1,720,976 research outputs found
Ecological feedback mechanisms and variable disturbance regimes: the uncertain future of Mediterranean macroalgal forests
Full text linkLoss of algal canopies can result in a shift towards a turf-dominated state, where variability in species life-history traits can determine new mechanisms of feedback, and influence the degraded system under variable regimes of disturbance. By focusing on rockpools dominated by Cystoseira brachycarpa, we tested the hypothesis that the alga Dictyopteris polypodioides could take advantage of extreme regimes of disturbance related to storms, and outcompete other turfs through a distinctive combination of life traits. Replacement of the canopy was initially driven by a mix of taxon-specific life-traits and resulting assemblages were susceptible to intense events of disturbance. Subsequently, D. polypodioides dominated removal quadrats, favored by density-dependent abilities to intercept more light and reach larger size than the rest of turf. These new positive feedbacks may contribute to maintain the modified state of the system and influence its ability to withstand extreme abiotic conditions
Analysis of an Autogenous Propellant Pressurization System for Nuclear Thermal Rocket
No full textSince the NERVA project, hydrogen has been the most proposed propellant for nuclear propulsion systems thanks to its overwhelming performance. However, its low density and the need to store it in cryogenic conditions, combined with the long duration of the foreseen interplanetary missions, led to the design of propulsion systems with enormous envelopes and the introduction of complex systems for propellant management. On the other hand, ammonia presents a clear loss of performance compared to hydrogen due to its higher molecular weight, limiting the spectrum of missions made accessible by the use of this propellant in a nuclear thermal propulsion system. However, the greater density of ammonia and its high vapor pressure at around 373 K allow for much more compact and less complex propellant management system configurations. This work proposes a new propellant management configuration for an ammonia-fueled nuclear thermal propulsion system for a class of missions involving cargo displacements between LEO and the lunar orbits of interest for future space programs. The suggested configuration maximizes the advantage deriving from the self-pressurization of ammonia by exploiting the thermal power lost by the nuclear reactor towards the vacuum space due to the escaping particles. In this layout a tank containing ammonia in saturated conditions is placed near the nuclear reactor and receives an input thermal power proportional to the dose of gamma rays and neutrons absorbed by the ammonia and the tank walls. This thermal power accelerates the vaporization process of the saturated ammonia, thus increasing the pressure in the tank. A pressure regulator valve exploits this overpressure to pressurize the ammonia propellant contained in a run tank to the level required by the mission by connecting the two ammonia volumes. The pressure achieved inside the run tank pushes the propellant with an adequate mass flow rate inside the nuclear reactor. The Homogeneous Equilibrium Model and the Two-Temperature Model describe the coupled dynamics of these two tanks, while a study of neutron and photon transport performed with the Monte Carlo code OpenMC provides the thermal power input received from the pressurizing ammonia tank. The developed analysis shows how this propellant management system can provide a constant mass flow to the nuclear reactor without using a turbopump assembly. This capacity depends on the relative distance between the tank and the nuclear fission reactor. Another advantage of the proposed concept concerns the reduction of the mass of the radiation shield: it does not have to protect the tanks from nuclear radiation, as happens for hydrogen-based projects, and therefore, its dimensions are governed exclusively by the radial size of the payload. For some missions, this equates to having a smaller shield size and, consequently, a lower mass
I canonici regolari di Santa Maria Forisportam, l’abate Giovanni Alfonso Puccinelli e due pale di Guercino per Lucca
Full text linkIl saggio prende in esame le due pale d'altare dipinte da Guercino per la chiesa di Santa Maria Forisportam a Lucca su committenza dell'abate Giovanni Alfonso Puccinelli e della famiglia Mazzarosa. Attraverso il reperimento di nuovi documenti di archivio la ricerca ha ricostruito il ricco tessuto di relazioni che legano i prestigiosi committenti e la chiesa lucchese al pittore di Cento
Nuclear Thermal Propulsion for Earth Orbit and Interplanetary Missions: Challenges and Issues
