1,721,071 research outputs found
Linear Analysis of Vortex-Shedding Induced Longitudinal Oscillations in Segmented Solid Rocket Motors
No full textA linear model for the analysis of longitudinal fluid dynamic oscillations in solid rocket motors is developed and applied to the Ariane 5 boosters. The natural frequencies and mode shapes are corrected as proposed by Culick in order to account for the effects of mean flow, mass addition from the burning grain. pressure coupling, nozzle admittance. and internal geometry of (he combustion chamber. In segmented solid rocket motors, the main driving source of acoustic oscillations is often represented by the coupling between the vortices shed by (he inhibitors at the intersegments and the acoustic field in the combustion chamber. This interaction is analyzed using Flandro's model, which provides the vortex-related contribution to the amplification/damping of the acoustic oscillations. The natural acoustic frequencies are considered as known inputs for vortex development in the shear layer, which is described by means of classical linear stability theory of parallel flows. The risk assessment of vortex-induced fluid dynamic instabilities in a small scale and full size configuration of the Ariane 5 boosters is carried out for the leading modes at several burn times. Model predictions are in good agreement with the available experimental results for the scaled configuration
Ion Beam Modeling in FEEP Thrusters
No full textA numerical model of the Ion beam associated with the operation of Field Emission Electric Propulsion thrusters Is presented. The objective oft be study Is to develop a computational tool capable of predicting the effects or thruster configuration parameters (geometry, voltage distribution between the electrodes) on thruster performance (beam divergence, ion exhaust velocity). A two-dimensional, steady model bas been adopted. The electric potential, electric field and the trajectories of the emitted particles are evaluated In a domain which extends from the emitter tip to a point where either beam neutralization takes place or the electric field becomes sufficiently low to assume that the trajectories can be reasonably approximated as straight lines. A simple emission model has been assumed, neglecting the detailed, microscopic description of the physical emission mechanism. The electric field Is evaluated by a finite element scheme; boundary conditions are provided by an auxUiary technique based on the placement of fictitious charges In proximity to the electrode surfaces. The beam Is simulated through a discrete number of charged particles; the effects or the associated space charge density distribution on the electric field Is also considered. The results of the simulations or typical thruster configurations are presented and discussed. The need to take the space charge effects Into account to attain a sufficiently accurate solution Is Indicated. Neutralization at a finite distance must be taken Into account to fully appreciate the effects or different voltage distributions between the electrodes, and It Is shown to Increase beam divergence. The capabUity or a thruster configuration with an additional, decelerating neutral electrode to focus the ion beam Is also Investigated
Linear Analysis of Vortex-Shedding Induced Longitudinal Oscillations in Segmented Solid Rocket Motors
Full text linkA linear model for the analysis of longitudinal fluid dynamic oscillations in solid rocket motors is developed and applied to the Ariane 5 boosters. The natural frequencies are corrected as proposed by Culick in order to account for the effects of mean flow, mass addition from the burning grain, pressure coupling, nozzle admittance, and internal geometry of the combustion chamber. In segmented solid rocket motors, the main driving source of acoustic oscillations is often represented by the coupling between the vortices shed by the inhibitors at the intersegments and the acoustic field in the combustion chamber. This interaction is analyzed using Flandro's model, which provides the vortex-related contribution to the amplificationldamping of the acoustic oscillations. The natural acoustic frequencies are considered as known inputs for vortex development in the shear layer, which is described by means of classical linear stability theory of parallel flows. The risk assessment of vortex-inducedfluid dynamic instabilities in a small scale and full size configuration of the Ariane 5 boosters is carried out for the leading modes at several burn times. Model predictions are in good agreement with the available experimental results for the scaled configuration
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
A Numerical Tool for the Investigation of Cavitating Flows in Turbopump Inducers
No full textA numerical tool for the simulation of cavitating flows is presented in this paper. The numerical model is based on the 2D Euler/Navier-Stokes equations with a barotropic state law, which are solved in body fitted coordinates using a robust spatial and temporal differencing scheme. The model has been validated comparing numerical and experimental results on ogival and hemispherical axisymmetric head forms at different cavitation numbers. Simulations of the flow around a NACAOO 15 airfoil have also been successfully performed. Then, 2D simulations of the blade-toblade flow at a fixed radial position of a 9° Helical Inducer have been carried out at various cavitation numbers. Water has been assumed as the working fluid. Cavitating regions of increasing size have been computed, until the cavitation bubble covers the entire blade profile; a good agreement is observed between the computed cavitation number and the values obtained both experimentally and through an empirical correlation for the case at which choked conditions are attained
Development of a Numerical Model of the Nozzle Flow in Low Power Arcjet Thrusters
