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Pressure field and wake modes analysis of an oscillating cylinder
No full textThis work proposes a numerical investigation of the main flow field characteristics and pressure field analysis around a freely oscillating rigid circular cylinder immersed in a high Reynolds number flow in the subcritical range. The cylinder is characterized by high value of mass ratio and mass damping, while the damping itself is quite low. Here the numerical results are compared with experimental data obtained in the Politecnico di Milano wind tunnel under the same fluid dynamic and mechanical conditions. The preliminary check of this numerical setup was provided by considering the case of a fixed cylinder in the subcritical region. Based on this benchmark, the full setup was checked by considering fluid dynamic conditions outside the lock-in region.
Finally, a number of points were investigated on the cylinder's steady state response curve in the lock-in region. The numerical model yielded good results in terms of cylinder amplitude response, aerodynamic forces and pressure field analysis, in agreement with the results of the experiment. Analysis of the numerical reconstruction of the flow field evolution is therefore considered to yield further information on the vortex shedding mode, especially in the transition region between 2S and 2P mode
Hydro-abrasive erosion in Pelton turbine injectors: A numerical study
Get PDFNumerical simulations were performed to investigate how the design and the operation conditions of a Pelton turbine injector affect its vulnerability to hydro-abrasive erosion, alongside with its flow control capacity. Use was made of a Volume Of Fluid (VOF) model for simulating the free nozzle jet, a Lagrangian particle tracking model for reproducing the trajectories of the solid particles, and two erosion models for estimating the mass removal. The comparison against earlier studies and the experimental evidence, integrated with a careful sensitivity analysis, gave strength to the reliability of the numerical model. Nozzle seat and needle were the injector components most vulnerable to erosion. As the valve was closing, the erosion of the needle strongly increased, whilst that of the nozzle seat remained broadly constant. The influence of the injector design was also explored, suggesting that a reduction of the needle vertex angle is likely to enhance the risk of erosive wear. Finally, it was found that the possibility to condense the effects of the needle stroke and the needle vertex angle in a single parameter (i.e. the effective opening area) is no more allowed when hydro-abrasive erosion is considered, thereby assessing the need for case-specific wear prediction analyses
VIBRAZIONI INDOTTE DA DISTACCO DI VORTICI SU UN CILINDRO CIRCOLARE
No full textL’interazione tra un cilindro oscillante ed una corrente stazionaria è un tema che continua a destare molto interesse nonostante sia ampiamente studiato. Infatti set up caratterizzati da alti valori di mass-ratio, mass-damping e numero di Reynolds sono ancora poco studiati nonostante siano di significativo interesse pratico. Sfruttando le caratteristiche della Galleria del Vento del Politecnico di Milano da qualche anno si sono avviate ricerche in questi range sperimentali. In particolare, in questo lavoro, si è approfondita l’analisi cinematica del campo di moto a valle di un cilindro oscillate al fine di verificare la transizione della modalità di distacco di vortici da 2S a 2P, in corrispondenza del salto tra l’initial branch e l’upper branch. Il significativo miglioramento dei risultati rispetto ai precedenti lavori è stato ottenuto sviluppando sia il sistema di acquisizione sia le tecniche di elaborazione delle immagini e di post-processing dei dati cinematici
Numerical Simulation of an Oscillating Cylinder at High Reynolds Number
No full textIn this work, we propose a numerical investigation of the main flow field characteristics around a free oscillating rigid circular cylinder that is immersed in a turbulent flow (Re≈5∙〖10〗^4). The cylinder is characterized by high value of mass ratio and mass damping (m^*=145; ξ=0.6÷1.14∙〖10〗^(-3);m^* ξ=0.1÷0.25). Then the numerical results are compared with previous experimental data obtained in the wind tunnel under very similar fluid dynamic conditions. There are few works in literature that consider both numerical and experimental results under these conditions. This is probably due to the experimental facilities limitations and the computational difficulties correlated to modeling the flow with high Reynolds number. With the aim of matching the experimental analysis, developed in the last years in the Politecnico di Milano Wind Tunnel on a suspended oscillating cylinder, with numerical results, we set-up a numerical URANS model through a CFD commercial code using a k-ω SST turbulence model in a 3D domain.
The numerical setup is characterized by the use of the DFBI-Morphing (Dynamic Fluid Body Interaction) model that allows the modeling of the body motion in response to fluid forces treating the cylinder as a mass-damping-spring system by introducing spring and damping forces acting on it.
A preliminary check of this numerical setup was provided by a benchmark involving a simple case of fixed cylinder at the same Reynolds number, where the movements of the cylinder were disabled. The numerical results of this case have been compared with experimental and numerical results reported in literature by means of Drag and Lift coefficients and Strouhal number at high Reynolds (Re≈5∙〖10〗^4). After that benchmark, the full setup has been checked by considering specific fluid dynamic conditions where the cylinder was out of the lock in region in which the oscillations of the cylinder are negligible.
Finally we have investigated two points of the steady state oscillating response curve of the cylinder in the lock in region. The numerical model gave good results in terms of amplitude response of the cylinder and aerodynamic forces if compared with experimental results.
The analysis of the numerical reconstruction of the flow field evolution are therefore considered to have more information on the vortex shedding mode especially in the transition region between 2S and 2P mode
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
Fluid-Structure Interaction and Co-Simulation: Analysis of a Beam-Supported Sphere for VIV Application
No full textThe recent developments in numerical tools and computing resources seem to provide a suitable environment to perform numerical analyses of Fluid-Structure Interaction problems. The Co-Simulation technique, in particular, develops the idea of coupling a CFD software with a structural one in order to simulate complex FSI phenomena with a partitioned approach, stressing the concept of software modularity. In this way, it is possible to adopt software tools at the cutting edge of technology. Nonetheless, several difficulties may arise in the choice of the partitioning scheme and of the algorithmic details for the step-by-step time integration.
This paper deals with the application of the Co-Simulation technique to a benchmark case experimentally investigated in previous works: the vortex-induced vibrations (VIV) of a beam supported sphere (that is, a sphere fixed to the end of a slender cantilever beam) in a free surface flow. This problem is challenging although apparently simple and it seems quite absent from literature so far. In this paper, the computational issues are thoroughly investigated and the model is validated by comparison with the experimental data. In this way, a robust framework is created in order to deal with VIV problems
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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