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This study presents an integrated analysis of the seakeeping data collected during full-scale trials of a medium-size planing pleasure-boat in mild weather conditions. The present work is part of a more general project whose aim is to investigate the correlation between the sea state and the performance of a planing hull in terms of seakindliness, safety and powering. Full scale heave and pitch transfer functions, derived from the proposed general integrated analysis, are compared to results obtained from model scale tests in regular head waves. A focal point in the consistency of the results is found in the influence of the quality of the reconstructed directional wave spectrum derived from a standard directional wave buoy, in particular for the mild weather conditions here assumed as target for standard end-users of the boat.

The numerical predictions of a model scale propeller working in uniform and oblique flow are presented. Both non-cavitating and cavitating flow conditions are numerically investigated using homogeneous (mixture) model. Two previously calibrated mass transfer models are alternatively used to model the mass transfer rate due to cavitation. The turbulence effect is modelled using the Reynolds Averaged Navier Stokes (RANS) approach. The simulations are performed using an open source solver. The numerical results are compared with the available experimental data. For a quantitative comparison the propeller thrust is considered, while for a qualitative comparison, snapshots of cavitation patterns are shown. From the overall results it seems that, with the current simulation approach it is possible to predict with a reasonable accuracy the propeller performances. Nevertheless, for detailed reproduction of complex cavitation phenomena, such as bubble cavitation for instance, a more sophisticated modelling approach is probably required.

Optimization of modern hulls when moving in a seaway puts new demands on the computational methods used. Nonlinear effects become important for wave loads and added resistance in waves in presence of large motions. The purpose of this paper is to present a method which aims to fill the gap between RANSE methods and partly nonlinear panel methods. The method solves the fully nonlinear free surface time-domain potential flow problem including a hull undergoing rigid body motions. Nonlinearities under the hypothesis of potential flow are taken into account, i.e. higher and lower frequency components, hull shape above calm water line and interaction between incoming, radiated, diffracted, reflected and ship generated waves. The potential flow method alone cannot handle roll motion since roll is dominated by viscous effects. Two methods to include roll damping within the potential flow code are used: the first one obtains roll damping coefficients through inertial and geometric characteristics of the ship. The second one uses model test results. Numerical results using both methods are compared. The code has already been tested in head seas. In this paper, numerical simulations of roll decay and roll motion in beam sea are compared to model test results.

An extensive campaign of model and full scale experimental tests and numerical computations is being undertaken within the framework of the Project “SOPHYA - Seakeeping Of Planing Hull YAchts”, co-financed by Friuli-Venezia Giulia Region in the field of joint industrial and academic research. The project is aimed at the prediction and optimization of the hydrodynamic performances of a 65 ft motoryacht by MonteCarlo Yachts SpA in calm water and in waves. In this paper, selected results of the numerical computations for the calm water case, conducted jointly by HyMOLab-University of Trieste and by the University of Zagreb, are presented. RANS, Volume-of-Fluid and Level Set simulations are carried out in combination with an automatic mesh-generation tool, developed in this project within the OpenFOAM/foam–extend frameworks. The numerical results are compared with experimental data obtained within the project at the towing tank of the University of Naples.

In ocean and offshore engineering, Vortex and Wake Induced Vibrations (VIV and WIV respectively) are serious issues related to the design and operational safety of offshore installations/structures. VIV occur when vortices shed by a blunt freely-moving structure in steady (or unsteady) flow induce an oscillatory force on the structure, mostly in the direction perpendicular to the ambient flow. WIV take place when the oscillatory wake shed on the leeside of a structures hits a secondary element of the structure, inducing an oscillatory force on the latter. VIV of a single elastically-mounted 2D cylinder has already been investigated by the authors and here used as reference case. In this work, the crossflow motion of two elastically-mounted 2D cylinders in tandem arrangement is investigated via computationally efficient CFD URANS- based simulations. Two relevant cases are presented, the first one in which the upwind cylinder is fixed and the downwind cylinder is free to move and the second one where both are free to move. Considering the strong complexity of the phenomena involved in VIV and WIV, considering also the relatively low computational effort (2D-RANS) adopted in this first step investigation, the results are in good agreement with experimental data, allowing a close insight in the coupling between wake and cylinder motion and providing a robust base for future more sophisticated simulations.

