1,720,961 research outputs found
Identification of Hydrodynamic Coefficient from Standard Manoeuvres
No full textThe purpose of the present paper is to outline a procedure for the identification of the hydrodynamic coefficients from experimental data, either full-scale or model-scale, on standard zigzag and turning circle manoeuvres. To this end a ship was selected for which experimental results from PMM model-scale tests and standard manoeuvres from full-scale tests were available and two sets of standard manoeuvres were considered, zigzag and turning circle. The most influent hydrodynamic coefficients (resulting from an initial sensitivity analysis) were the unknown parameters for the identification procedure. This procedure was based on the iterative use of our manoeuvring simulation program (SIMSUP) within the framework of a numerical optimization package (FRONTIER), having as objective to be minimized an error function made up of the main parameters of the two manoeuvres. The proposed identification procedure allows to refine the predictions based on the preliminary estimation of the coefficients through the regression formulas incorporated in SIMSUP, even if it was not yet possible to identify univocally a pre-determined set of hydrodynamic coefficients. Among the benefits of the proposed procedure is the feasibility to exploit any available experimental data-base of standard manoeuvres in order to extract a set of hydrodynamic coefficients, ready to be used for computer simulations or to build up regression formulas
Design Project of a Trimaran Multipurpuose Frigate – Study of the Hydrodynamic Aspects
No full textObject of this study is the design project of a trimaran multipurpose frigate with the aim of evaluating advantages/disadvantages of a multihull configuration for new combat ships in comparison with a corresponding traditional monohull vessel.
As a first step, an analysis of existing technical literature was conducted in order to collect information about different aspects related to the project of a trimaran, then the main characteristics of the mono-hull chosen as reference were defined.
On the basis of the same operational requirements adopted in the definition of the corresponding monohull and of the information deducted by literature, the main characteristics of a trimaran configuration, identified as target vessel, were defined (i.e. dimensions and hull geometry, main hull and outrigger positioning, internal compartments definition, lay-out of general and engine arrangements, hull appendages).
Once defined the target vessel, 9 different configurations obtained changing outriggers position and hull forms were assumed in order to investigate the influence of these variations on main trimaran performances and in particular to analyse wave interference effects strictly depending on the distance between the hulls and their respective position.
Therefore 10 different configurations were obtained and for these studies of propulsion, seakeeping, manoeuvrability and stability were developed in order to compare the performance of the trimaran with the corresponding monohull.
From the results obtained it can be deduced that a trimaran offers a greater bridge area with a consequent improvement of the layout while the reduced breath of the main hull causes some problems in internal arrangement. With regard to propulsive performance a lower power is required at higher velocities but not at lower speed; the trimaran configuration moreover offers improved seakeeping even if special care is required to evaluate impacts on traversal connection between hulls. Considering manoeuvrability, the course stability of the trimaran is greater than that of an equivalent monohull with a consequent reduction of the turning ability; finally, the intact stability of the vessel is improved, while some problems can be related to damage stability.
In conclusion of this analysis concerning advantages/disadvantages of a trimaran configuration, it can be concluded that it offers some aspects of great interest for future development
Wave Drift Forces Estimation for the Preliminary Design of Dynamic Positioning Systems
No full textIn present work, a simplified methodology able to provide a sufficiently accurate assessment of wave drift forces is presented, with the aim of providing a procedure which may be applied in the preliminary stages of the design, when the technical data of the ship (such as the hull forms) are not yet available, and only main hull parameters are present. In particular, the 3D panel seakeeping program PRECAL, developed at Cooperative Research Ships (CRS) is utilised. At first, its capabilities are verified comparing numerical results with literature data, showing the code reliability. Then, a series of simplified ellipsoidal hull forms have been analysed and compared with more complex hull forms in order to identify a criterion of equivalence, which may be used when a detailed description of the hullform is not available. Results show that the proposed simplified methodology is reliable and enough accurate to allow a proper sizing of the propulsion and manoeuvring systems of a ship with dynamic positioning requirements in the preliminary stage of the design
Hydrodynamic coefficients regressions analysis and application to twin screw vessels
No full textThe problem of ship manoeuvrability has reached nowadays a significant consideration, both for merchant ships, with the adoption of IMO standards, and naval ships, with the production of various documents by NATO Specialist Teams. In literature many works regarding manoeuvrability of single screw slow/medium speed ships can be found, while a lack of information exists for what regards twin screw ships (cruise ships, RoRo ferries, megayachts, naval vessels), characterised by different form coefficients and more complex stern configuration due to appendages like skegs, shaft lines and brackets that can strongly affect manoeuvrability behaviour. In this hydrodynamic coefficients of a series of twin screw naval vessels have been evaluated considering different semi-empirical formulations proposed by various authors and standard manoeuvres (turning circle and zig-zag) have been simulated with the aid of a simulator tool. Discrepancies of predicted manoeuvres with respect to experimental full scale ones have been analysed considering parameters involving stern appendages and hull main dimensions in order to select most suitable regressions to study twin screw vessel manoeuvring performance in the earlier design stages. Moreover, linear hydrodynamic coefficients corrections related to stern appendages contribution have been developed on the basis of results of a system identification technique and of an analysis of a set of experimental data at PMM for a ship model equipped with different stern appendages configurations
Carena Trimarano per Fregata Multiruolo
No full textArticolo su rivista Tecnologie Trasporti Mare, gennaio-febbraio 200
Application of System Identification for the Improvement of Manoeuvrability Prediction for Twin-Screw Ships
No full textInterest in ship manoeuvrability has increasingly grown during last decades, both for merchant and naval ships resulting in the adoption by IMO of RESOLUTION MSC137 (76) and in publication by NATO of ANEP 70. Ship manoeuvrability characteristics prediction is strongly related to the accuracy in determination of the hydrodynamic coefficients which specify the mathematical models used in simulation tools. Hydrodynamic coefficients are usually estimated, in preliminary design stages, from regression formulas based on existing data, which may lead to inaccurate predictions for non conventional vessels or ships exceeding experimental data base parametric range. In this work techniques already developed by the authors have been applied systematically to a series of seven twin screw naval ships of different type, in order to improve the current prediction capabilities of the available simulators. From the comparison of results before and after system identification, it can be noticed that a considerable improvement in the simulations quality has been experienced, and a simple procedure for calibrating the existing regressions on the basis of results obtained is proposed. Moreover, the very important influence of the hull appendages configurations for this kind of ships is underlined, and a tentative method for its assessment is described
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
Identification of Hydrodynamic Coefficients from Standard Manoeuvres for a Series of Twin-Screw Ships
No full textInterest in ship manoeuvrability has increasingly grown during last decades, both for merchant and naval ships; resulting in the adoption by IMO of RESOLUTION MSC137(76) and in publication by NATO of ANEP 70. Ship manoeuvrability characteristics estimation is strongly related to the accuracy in determination of the hydrodynamic coefficients which specify mathematical model used in simulation tools. Hydrodynamic coefficients are usually estimated, in preliminary design stages, from regression formulas based on existing data, which may lead to inaccurate predictions for non conventional vessels or ships exceeding experimental data base parametric range. In this work a previously developed method for hydrodynamic coefficients identification from standard experimental manoeuvres is systematically applied to a series of twin-screw ships, with the primary aim of investigating the possibility of improving the prediction capability of existing manoeuvring simulators for this specific ship type, and the general aim of establishing a low-risk procedure for ship manoeuvrability prediction in the preliminary design stages
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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