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Ianniello S.,CNR Italian Ship Model Basin
21st International Congress on Sound and Vibration 2014, ICSV 2014

This paper aims to numerically demonstrate that the underwater noise generated by a marine propeller is an essentially nonlinear problem and that, unlike the analogous aeronautical configurations, the nonlinear flow noise sources play a dominant role independently of the low rotational speed of the blade. To this aim the prediction of the thickness noise component only (in air) is carried out on two typical blade models corresponding to a helicopter rotor and a marine propeller, in order to assess how the main structural and geometrical differences between these two bodies (the aspect ratio, the twist and thickness distribution along span) can affect the resulting noise in the far field. Source

Marino E.,University of Florence | Lugni C.,CNR Italian Ship Model Basin | Borri C.,University of Florence
Computer Methods in Applied Mechanics and Engineering

We present a novel numerical procedure for the prediction of nonlinear hydrodynamic loads exerted on offshore wind turbines exposed to severe weather conditions. The main feature of the proposed procedure is the computational efficiency, which makes the numerical package suitable for design purposes when a large number of simulations are typically necessary. The small computational effort is due to (i) the use of a domain-decomposition strategy, that, according to the local wave steepness, requires the numerical solution of the nonlinear governing equations only on a limited number of reduced regions (sub-domains) of the whole space-time domain, (ii) the choice of the particular numerical method for the spatial discretization of the governing equation for the water-wave problem. Within the potential flow assumption, the Laplace equation is solved by means of a higher-order boundary-element method (HOBEM). For the time evolution of the unsteady free-surface equations the 4th-order Runge-Kutta algorithm is adopted. The compound solver is successfully applied to simulate nonlinear waves up to overturning plunging breakers, that may cause severe impact loads on the wind turbine substructure.Emphasis is finally given to wind turbine exposed to realistic environmental conditions, where the proposed tool is shown to be capable of capturing important nonlinear effects not detected by the linear models routinely adopted in the design practice. © 2012 Elsevier B.V. Source

Dessi D.,CNR Italian Ship Model Basin
Mechanical Systems and Signal Processing

In this paper, a new technique for determining a load field (e.g., pressure) on the basis of a few global measurements (e.g., forces) is presented. This technique is based on a combination of proper orthogonal decomposition (POD) and polynomial spline approximation with integral constraints and is here illustrated with respect to the prediction of the wave load distribution along a slender floating body like a ship. The input data are provided by the time-histories of the lumped vertical forces acting on several longitudinal portions (i.e., segments) of a scaled model of a fast ship. To achieve a better understanding of the accuracy of this technique, the segment forces are not experimentally but numerically obtained by integrating the sectional forces over the length of the segments. These forces, calculated with a strip-theory approach precessing experimental data relative to ship motion in waves, provide also the target distribution for validating the present procedure. The set of lumped hydrodynamic forces defines the vector process to which POD is applied. The components of the identified POD modes provide the integral constraints for the spline polynomials approximating the continuous basis functions of the sectional load distribution. The sectional load distribution along the hull is then reconstructed, showing good agreement with the original load data. Finally, the robustness of the proposed technique is investigated by studying the propagation of bias errors and additive white noise from input data to final results. © 2014 Elsevier Ltd. Source

OFF, an open source (free software) code for performing fluid dynamics simulations, is presented. The aim of OFF is to solve, numerically, the unsteady (and steady) compressible Navier-Stokes equations of fluid dynamics by means of finite volume techniques: the research background is mainly focused on high-order (WENO) schemes for multi-fluids, multi-phase flows over complex geometries. To this purpose a highly modular, object-oriented application program interface (API) has been developed. In particular, the concepts of data encapsulation and inheritance available within Fortran language (from standard 2003) have been stressed in order to represent each fluid dynamics "entity" (e.g. the conservative variables of a finite volume, its geometry, etc...) by a single object so that a large variety of computational libraries can be easily (and efficiently) developed upon these objects. The main features of OFF can be summarized as follows: Programming LanguageOFF is written in standard (compliant) Fortran 2003; its design is highly modular in order to enhance simplicity of use and maintenance without compromising the efficiency; Parallel Frameworks Supported the development of OFF has been also targeted to maximize the computational efficiency: the code is designed to run on shared-memory multi-cores workstations and distributed-memory clusters of shared-memory nodes (supercomputers); the code's parallelization is based on Open Multiprocessing (OpenMP) and Message Passing Interface (MPI) paradigms; Usability, Maintenance and Enhancement in order to improve the usability, maintenance and enhancement of the code also the documentation has been carefully taken into account; the documentation is built upon comprehensive comments placed directly into the source files (no external documentation files needed): these comments are parsed by means of doxygen free software producing high quality html and latex documentation pages; the distributed versioning system referred as git has been adopted in order to facilitate the collaborative maintenance and improvement of the code; CopyrightsOFF is a free software that anyone can use, copy, distribute, study, change and improve under the GNU Public License version 3. The present paper is a manifesto of OFF code and presents the currently implemented features and ongoing developments. This work is focused on the computational techniques adopted and a detailed description of the main API characteristics is reported. OFF capabilities are demonstrated by means of one and two dimensional examples and a three dimensional real application. Source

Bouscasse B.,CNR Italian Ship Model Basin | Broglia R.,CNR Italian Ship Model Basin | Stern F.,University of Iowa
Ocean Engineering

The present work is about the seakeeping behavior of a fast catamaran advancing in head sea. To this aim seakeeping tests with transient, regular and irregular waves are performed in the rectilinear water tank of CNR-INSEAN. Seakeeping transient tests are used to provide the response amplitude operator for a wide range of wave lengths and several speeds of advancement. These tests allow identifying the Froude number at which the maximum vertical response occurs and an analysis of natural heave and pitch frequencies. A comparison with theoretical predictions is provided. Regular wave tests are used to assess nonlinear effects on the hull motions, as well as the added resistance generated by the increasing steepness and dependency on both the Froude number and wave lengths: therefore several ship speeds, several steepness and wave lengths of the incident wave system are considered. For a similar Froude number range, irregular wave tests are also pursued to investigate the seakeeping properties of the vessel in real scenarios. The estimation of added resistance in wave, its dependency on the Froude number, the investigation of nonlinear effects and the analysis of increased resistance in a real scenario are addressed as well. To make sure the present experimental campaign is valid, it is checked with repeatability analysis and matched with ad hoc experimental data collected at TU Delft. The whole set of measurements is a valuable database for both hydrodynamic studies of high speed catamarans and CFD validation. © 2013 Elsevier Ltd. Source

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