data suggest that: 1. the integrated use of data from in vitro and in silico approaches used in this study support the hypothesis that embryonic exposure to FLUCO and Eth can lead to the same AO (craniofacial abnormalities) acting with different MIEs but both converging on the same AOP by altering the RA production (Eth) and the RA catabolism (FLUCO); 2. the hypothesis that substances with different MoAs but acting on the same pathway could produce an additive effect also at concentrations considered not effective is supported; 3. the obtained results highlight the potential additive effect that could occur after exposure to azoles and ethanol, suggesting a precautionary position in alcohol consumption during azoles exposure in pregnancy. The overall view of the obtained results support the need of a cumulative risk assessment not only for chemicals grouped on the base of similarities in chemical structure or derived from mechanistic considerations but also for chemicals differently acting on the same biological pathway
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
Mixture effect of two antifungal triazoles (triadimefon and cyproconazole) in rat and Amphibian embryos cultured in vitro
No full textThe present work aims to evaluate the teratogenic potential of
two triazole-derived fungicides (triadimefon, FON, cyproconazole,
CYPRO) alone, as well as their action in a mixed solution on Amphibian
(Xenopus laevis) and rat embryos cultured in vitro. On the basis
of the morphological and molecular interclasses similarities at the
phylotypic stage, low vertebrate animal models have been proposed
as alternative to mammals in assessing the developmental
toxicity of xenobiotics. Triazole derivatives represent a large class
of molecules with antifungal properties, used in agriculture and
human-veterinary therapy. Experimental studies indicate all the
tested triazoles as teratogenic in rodents, rabbits, amphibians and
tunicates, and neurulation as the most sensitive period for inducing
craniofacial defects thus affecting the organization of embryonic
transient structures (branchial arches). Rat embryos were exposed
during the whole culture period (from early neurulation till phylotypic
stage) to 7.8–250Mof two agrochemical triazoles (FON and
CYPRO) alone, or to their mixture (MIX1: FON 7.8M plus CYPRO
7.8M). At the end of the culture period embryos were morphologically
examined and immunostained to evaluate neural crest cell
migration. X. laevis embryos were exposed during early neurulation
stages to FON and CYPRO alone (3.9-250M), to MIX1, and
to MIX2 (FON 3.9M plus CYPRO 3.9M). At the end of the exposures,
embryos were washed and allowed to reach the phylotypic
stage or the larval stage 47 to evaluate neural crest cell migration
and craniofacial morphology. The obtained results confirmFONand
CYPRO as teratogenic and showed a clear mixture effect in both
species. This suggests that the teratogenic mechanism is common
for the two molecules and that it is able to act in different Vertebrate
species, indicating X. laevis as an adequate alternative animal model
for teratogenic screening of mixtures obtained from molecules of
this chemical class
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