No full textFrom the late 1950s to date, nuclear thermal propulsion has the purpose of making possible missions impossible to accomplish with chemical propulsion. The use of hydrogen as a propellant and a fuel resistant to very high temperatures guarantees exceptional propulsive performance. Since the NERVA project, a propulsion system with such capabilities has proven to be the most suitable to carry out missions to Mars. Even today, the literature on nuclear thermal propulsion systems appears to be full of Mars mission studies. Little attention is given to other missions potentially made accessible by this type of propulsion. The purpose of this work is to investigate different mission scenarios that could be made accessible by nuclear propulsion and to identify the most efficient configurations of the propulsion system for each scenario. Some mission scenarios are presented, covering both interplanetary and earth orbit missions. The specific requirements that each mission imposes on the propulsion system are determined. Particular attention is given to the analysis of the safety requirements, which constitute the most stringent constraints to be respected for the use of nuclear reactors in space. A literature review identifies the most promising fuels and propellants, with a focus on the recently developed high temperature fuels. A preliminary design determines the mass, size and materials of the components of the propulsion systems satisfying the imposed requirements for the diverse missions. All the proposed configurations have associated different combinations of fuels and propellants among the one chosen from the previous literature reviews. For each mission, a trade-off is made based on the performance offered by the various propulsion systems proposed. Particular attention is given to the parameter of the system specific impulse, which weighs the specific impulse on the mass of the propulsion system. This parameter is very important in systems using nuclear propulsion, where the mass of the engine occupies a large part of the total mass of the system. From this preliminary trade-off, the nuclear thermal propulsion system configurations worth to be studied in detail and optimized emerge for each identified mission. Finally, the possible benefits in terms of system specific impulse derived by the use of the fission reactor also for power generation purposes are discussed
Design of a Modular 1U Propellant Tank for CubeSats HTP Monopropellant Propulsion System
No full textNowadays, the demand of providing CubeSats for a propulsion system is increasing parallel to their growing market and applications. However, today there exist few nearly off-the-shelf European propulsion solutions. In this context, the European Space Agency launched a call to identify the most promising European Propulsion Systems for CubeSats. The University of Pisa participated to this ESA call with the CHIPS project (CubeSat HTP Innovative Propulsion System) focused on the design, manufacturing, and testing of an affordable chemical monopropellant propulsion system for CubeSats which uses hydrogen peroxide as the propellant. The CHIPS baseline design fits in 2U, and it has to generate a maximum thrust of 0.5 N with a specific impulse of up to 160 s. To fulfil the physical constraints, performance requirements, and limited budget a blowdown configuration has been devised with a beginning-of-life pressure of about 24 bar. The hydrogen peroxide concentration has been selected up to 98% wt. The identification of miniaturized off-the-shelf components (COTS) compatible with such as high concentrated H2O2 and operating at high pressures is challenging. In particular, the market seems to lack of tank COTS able to satisfy the CHIPS requirements. For this reason, a design and development activity for the tank was carried out during the CHIPS project, since this component represents one of the most critical parts. The purpose of this paper is to present the design of an innovative tank for CubeSats. The preliminary idea is to confine the storage system to 1U to allow for a modular and easy-to-interface subsystem. The main design driving factors are to maximize the propellant volume and reduce the mass and the cost. The first major issue is the selection of compatible materials with HTP, to guarantee a low decomposition rate and long-term storage capability. Different manufacturing processes compatible with the selected materials have been evaluated, which affect the achievable shapes and thus the volume optimization process. Various shapes, such as cylindrical and cuboidal ones, have been analysed, in terms of both the propellant volume fraction and the provided performance. A trade-off between the selected materials, processes, and different propellant management devices, such as piston, bladder, bellow, and diaphragm, has been carried out to achieve the best configuration that fulfils the design objectives. The outcome of this design process aims to give a modular 1U tank solution for CubeSat not only for the CHIPS project but extendable to other monopropellant propulsion system applications
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Full text linkWe 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
Full text linkWe 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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