DHI (Danish Hydraulic Institute) and HyMOLab (Hydrodynamics and Met-Ocean Laboratory of the Dept. of Engineering and Architecture of the University of Trieste) have undertaken a joint applied research project with the aim to develop a state-of-art wind-wave forecast service at mid resolution for the Mediterranean Sea and at very high resolution for the Adriatic Sea. Weather routing, civil protection, coastal engineering, oil&gas and renewable energy fields, the planning of operations at sea, ... are just few among the multiple potential applications of this service. The meteorological model used in this study is WRF-ARW, one of the most widely used state-of-the-art open-source non-hydrostatic model. Global Forecast System (GFS) dataset provides the boundary and initial conditions. MIKE21-Spectral Waves is used as wave model with resolution ranging from 0.1 to 0.03 approximately. The use of a local area meteorological model guarantees higher levels of resolution and accuracy in an area such as the Mediterranean Sea where the complex orography and coastline induce short-time/small-space weather scales. The model chain runs daily (or twice a day on demand) on the High Performance Computing (HPC) infrastructure of HyMOLab. The validation of the entire model chain and specifically the forecast data obtained for the sea state is continuously updated according to new available data from satellites and buoys. Anyway, a major verification of the performance of the model chain against historic data (hindcast) is almost mandatory. For this aim, we performed a multi-decade test obtaining very good statistical parameters for the entire model chain performance. In this context the hindcast dataset developed by DHI and HyMOLab consists of 35 years of hourly data for the period 1979-2013, with the same model chain. The CFSR d093.0 hourly dataset with a spatial resolution of 0.5 provides the boundary and initial conditions. The atmospheric and wave models performance is checked against six satellite datasets, missions Envisat, ERS-2, Geosat FO, Jason-1, Jason-2, Topex-Poseidon, using a moving window technique procedure. Wave data close to coast are compared with available data from more than 20 buoys. The paper describes the validation procedure adopted for the hindcasted data. Furthermore the forecast service is described too, with specific emphasis to the very high resolution adopted in the Adriatic Sea.

The modelling of large individual waves for the computation of loads on ships and offshore structures in extreme weather conditions is still a challenging problem. Since the early 50s the predictions of loads on fixed offshore structures and motions of compliant or sailing structures due to surface waves are commonly made by computations on the basis of the statistical/spectral description of the sea elevation and of a linearized response model. Quadratic Transfer Functions or fully non-linear methods are used only in specific cases. The linear approach is recognized to work reasonably well for the so-called operational conditions, assuming that hydrodynamic and dynamic nonlinear effects can be neglected. On the other hand, it is also recognized that the modelling of large amplitude motions and the modelling of waves in the so-called survival conditions, i.e. extreme wave conditions, cannot recast a linear approach. In these conditions the wave-wave interaction plays a fundamental role (energy transfer, down-shift, etc) in the actual deterministic or spectral representation of the wave/flow field and thus in the related loads on the structure. In the present paper the nonlinear aspects related to the behavior of steep focusing breaking and non- breaking waves are analyzed by means of numerical simulations and new experiments. The experiments are carried out at the wave flume of the Laboratory of Maritime Engineering (LABIMA) of the Dept. of Civil and Environmental Engineering of the University of Florence. The computations are carried out at Hydrodynamic and MetOcean Laboratory (HyMOLab) of the Dept. of Engineering and Architecture of the University of Trieste. The paper focuses the attention on the comparison between the results obtained with a state-of-art viscous flow simulation and laboratory experiments, with particular emphasis on the spectral energy exchange between component waves of a non-breaking and breaking focusing wave train. This study is carried out as part of the research project “OpenViewSHIP Development of an integrated computational ecosystem for the hydrodynamic design of the hull-propeller system”, co-financed by Friuli Venezia Giulia Region in the field of industrial application of open-source CFD and High Performance Computing.

