Abstract
A three degrees-of-freedom model based on the potential flow theory was implemented to represent the motion of a slender cylindrical buoy under waves. The model calibration was performed by means of the comparison between the model results and the experiments performed at the Laboratory of Hydraulic Engineering of the University of Bologna (Italy). The dynamics of the floating cylinder, placed at the mid-section of the wave flume and anchored at the bottom through a mooring system of four catenaries, were obtained through videography analysis, providing surge, heave and pitch motions. The implementation of the mathematical model consisted of two main parts: The first has been developed in the frequency domain by applying NEMOH to assess the hydrodynamic coefficients of the object, i.e., the excitation, radiation and added mass coefficients; then, the used mooring system was included in the time-domain model, solving the motion of the floating cylinder, by calibrating the mooring coefficients by comparing the results with the data. The simplicity of the implemented model is a very important feature, and it should be used as a preliminary study to understand the response of moored floating cylinders and others floating bodies under waves.
Highlights
Interaction between water waves and floating objects has been extensively studied in naval, ocean and coastal engineering, and recently quite a remarkable amount of research [1,2,3,4] has been devoted to floating prototypes for wave energy conversion (WEC); this is due to the increasing investigations in renewable energy resources exploitation derived from the need for a new global energetic model, aiming to reduce fossil fuel consumption according to the IPPC [5]
This study aims to present a properly developed three degrees-of-freedom (DoF) model, based on potential flow theory, in order to represent the motion of a floating body calibrated through the comparison with laboratory data
The vector Sj represents the object area, the matrices Rd,jk, Kjk, fexc,j and Cd,jk include the hydrodynamic coefficients of radiation, hydrostatic stiffness, wave excitation and drag coefficients: the first three are computed in the frequency domain and the latter coefficient has been obtained from the literature [41], resulting in (0.84; 0.98; 0.0) for surge, heave and pitch modes, respectively
Summary
Interaction between water waves and floating objects has been extensively studied in naval, ocean and coastal engineering, and recently quite a remarkable amount of research [1,2,3,4] has been devoted to floating prototypes for wave energy conversion (WEC); this is due to the increasing investigations in renewable energy resources exploitation derived from the need for a new global energetic model, aiming to reduce fossil fuel consumption according to the IPPC [5]. The analysis of floating device responses to waves is often supported by numerical computations, generally based on Morison approaches, boundary element methods (BEM), computational fluid dynamics (CFD) or smooth particle hydrodynamics (SPH) models. Frequency-domain models, based on linearized potential flow theories, are often used in a proof-of-concept, hypothesized during the design phase of the floaters and under the assumptions of negligible fluid viscosity and nonlinear effects This approach, largely adopted by marine engineering, has been successfully used to simulate many interaction problems between non-breaking waves and floating bodies with dimensions much smaller than the wave length; among the others, the recent studies by [10,11,12,13,14] have been reviewed, together with the development of commercial codes, such OrcaFlex [15] and ANSYS Aqwa [16]. Some conclusions close the paper, highlighting the implemented modelling approach as a simple and useful tool to support preliminary studies to design floating bodies and understand the response of a moored floating cylinder under waves
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