1,720,958 research outputs found
Assessment and Optimization of Dynamic Stall Semi-empirical Model for Pitching Aerofoils
Get PDFDynamic stall is a phenomenon affecting aerofoils in unsteady flows which is particularly relevant in the rotary-wing field. Semi-empirical models are simplified tools to simulate this phenomenon, especially during preliminary design phases and for aeroelastic assessments. However, they need a large number of tuning parameters to provide reliable estimations of unsteady airloads. To face this problem, a parameter identification procedure based on sequential resolutions of optimization problems using a genetic algorithm is developed and it is applied to the state-space formulation of a modified version of the so-called "Second Generation” Leishman-Beddoes model. The effects of the optimal parameters on the model prediction capabilities are discussed and the variability of the parameters with reduced frequency is studied. The estimations of the unsteady airloads obtained by applying the optimization of parameters show a great improvement in the correlation of the experimental data if compared to the predictions obtained by using the parameters provided in the literature, especially for pitching moments where the negative peaks are very well described. These improvements justify the need for optimization to set the parameters
Flutter analysis of propfan-open rotors
Get PDFThis work aims at showing the importance of flutter predictions in the preliminary design of propfan–open rotors. Both single rotating propellers and contrarotating open rotors are investigated. The related structural subsystems are modeled through finite element analyses; the aerodynamic subsystems exploit a finite volume, full potential formulation suitable for unsteady transonic flows. An ad hoc technique is developed for simulating the flowfield around a contrarotating open rotor configuration. The effectiveness of the proposed aeroelastic analysis is successfully assessed for single propellers through comparisons with reference numerical and experimental data available in the literature, as well as against Euler flow-based solutions. The results for contrarotating open rotors cannot be validated because of the lack of corresponding open-literature data
Prediction of Helicopter Rotor Loads and Fatigue Damage Evaluation with Neural Networks
Get PDFIn recent years, machine learning algorithms have experienced rapid advancement, driven by the exponential growth of data availability and computational capabilities. Among these algorithms, artificial neural networks stand out as one of the most renowned and effective classes, possessing the ability to discern relationships within data. In this study, we harness neural networks to deduce the relationship between flight mechanics parameters and resulting loads in an articulated rotor configuration. The accuracy of these algorithms hinges closely on the quality of the dataset used for training. Given that rotor loads manifest as time-periodic signals with precise harmonic content, we train dedicated neural networks to predict each harmonic individually. Subsequently, the load time history is reconstructed post hoc by amalgamating predictions from each individual network. Various network architectures are explored, and a sensitivity analysis is conducted on hyper-parameters to determine the optimal configuration for this specific application. Moreover, these predictions serve as input for a fatigue damage calculation algorithm
Flutter analysis of open rotors
Get PDFLAUREA MAGISTRALEQuesto lavoro di tesi è volto a dimostrare l’importanza dell’analisi
aeroelastica, e.g. predizione del flutter, nell’analisi preliminare di Contra
Rotating Open Rotor (CROR). A tal fine, il sistema strutturale è
modellato con i programmi ad elementi finiti NASTRAN® e ANSYS®.
Viene condoto un confronto delle frequenze proprie e delle forme modali
usando sia modelli ad elementi solidi (tetraedri) sia ad elementi
di piastra. Il sistema aerodinamico è modellato con il solutore a potenziale
a volumi finiti ST . Il programma risolve correnti transoniche
non stazionarie grazie ad un’approssimazione indipendente per i campi
della densità e del potenziale di velocità. Questo è anche in grado
di risolvere correnti non isentropiche grazie ad una nuova forma della
condizione di Kutta [53–55]. È stata sviluppata una tecnica ad hoc
per simulare le correnti attorno a due corpi portanti in moto relativo
(cioè lo stadio anteriore e posteriore). Le analisi di flutter sono effetuate
con il metodo K-E(fficiente) (già usato nel toolbox aeroelastico
di NASTRAN®) e con un metodo non lineare di inseguimento degli
autovalori in modo da tracciare i diagrammi V - g e V -w.
La validazione della tecnica proposta per l’analisi aeroelastica di Single
Rotating Propfan è ampiamente supportata dal confronto dei risultati
ottenuti con i dati sperimentali e numerici presenti in letteratura e
con quelli ottenuti dal solutore euleriano AeroX [44, 58]. Non è stato
possibile validare le tecniche per il CROR poichè non sono presenti
dati in letteratura e non sono reperibili altri strumenti di analisi.
Parole chiave: aeroelasticità, contra-rotating open rotor, regime transonico,
flutter, aerodinamica a potenziale.The thesis is aimed to show the importance of aeroelastic analysis,
e.g. flutter prediction, in the preliminary design of Contra Rotating
Open Rotor (CROR). To this end, the structural sub-system is modelled
with the Finite-Element (FE) solvers NASTRAN® and ANSYS®.
A comparison of natural frequencies and mode shapes is performed using
both tetrahedral and shell element models. The aerodynamic subsystem
is modelled with the Finite-Volume (FV) full potential solver
ST . The solver model unsteady transonic flows by means of an independent
approximation of the density and velocity potential fields. It
can also model non-isentropic flows thanks to a new form of Kutta
condition [53–55]. An ad hoc technique is developed for simulating the
flow field around two lifting bodies with relative motion, i.e. the front
and rear propeller stages. The flutter analyses are carried out using
the so-called K-E(fficient) method (also used in NASTRAN® aeroelastic
toolbox) and a root-tracking non-linear method for computing the
V - g and V -w diagrams.
The effectiveness of the proposed aeroelastic analysis for Single Rotating
Propfan (SRP) is successfully assessed by tackling a set of realistic
dynamic problems and by comparing the results with reference
experimental and numerical data available in literature and with the
results obtained by the Euler flow solver AeroX [44, 58]. The results of
the CROR cannot be validated because there are neither data available
in the literature nor other alternative procedures.
Key words: aeroelasticity, contra-rotating open rotor, transonic flow,
flutter, full potential
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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