The Harmonic Balance method for incompressible, turbulent periodic flows is presented in this paper. Using the Fourier series as a decomposition method for primitive variables, the transformation from a single transient problem to a number of coupled steady state snapshots is achieved. Depending on the number of harmonics in the Fourier series, n, a set of 2n + 1 coupled equations is assembled, yielding the solution for the whole representative period rather than a single time instant. The method is implemented in the Finite Volume framework within the open source software OpenFOAM and designed for turbomachinery applications. Two turbomachinery test cases are presented in this paper, a KP505 propeller in open water and ERCOFTAC centrifugal pump. Additionally, a CPU time comparison will be presented, demonstrating the speed-up achieved by using Harmonic Balance.

This work is currently being carried out at the Met-Ocean Laboratory (MOLab) of the Dept. of Engineering and Architecture of the University of Trieste [1] and at Danish Hydraulic Institute-Italia (DHI) with the aim to develop and validate a state-of-the-art model chain for medium resolution (0.10° approximately) wind and wave hindcast simulations over the Mediterranean and Black Seas, consisting of 30 years long hourly data, ready to be used for engineering and environmental applications or even to be used as input for local nested higher resolution simulations (of order of few kilometers or less). The meteorological model used is WRF-ARW [2,3,4], (one of) the most widely used state-of-the-art open-source non-hydrostatic model. The CFSR d093.0 dataset [5,6], hourly data with a spatial resolution of 0.5°, is used as boundary and initial conditions. WWIII [7] and MIKE21 [8] are used as wave models with resolution ranging from 0.1° to 0.03° approximately. The paper will show the results of the preliminary validation tests, both wind and wave, carried out over an entire year, comparing the data of several field stations.

This paper presents the work on a fully nonlinear body-exact unsteady three-dimensional boundary element method (BEM) for free-surface flows with forward speed based on potential flow theory. The BEM is coupled with free-surface markers updated in a semi-Lagrangian way. Time-dependent nonlinear boundary conditions are applied on the instantaneous position of the free-surface and the wetted surface of the body and the boundary value problem (BVP) is solved at each time step in an earth-fixed reference frame, thus using a moving grid technique. A fourth-order Runge-Kutta scheme is used for the time-integration of the free-surface boundary conditions controlling the free-surface markers. The ongoing development of this method is done with the outlook of computing ship motions although this paper only discusses captive hull computations with forward speed in calm water for the purpose of validating the forward speed effects in a controlled manner, thus keeping the same mathematical model and approach developed for the wave-body interaction.

In this work a new optimization procedure aimed at the fore body and bulb shape optimization for the best wave resistance performance of a hull in calm water is presented. A parametric CAD and a CFD solver are linked and driven by an optimization procedure that generates hull shapes and runs a wave resistance code automatically, comparing the results obtained. A special care has been given to the algorithm used for the parametric generation of the fore body hull shape, especially at the bulb tunnel. The test case here presented is related to a cruise ship from Fincantieri Shipyard.

The use of the CFD in the America's Cup campaigns represents one of the major innovations introduced in the last years in order to help the designers in analysing the yachts performances. The high cost of the software is one of the drawbacks that more affect this kind of analysis aside the validation of the numerical results. Recently the development of reliable CFD packages in the ambit of open source projects made these investigations affordable to the low budget companies brave enough to invest their resources in less friendly but more powerful frameworks. This work is aimed at the comparison between the experimental measures performed at the Chalmers University of Technology and the numerical results obtained using the open source viscous solver OpenFOAM® for an America's cup yacht keel. The yacht keel was investigated in two configurations: one with and one without the two winglets. A grid dependence study was made in either of the two configurations in order to get a computational grid well suited for the adopted turbulences models, which are the k-w SST and k-e.

