1,720,974 research outputs found
Formulazione in Complementarietà Lineare per Risolvere Problemi di Lubrificazione con Cavitazione in Regime Elastoidrodinamico
No full textIn questa tesi viene descritta una formulazione per lo studio di problemi di lubrificazione basata sul concetto di complementarietà lineare.
Questa formulazione si fonda sull'equazione di Reynolds riscritta in termini di pressione e frazione gassosa. La corretta descrizione del fenomeno della cavitazione, così come l'individuazione delle interfacce tra zone cavitate e zone attive, è ottenuta automaticamente grazie alla natura complementare di queste due variabili. Per questo motivo è possibile descrivere correttamente il problema fluidodinamico su tutto il dominio utilizzando un unica equazione, valida sia nelle zone attive che in quelle cavitate. Questo approccio garantisce inoltre la conservazione della massa senza la necessità di imporre opportune condizioni alle frontiere tra zone attive e zone cavitate.
La formulazione complementare dell'equazione di Reynolds, sia per casi monodimensionali che per casi bidimensionali, è analizzata in dettaglio. Inoltre, sono stati considerati vari modelli per una simulazione accurata della reologia e delle proprietà del lubrificante.
L'accoppiamento della formulazione complementare per la soluzione dell'equazione di Reynolds alla teoria Hertziana del contatto permette la soluzione di problemi elastoidrodinamici. Alternativamente, è possibile utilizzare anche un modello di deflessione elastica ottenuto da modelli strutturali a Elementi Finiti. In questo modo è possibile considerare correttamente le caratteristiche elastiche di componenti meccanici non modellabili in modo soddisfacente tramite la teoria di Hertz, quali ad esempio testa e piede di biella di motori a combustione interna.
La formulazione presentata è stata discretizzata ed implementata numericamente utilizzando uno schema a Elementi Finiti. Questa implementazione si è rivelata estremamente robusta e versatile, inoltre non richiede specifiche condizioni sulle dimensioni degli elementi della griglia di calcolo. Particolare attenzione è stata dedicata allo sviluppo del codice numerico al fine di massimizzare la velocità computazionale.
Il metodo sviluppato è capace di risolvere con efficienza una vasta gamma di problemi e di implementare qualsiasi modello reologico ed elastico risulti necessario. Un particolare vantaggio rispetto ai modelli classici è offerto nel caso di problemi di lubrificazione in regimi isoviscosi rigidi ed in presenza di forti gradienti di altezza del meato di lubrificante.This thesis describes a formulation for the analysis of lubrication problems that handles the cavitation phenomenon with the concept of linear complementarity.
This formulation is based on the Reynolds equation, appropriately recasted in terms of pressure and void fraction. The correct detection of the cavitation and the location of the boundaries between cavitated and active areas is guaranteed by the complementary nature of the two chosen variables. Therefore, it is possible to describe the hydrodynamic problem all over the domain using a single equation, which is valid both in the active and in the cavitated zone. This approach naturally guarantees the mass conservation.
A detailed analysis of the proposed complementary formulation of the Reynolds equation is presented, both for one dimensional and two dimensional cases. In addition, various rheological models are considered in order to simulate accurately the lubricant behaviour and properties.
The Hertz contact theory has been introduced for the consideration of the elastic deflection of the contact bodies. As an alternative, it is possible to handle the elasticity of the solids by importing the compliance matrix generated by a external structural Finite Element model. This second method is suitable for the EHL analysis of various common mechanical components, such connecting rods big end and small end bearings, that cannot be satisfactorily modelled with the Hertz contact theory.
A numerical method, based on the Finite Element framework, has been employed to solve the complementarity formulation of the Reynolds equation. The developed implementation is versatile and robust and does not require particular conditions on the dimensions of the elements of the mesh. The numerical development focused on the maximization of the computational speed.
The proposed formulation is capable to solve efficiently a wide range of problems and to consider various rheological and elastic models. Particular advantages, with respect to standard finite difference approaches, have been found in the cases of isoviscous and rigid lubrication regimes and in the presence of steep film thickness gradients
Exploring the nexus between the standardized nursing terminologies and the unfinished nursing care phenomenon: An empty systematic review
No full textPurpose: To identify and synthesize evidence regarding the documented relationship between the standardized nursing terminologies and the unfinished nursing care phenomenon. Data sources: A systematic review according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. PubMed, Scopus, and Cumulative Index to Nursing and Allied Health Literature Complete databases were last consulted on November 27, 2023. The review included primary quantitative studies that reported an association between recognized standardized nursing terminologies and unfinished nursing care. Two researchers completedtitle and abstract and full-text screening. Data synthesis: Our search identified 149 citations. A full-text review of one paper was undertaken. No studies met our inclusion criteria. We report an empty review. Conclusions: Standardized nursing terminologies and Unfinished Care are two sides of the same coin: despite their potential commonalities, no studies have documented their potential links. Digital systems, such as electronic health records and decision support systems, could foster this linkage. Implications for nursing practice: This review suggests that linking the conceptual frameworks can promote the diffusion of standardized nursing terminologies in clinical practice and increase accuracy in the measurement of Unfinished Care. This synergy could promote the contribution of nursing knowledge to patient care, nursing visibility, and be beneficial to clinical nurses, managers, and healthcare systems to international level
Microgeneratori ad effetto Peltier per dispositivi di contabilizzazione termica
