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The numerical predictions of a cavitating model scale propeller working in uniform and oblique flow conditions are presented. The cavitating phenomena are numerically reproduced using a homogeneous (mixture) model where three previously calibrated mass transfer models are alternatively used to model the mass transfer rate. 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 available experimental data. For a quantitative comparison the propeller thrust is considered, while for a qualitative comparison, snapshots of cavitation patterns are shown. The thrust values obtained with the three different mass transfer models are very close to each other, however differences in the predicted cavitation patterns are observed. Moreover, some discrepancies between the numerical results and experimental data are reported.

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.

A novel single-degree-of-freedom multi-frequency model (sdof-mf) for the prediction of the Vortex Induced Vibrations (VIV) of an elastically mounted circular cylinder in two-dimensional cross flow is presented. The proposed model treats the total hydrodynamic force as sum of conventional Morison-like inertia and drag terms related to the cylinder motion in still fluid plus additional harmonics that account for the lift force induced by vortex shedding. Amplitudes, frequencies and phase lags of these harmonics are identified using a Parameter Identification (PI) procedure applied to time domain data of vortex induced forces, here obtained via CFD simulations. The proposed sharp identification via PI of the independent frequencies of the vortex shedding fluid force is the peculiarity of the proposed method. The model is assessed considering a wide range of flow regimes, including lock-in conditions. From the overall results, the proposed sdof-mf model exhibits promising but consistent capabilities in the reproduction of the vortex shedding forces and cylinder motion, in terms of both amplitudes and frequencies.

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.

Pursuing the industrial goals of the project SOPHYA (Seakeeping Of Planing Hull YAchts, POR FESR 2014 2020 - 1.3.b Friuli Venezia Giulia Region), in particular those aimed at reducing the time-to-results of open source CFD simulations of planing hulls, a fully automated pre-processing procedure has been developed and tested. The procedure is based on open-source tools available within the OpenFOAM library. The pre-processing time is drastically reduced making the procedure fully-automated and user friendly, still making it robust and accurate. The novelty of the mesh generation procedure stands in the selection of the best mesh among a family of grids that share the same background octree-based mesh and differ from each other for the boundary-layer cells. This procedure allows to obtain high quality meshes with reduced number of cells that in turn allows a further reduction of the time-to-result through reduction of the CPU time. In addition to the mesh generation, the developed procedure provides a ready-to-run case folder for planing hull simulations. The proposed procedure allows a marked reduction of input parameters and file edits, along with better mesh quality and reduced risk of user error. The automated procedure is fully customizable and ultimately allows users to significantly reduce both pre-processing and computation time, a key aspect in CFD investigations at the design stage.

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.