Last Louis Vuitton Cup and XXXI America’s Cup revealed that while the hull shapes were strongly similar for all the top teams’ boats, the bulb shapes were quite different, even if boat performances were very close one to each other. XXXII America’s Cup will adopt Version 5 of AC Rule, which is more restrictive than Version 4 as far as the hull shape, leaving the same degree of freedom to the design of appendages, so the bulb shape optimization will become an even more significant task in the development of a winning yacht. Currently CFD gives the way to lower the number of real life trial and error experiments. It is used to study the flow field of different geometries, giving results on the best one. What if we want to get the optimized solutions? In some cases, this is straight to obtain, we only need a few (less than 10) iterations. However, when the system gets more complex, the use of optimization software is required. The use of modern Computational Fluid Dynamics (CFD) codes coupled with automatic process integration and optimization tools allow us to explore the design space by evaluating a large number of design variations in a fast and smart way. It is important to consider not only the drag forces, but also the motions in waves and the related total mean resistance, considering that the location has changed and the weather/sea conditions are different from the Hauraki Gulf. In this work seakeeping tests of an IACC fully appended scale model have been carried out at the towing tank of the University of Trieste. The model has been tested in head waves, with wave lengths that match the encounter frequencies at Valencia site in upwind sailing conditions, calculated with an heading angle of 150 degrees. Three bulb shapes of different length have been designed using an automatic optimization procedure that, for each length chosen, minimizes the drag in calm water of the bulb and gives the lowest VCG. These bulbs have been used during the seakeeping tests to check the influence of their shape, mostly the length and related mass and added inertia, on the performance of the vertical motions of the yacht in waves.

In this paper we investigate the so-called harbour oscillations problem by means of efficient time-domain high-resolution numerical simulations. The method proposed here is able to handle an arbitrary geometry of the docks/piers and variable water depth, so that reflection, diffraction and refraction effects are taken into account simultaneously. Partial absorption from selected walls/protections can also be introduced and tuned locally. A very efficient technique for the absorption of outgoing waves is also implemented. Due to the specific parallel implementation of the code according to the Message Passing Interface (MPI), the numerical scheme is able to handle quite large physical domains with extremely good computing performances (e.g. 600m x 600m on a 20 CPU’s cluster with 40 Gb of RAM) with high resolution grids the allow the analysis with cells size of approximately 1mx1m in full scale. Some “academic” test cases are presented first, comparing the results to analytical and experimental data available in the literature. As a practical case study, the method has been applied to the harbour of Trieste (Italy-North Adriatic Sea) with actual sea spectra from North-West to South-West.

The application, tuning and optimisation of a third generation SWAN wave model to the Italian Seas is here analysed in terms of statistical and deterministic properties. Specifically we focus on the capabilities of the model to reproduce experimental data from wave buoys with the main aim to quantify the confidence limits of the forecasts when used for operational purposes at sea. The model was run to simulate a continuous 12 months period (November 2004 – October 2005), forced with wind fields derived from an 0.25 deg resolution ETA model over the entire Mediterranean Sea. The numerical results of significant wave height, peak period and mean wave direction are compared with the data available from 13 wave buoys of Rete Ondametrica Nazionale – RON (Italy), both in a statistical (Wave Scatter Diagrams) and deterministic (time domain) analysis.

This paper summarizes the results of experimental tests conducted at the Danish Hydraulic Institute (DHI) in the frame of the EU-IHP-ARI project “Wave Forces on an Array of Circular Cylinders: Experimental Investigation on the Higher Order Near Trapping”. The aim of this research project was to study in detail the so-called “near trapping”, specifically the second order near trapping. This phenomenon regards diffraction effects on TLP like structures in waves or, in general, on arrays of vertical cylinders that encircle a portion of free surface. The result of the wave-structure interaction is a rather large amplification of the oscillations of the free surface in the inner domain. These magnifications are particularly large at the fluid-body interface with associated large induced pressures; they show their maximum effect for specific ratios of the diffraction parameter ka and of the column radius a to the distance between the column axis d. Second order near trapping is a double frequency phenomenon that is expected to occur when double frequency component waves exhibit a wavelength comparable to that of linear waves giving first order near trapping. The paper describes the set-up of the experiments, the tests conducted and finally presents the most meaningful results obtained.