No full textLa generazione elettrica mediante dispositivi ad effetto Peltier è efficace e competitiva in situazioni di alimentazione di apparecchi delocalizzati a basso assorbimento di potenza.Lo studio svolto riguarda l'accoppiamento di microgeneratori ad effetto Peltier e contatori di consumo termico applicabili alle terminazioni radianti di un impianto di riscaldamento centralizzato con distribuzione a colonne montanti. Misurare il calore fornito alle varie unità abitative di un condominio può infatti richiedere l'installazione di un gran numero di contatori, fino ad uno per ogni corpo radiante, rendendo così onerosa la manutenzione relativa alla sostituzione periodica delle batterie e pregiudicando l’affidabilità generale del sistema di contabilizzazione.L'effetto Peltier può essere sfruttato per alimentare un contatore di calore in virtù della differenza di temperatura esistente tra il fluido termovettore dell’impianto, che scorre nel corpo radiante monitorato, e l’ambiente riscaldato. Si può così produrre l'energia elettrica necessaria a rendere il contatore un dispositivo stand-alone autoalimentato, senza bisogno di connessioni alla rete elettrica o di batterie chimiche da sostituire periodicamente. La minore manutenzione richiesta può così favorire la diffusione della contabilizzazione dell'energia per il riscaldamento, che è fondamentale per incentivare gli utenti al risparmio energetico.Il presente studio si è focalizzato sulle strategie di dimensionamento ed ottimizzazione del microgeneratore in oggetto, utilizzando a tal scopo metodologie di modellazione a parametri concentrati e considerando diverse configurazioni del dispositivo. Il progetto è stato sviluppato prendendo a riferimento componentistica commerciale ovunque possibile, in modo da garantire ridotti costi di fabbricazione ed affidabilità adeguata
A complementarity formulation to study lubricated contact problems in the presence of cavitation
No full textA mass-conserving formulation of the Reynolds equation has been recently developed using the concept of complementarity [1]. The mathematical derivation of the Linear Complementarity Problem (LCP) implemented in the solver favoured in [1] overcomes the drawbacks previously associated with the use of such complementarity formulations for the solution of cavitation problems in which reformation of the liquid film occurs. In the present paper, the proposed methodology, already successfully applied to solve textured bearing and squeeze problems in the presence of cavitation in a one dimensional domain and for incompressible fluids [1], has been extend to a two dimensional domain and the fluid compressibility has been included in the formulation. The evolution of the cavitated region and the contact pressure distribution are studied for a number of different configurations. Some of the results obtained with the proposed scheme are critically analysed and compared with the predictions obtained using alternative formulations (including full CFD calculations). The stability of the proposed algorithm and its flexibility in terms of the implementation of different compressibility laws is highlighted
Achievement of a uniform contact pressure in a shaft–hub press-fit
No full textIn this article, the achievement of a uniform elastic contact pressure in a frictionless, keyless, shaft-hub interference fit obtained by properly shaping the mating profiles is examined. The peculiarity of the hub mechanical response according to which, under the effect of a uniform pressure applied to the hub bore, the bore axial profile moves radially without any distortion, is exploited to simplify the determination of the mating profiles that return a uniform pressure. In particular, the hub radial deflection may be computed with a simple plane model, whereas only the shaft radial deflection requires a more complex analysis in cylindrical coordinates. Explicit approximate expressions are reported for the shapes to be conferred to the mating profiles to achieve a uniform pressure. Selected examples are presented to clarify the proposed design procedure and to preliminarily explore the effect on the pressure profile of simple shape errors
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
OPTIMIZATION METHODOLOGY FOR INNOVATIVE AUTOMOTIVE CRASH ABSORBERS
No full textThe simulation of vehicle crash impacts requires accurate and computationally expensive Finite Element analysis. An effective procedure consists in considering and establishing which improvement can be made on an equivalent sub-model of the full vehicle. In this way, all the analysis can be performed on smaller models, thus saving computational time. A full vehicle simulation is required only at the end of the design process to validate the results of the sub-model analysis.A software based on a genetic optimization algorithm has been developed in order to optimize the geometrical parameters of a variable-thickness crash absorber. A numerical study on the folding of thin-walled aluminum tubes with variable-thickness has been performed in order to achieve the maximum energy absorption-to-mass ratio. Moreover, the performance in terms of folding length and crush load peaks have been considered.Different optimization strategies have been implemented to find out which solution guarantees the achievement of the optimization target with the lowest computational cost.The results show how the approach proposed by the authors allows an efficient variable-thickness crash absorber to be obtained. In fact it performs better in term of crash behavior and energy dissipation-to-mass ratio, with respect to the original constant_thickness model
Stress Concentrations at the Rounded Edges of a Shaft-Hub Interference Fit Expressed in Terms of a Coefficient Normalizing the Coupling Geometry and the Young’s Modulus Effects
No full textThe elastic stress concentrations developed from the keyless, frictionless, static press-fit of a solid shaft into a hub with bore rounded edges are addressed. Derived from an analytical approach, a normalising parameter Φ is employed that accounts for the combined effects on the hub stress concentration of the fillet radius of the hub bore, the shaft radius, the interference, and the Young’s modulus. Compiled with the aid of Finite Elements, several design charts are presented that report the elastic stress concentrations within the hub versus the normalising parameter Φ. Each curve is valid for prescribed ratios of a) the hub inner radius to the outer radius, and b) the fillet radius to the shaft radius. An approximating expression of ample validity is also presented for a prompt evaluation of the hub stress concentration factor
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
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