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MODELLAZIONE DELLA RAPIDA DINAMICA DELLE BOLLE DI CAVITAZIONE ALL’INTERNO DI UN MEZZO D’ACQUA CONTINUO
No full textIl termine "cavitazione" è comunemente utilizzato per identificare la formazione di cavità di vapore in un liquido (cioè piccole zone senza liquido chiamate "bolle" o "vuoti") originate da forze che agiscono sul liquido stesso. Come noto, l'evoluzione di una bolla di cavitazione può essere termodinamicamente descritta combinando la legge di Clausius-Clapeyron, l’equazione di Kelvin e l’equazione di Rayleigh-Plesset. Le principali proprietà termo-fisiche dei fluidi, indispensabili per un'analisi dettagliata di cavitazione dei flussi, sono cinque: pressione di vapore, entalpia di vaporizzazione, tensione superficiale, viscosità e densità. In particolare, la temperatura di lavoro del fluido operatore determina un'influenza fondamentale su tutte queste proprietà. Più in dettaglio, lo stato termodinamico della fase vapore conseguente ad un transitorio di cavitazione è stato modellato applicando la correlazione dimostrata tra l’equazione di Kelvin e la legge di Clausius-Clapeyron. La validazione di questa teoria è stata effettuata confrontando i risultati numerici e i dati sperimentali relativi all'acqua disponibili in letteratura. Inoltre, due differenti tipi di cavitazione possono essere generalmente identificati: inerziale (o transitoria) e non inerziale. La cavitazione inerziale è il processo in cui un vuoto, o una bolla, in un liquido collassa rapidamente, producendo un'onda d'urto. Il modello di Rayleigh-Plesset è stato applicato per descrivere la partizione di energia della bolla in termini di energia di rimbalzo ed energia acustica durante il collasso in ambiente di microgravità. In questo caso, un ottimo accordo è stato ottenuto nel confronto tra i risultati numerici ed i dati sperimentali ESA presenti in letteratura. Per le bolle di cavitazione generate attraverso la tecnica del laser pulsato, soprattutto per le pressioni inferiori del liquido, le bolle presentano una doppia parete dovuta alla formazione del plasma che anticipa la nascita della bolla. E’ importante notare che il raggio massimo della bolla primaria corrisponde alla perturbazione acustica massima generata dal plasma nel liquido circostante. La cavitazione non inerziale è il processo in cui una bolla in un fluido è costretta ad oscillare in dimensione a causa di qualche forma di apporto di energia, come ad esempio un campo acustico. Anche per questa condizione, predizioni numeriche sono state effettuate al fine di valutare l'influenza della frequenza di oscillazione e dell’ampiezza del raggio della bolla sulla pressione percepita (o misurata) e sulle proprietà termodinamiche (in particola modo in termini di pressione e temperatura) del contenuto di vapore all'interno della bolla.The term “cavitation” is commonly used to identify the formation of vapor cavities in a liquid (i.e. small liquid-free zones called "bubbles" or "voids") originated by forces acting upon the liquid itself.
As well known, the evolution of a cavitating bubble may be thermodynamically described combining Clausius-Clapeyron, Kelvin and Rayleigh-Plesset relationships. The main thermo-physical fluid properties, essential for a detailed analysis of cavitating flows, are five: vapor pressure, enthalpy of vaporization, surface tension, viscosity and density. In particular, the working temperature of the operating fluid determines a fundamental influence on all these properties.
More in detail, in this work, the thermodynamic status of the vapor phase originated during a cavitation transient has been modeled by applying the demonstrated correlation between Kelvin and Clausius-Clapeyron equations. The validation of this theory has been carried out by comparing numerical results and experimental data, related to water, available in literature.
Moreover, two different types of cavitation can be usually identified: inertial (so called transient) and non-inertial. Inertial cavitation is the process where a void or a bubble in a liquid rapidly collapses, producing a shock wave. The Rayleigh-Plesset model has been applied in order to describe the energy partition of the bubble during the collapse in microgravity environment, in terms of rebound energy and acoustic energy. In this case, a very good agreement has been obtained from the comparison between numerical results and ESA experimental data found in literature. For cavitation bubbles generated by the laser pulsed technique, especially for the lower liquid pressures, bubbles present a double wall due to plasma formation that anticipates bubble inception. It is important to note that the maximum radius of the primary bubble corresponds to the maximum acoustic perturbation generated by the plasma into the surrounding liquid. Otherwise, non-inertial cavitation is the process in which a bubble within a fluid is forced to oscillate in size or shape due to some form of energy input, such as an acoustic field. Also for this condition, numerical predictions have been done in order to estimate the influence of the oscillation frequency and amplitude of the bubble radius exerted on the driving pressure and on the thermodynamic properties (mainly in terms of pressure and temperature) of the vapor content inside the bubble
A Transient Multidimensional CFD Approach to the Analysis of a Control Valve Using Non-Newtonian Fluids
No full textIn this paper the flow through a control directional valve is studied by means of a Computational Fluid-Dynamics analysis under transient operating conditions. The mesh motion is resolved on a time basis as a function of the external actuation system. In the analysis, an open source fluid-dynamics code is used and both cavitation and turbulence are accounted for in the modeling. Moreover, the numerical model of the working fluid is modified in order to account also for the non-Newtonian fluids. The effects of the shear rate on the shear stress are accounted for both by using experimental measurements and correlations available in literature, such as the Herschel-Bulkley model. The analysis determines the performance of the control directional valve under different operating conditions when using either Newtonian or non-Newtonian fluids. In particular, the discharge coefficient, the recirculating regions, the flow acceleration angle and the pressure and velocity fields are investigated
Modeling the Axial Balancing Mechanism of Orbit Annular Hydraulic Machines
No full textA customized combined methodology based on both 2D CFD (computational fluid dynamics) and lumped parameters numerical modeling, useful for simulating the hydraulic behavior of orbit annular machines, has been developed and here presented. More in details, the predictive capabilities of this CAE (computer-aided engineering) tool can be applied for the study of both roller and gerotor architectures and considering both pumping and motoring operating mode. First of all, an in-house developed 2D CFD methodology, based on the integration of the stationary form of the Reynolds equation for the determination of the pressure distribution inside the lateral clearances bounded by the sides of the stator-rotor group and the valve plate, as well as the internal manifold surface, is firstly presented and applied. The same computational procedure has been also involved for the investigation of the leakages through the clearance between the valve plate and the balancing ring. After that, a lumped and distributed parameters numerical model has been involved for the simulation of a typical orbit roller motor operation. In this case, particular care has been devoted to the modeling of the axial leakage clearances, adopting analytical interpolation functions deducted from the numerical results calculated applying the previously described 2D CFD methodology. Finally, the whole CAE approach has been validated by means of a comprehensive numerical versus experimental comparison, obtaining a general good accordance for the overall operating field of this particular type of hydraulic unit
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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