It is well known that the periods of the natural modes of the inner free surface oscillations of a harbour are determined thoroughly by its plan shape. The amplitude of the induced standing or partially standing waves can be large when the outer sea spectrum shows non negligible energy at those frequencies. Depending on the relative size, the motions of the vessels in the harbour, ships or pleasure boats, can be large too, in particular roll, surge and yaw, so that up-downloading operations must be interrupted or small vessels must abandon their mooring for safety reasons. Thus the design of harbour or a marina takes on a great importance from an economic and safety point of view. In this paper we investigate the so-called harbour oscillations induced by offshore waves by means of a time domain approach. The method proposed here is able to handle an arbitrarily shaped plan view and variable water depth, so that reflection, diffraction and refraction effects can be taken into account. The mathematical model and the numerical scheme employed are presented in synthesis. The preliminary test cases regard 2 harbours with simple rectangular plan shape. Afterwards the method is applied to the harbour of Trieste (Italy-North Adriatic Sea) with regular waves coming from 270° to 330°. The effects on the wave pattern induced by the lengthening of a large pier used to berth modern cruise ships are investigated too.

This paper summarises the main results of experimental tests conducted at the Danish Hydraulic Institute in the frame of the EU-TMR-LSF project “A New Concept of Catenary Mooring System for FPSO”. The main interest of this research project is the analysis of the behaviour of a proposed mooring system for FPSO well suited for weather conditions characterised by a dominant swell and current and by a moderate almost perpendicular local sea and wind. The basic idea of this mooring design is to join the best characteristics of the turret and spread mooring systems to allow the FPSO to move within a specified offset so that the wave loads on the lines and on the vessel can be minimised. The experiments and the related analysis refer to the dynamic behaviour of the FPSO alone and FPSO+Shuttle tanker during offloading, in extreme and operational weather conditions respectively. In the case with the FPSO+Shuttle tanker the tension on the hawser is analysed too.

This paper shows some numerical results obtained at DINMA on the fully nonlinear interaction of regular waves with four bottom-mounted circular cylinders [D’Este et al. 2001]. The phenomenon of interest is the so-called near trapping, i.e. the large magnification of the amplitude of the free surface elevation and induced pressures in the zone delimited by the cylinders surface. Linear theory [Linton & Evans, 1990] and experiments [Contento et al. 2000] show that this amplification depends strongly on the ratios of the column radius to the incident wavelength (ka parameter) and of the column radius to the side of the square (a/d parameter). In this specific case we analyse the ka condition that leads to extremely large oscillations of the second Fourier components of the free surface elevation in the inner zone and of the related pressure [Malenica et al., 1999] even in small amplitude incident waves. In particular the interest is focused on the behaviour of the higher order terms of the forces and run-up for two different configurations of the wave tank while keeping the same hydrodynamic parameters (ka, a/d). From the preliminary results shown in the present work, it seems that first order in-line terms are only weakly affected by the dimensions of the basin whereas the first order side force on the lateral cylinder shows evident interference effects. At the same time, the amplitude of the second order terms of the forces show a smoother variation with time for the larger basin.

The design of a harbour or a marina is primarily aimed to a safe and comfortable anchorage and to easy and efficient cargo-passengers operations [1-2]. Prevailing weather conditions and their extremes at the site determine thoroughly the design of the plan view and the height of the breakwaters. Nevertheless, even in not extreme conditions, the geometry of the harbour can affect directly the onset and grow of standing or partially standing waves inside it, inducing large rolling, surge and yaw of the moored vessels with exceeding stresses on the mooring lines and causing the interruption of up-downloading operations. In this paper we investigate the free surface oscillations in an arbitrary shaped harbour by means of a Time Domain Simulation tool developed at DINMA [3-4] in the frame of the so-called Numerical Wave Tank approach. The method proposed here is able to handle the bottom bathymetry, so that the important refraction effects can be taken into account. The capability of the method is first checked in the case of 2 harbours with simple rectangular plan shape. Afterwards the method is applied to the harbour of Trieste (Italy-North Adriatic Sea) with regular waves coming from Southwest in open sea and from West at the harbour opening. In particular the study is focussed on the magnification of the wave amplitude due to the change of the main dimension of a large pier planned to berth cruise ships.