No full textA numerical model for the investigation of' the flow field in a low power electrothermal arcjet employing hydrazine as propellant is presented. The model is based on the axisymmetric, laminar, complete Navier-Stokes equations, thus Including both heat conduction and viscous effects. Electromagnetic effects- are simply modeled through a heat source term in the energy equation to take into account the ohmic heating due to the electric arc; so field equation for the electric potential is solved. The gas mixture deriving from hydrazine decomposition is assumed to be in thermodynamic but chemical equilibrium. The governing equations are solved by MacCormack's explicit technique. Computed results compare favorably with experimental data from CENTROSPAZIO MOD4 low power arcjet operating with simulated hydrazine
The Italian optimization tool for the gains-Italy model
Full text linkNel presente rapporto verrà descritto il processo di ottimizzazione sviluppato nell’ambito del progetto MINNI per produrre scenari di costi-efficacia per l’Italia. Particolare enfasi verrà data alla metodologia con cui verranno determinati i costi per individuare le strategie di controllo per ridurre gli effetti dell’inquinamento atmosferico sia in termini di impatto ambientale che sanitario. Dopo una descrizione dei diversi aspetti formali della procedura di ottimizzazione, verranno illustrate alcune delle possibili configurazioni del processo di ottimizzazione considerando due possibili scenari di policy il cui obiettivo è la riduzione al 2030 delle concentrazioni di PM2.5 portandole ad un livello massimo di 20 mg/m3. Gli approcci scelti seguono due procedure comunemente utilizzate: il metodo del “gap closure”
che consente di fissare un target tale da distribuire equamente tra le Regioni sforzi e benefici ambientali; e il metodo del
“valore limite” dove lo stesso valore assoluto è imposto a tutte le Regioni. Nel presente lavoro verranno presentati i risultati di questo primo processo di ottimizzazione in termini di costi totali per Regione, riduzione delle emissioni per inquinante e per settore sempre a livello regionale. Lo strumento di ottimizzazione sviluppato rappresenta il primo strumento a livello nazionale per supportare i politici locali e nazionali a sviluppare politiche di riduzione dell’inquinamento atmosferico con la necessaria flessibilità senza perdere di vista analisi di tipo costi-efficacia.This document describes the optimization framework of the MINNI project as it could be used for the development of cost-effectiveness air pollution control scenarios for Italy. Particular emphasis has been given on the methodology for finding cost-effectiveness control strategies that address both environmental and health impact indicators related to air pollution. In this document the various formal aspects of the optimization procedure have been described illustrating some of the possible optimization configurations through two different policy scenarios whose aim is to reduce PM2.5
concentrations at a level of 20 mg/m3 at the year 2030. The chosen approaches follow two commonly used procedures: the “gap closure” approach that allows to set feasible targets and that equally distributes efforts and environmental benefits among Regions and the “limit value” approach where the same common absolute limit value is imposed for all the Italian Regions. In this report all the optimization results will be presented in terms of regional total costs, emission reductions by pollutants and sectors at a regional level. This is the first Italian optimization tool that runs on a national level and aids all regional and national policy makers to contemplate policy options with the required flexibility, without losing sight of cost-effectiveness considerations
Flight Demonstration Opportunities for FEEP
No full textThe scientific community’s recent interest in Field Emission Electric Propulsion (FEEP) for drag-free spacecraft represents a major breakthrough for this technology. This new, previously unforeseen application opportunity is due to the capabilities of FEEP thrusters in the very low thrust range (about 1 mN). A major project, the Laser Interferometer Spaceborne Antenna (LISA), presently scheduled as a cornerstone in the ESA Horizon 2000+ programme, features FEEP as a mission-enabling technology. Several other concepts have been proposed, ranging from AU baselength interferometry, to a spaceborne constellation of antennas for synthetic aperture radiometry to be used for high accuracy weather forecasting, to noiseless microgravity laboratories. Also, a proposed small scientific mission (Galileo Galilei - GG), presently being studied under Italian Space Agency’s funding, addresses the Equivalence Principle validity with a spinning drag-free satellite. This renewed interest in FEEP has lead to reconsidering the application capabilities of this thruster, namely for small satellite AOCS and for ultra-high precision pointing of spacecraft in constellations. FEEP tests in space simulators were extensively carried out at both the Electric Propulsion Laboratory of ESA and at Centrospazio, covering all of the thruster’s subsystems. A flight demonstration feasibility study was carried out by Centrospazio to exploit the Biopan facility of ESA. Mainly intended for biological experiments, the Biopan can be mounted on the outer surface of its carrier satellite so that its biological samples are directly exposed to the hard vacuum and radiation environment of outer space.
After a brief survey of the present state of development of the FEEP thruster, this paper presents the results of a feasibility study of the FEEP experiment on Biopan, carried out by Centrospazio and Kayser Italia. The viability of a simple, low cost and readily achievable flight demonstration of this promising electric propulsion technology is outlined and discussed. A few other attractive flight demonstration opportunities, like the use of a Get Away Special on the Space Shuttle, and a dedicated drag-free microsatellite, are presented
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