This paper summarises the main results of experimental tests conducted at the Danish Hydraulic Institute in the frame of the EU-TMR-LSF project “A New Concept of Catenary Mooring System for FPSO”. The main interest of this research project is the analysis of the behaviour of a proposed mooring system for FPSO well suited for weather conditions characterised by a dominant swell and current and by a moderate almost perpendicular local sea and wind. The basic idea of this mooring design is to join the best characteristics of the turret and spread mooring systems to allow the FPSO to move within a specified offset so that the wave loads on the lines and on the vessel can be minimised. The experiments and the related analysis refer to the dynamic behaviour of the FPSO alone and FPSO+Shuttle tanker during offloading, in extreme and operational weather conditions respectively. In the case with the FPSO+Shuttle tanker the tension on the hawser is analysed too.

The paper will present the results obtained from the experimental tests, performed in the towing tank of the University of Trieste, on two fast fishing vessels models, having the same main dimensions, ratios and coefficients, but different body plan. The hulls were designed by using hard-chine type and round bilge sections respectively. The hull performances were compared as regards the towing and wave pattern resistance, the wave generation, the transversal stability and the rolling motions in regular beam waves. The experience confirmed that the hard chine sections reduce the rolling motion but slightly increase the wave and towing resistance, as reported also by other who analysed separately the two aspects. In this case, simultaneous comparisons of the different phenomena allow a better evaluation of the two hulls performances.

In this paper the effect of the excitation modelling on the fitting capability of the nonlinear roll motion equation to experimental data is studied. Several frequency dependent and constant effective wave slope coefficients are derived for five different scale models corresponding to different ship typologies by a Parameter Identification Technique. The frequency domain behaviour of the obtained coefficients is discussed and compared with linear diffraction (strip theory) results and I.M.O. suggested values. It appears that a mathematical modelling with constant damping parameters and frequency dependent excitation could give very good results. As regards the excitation parameters, a common trend for slender bodies is evidenced.

In this paper the effect of the excitation modeling on the fitting capability of the nonlinear roll motion equation to experimental data is studied. Several frequency dependent and constant effective wave slope coefficients are derived for five different scale models corresponding to different ship typologies by a Parameter Identification Technique. The frequency domain behavior of the obtained coefficients is discussed and compared with linear diffraction (strip theory) results and I.M.O. suggested values. It appears that a mathematical modeling with constant damping parameters and frequency dependent excitation could give very good results. As regards the excitation parameters, a common trend for slender bodies is evidenced.

Lo studio accurato della maggior parte dei fenomeni connessi con l’interazione fluido-struttura in presenza di onde gravitazionali di superficie libera richiede come background indispensabile la conoscenza perlomeno dei parametri fondamentali della causa prima. Infatti, sia che lo studio sia rivolto alla determinazione di quantità integrali della interazione (moti, forze, parametri concentrati, ...) sia che esso sia rivolto ad una valutazione puntuale della interazione su grandezze primitive (pressioni, velocità, elevazione di superficie libera, ...), è indispensabile conoscere le caratteristiche della forzante del sistema e l’incertezza relativa. Nel caso specifico delle onde regolari queste caratteristiche sono rappresentate o dalla cinematica d’onda indisturbata (campo di velocità associato ad un profilo d’onda) oppure, quando questo non è possibile come nella maggioranza dei casi, sono rappresentate da alcuni elementi chiave (periodo, altezza, profondità dle fondale) con i quali risalire alla cinematica d’onda attraverso consolidati modelli analitici (Airy, Stokes, Cnoidal, ...) o numerici (“vasca numerica”). Sulla base di questi dati, la correlazione tra le grandezze di interesse in uscita dalla interazione fluido-struttura e le grandezze in entrata trova una maggior consistenza scientifica. In questo lavoro vengono presentati i risultati più significativi di una vasta serie di misure relative alle onde generabili presso il Laboratorio Idrodinamico del DINMA. In particolare verrà indicato il processo che è stato adottato per la valutazione delle caratteristiche fondamentali delle onde, corredate di analisi di incertezza. Lo studio ha portato alla “mappatura” delle caratteristiche del sistema ondogeno-onde generate ed alla identificazione del modello matematico più adatto alla rappresentazione delle onde stesse.