Marilena Greco - Academia.edu (original) (raw)

Papers by Marilena Greco

The use of composite structures in offshore engineering has rev mped the problem of Fluid Structu... more The use of composite structures in offshore engineering has rev mped the problem of Fluid Structure Interaction (FSI) during underwater blasts because they enhance the FSI effec t and increase the shock resistance of underwater structure s. The practical applications of these materials range from wa rfare to offshore infrastructures and to the deep sea fuel tr ansfer installations ([1],[2]). Nonetheless, the techniques tha t are usually applied to study the FSI rarely make use of the tr u coupling between the fluid dynamic problem and the structura l one. Most frequently, the hydrodynamic-pressure approximatio n proposed by Taylor [3] for plates interacting with explosion waves is included in the structural response ([2], [ 4]). This takes into account the reflected wave pressure and the damping effect due to the absorption of energy from the st ructure, but it is still a simplified approach. Only for air explosions, the complete coupling is actually modelled ([5 ],[6]) and only in ...

Volume 7A: Ocean Engineering, 2017

Parametric resonance (PR) in roll is of concern for fishing vessels, especially in head-sea waves... more Parametric resonance (PR) in roll is of concern for fishing vessels, especially in head-sea waves. Here the effect of passive anti-roll free-surface tank is investigated experimentally and numerically on realistic fishing-vessel geometry at zero forward speed. On the numerical side, the onboard tank is simulated using an open source computational fluid dynamic (CFD) development platform, OpenFOAM (Open Field Operation and Manipulation). The internal flow solver is coupled with the seakeeping solver, which is based on the weakly nonlinear method proposed in [1]. Experimentally, two different relevant scenarios were examined: 1) a 2D rectangular tank with shallow-water filling depth was forced to oscillate in roll and the loads induced on the tank were measured; 2) a fishing vessel was tested in a towing tank, prescribing regular head-sea waves. The vessel was examined both without and with anti-roll tanks and a mooring-line system was designed so to control the horizontal motions wit...

Ocean Engineering, 2021

A fully nonlinear two-dimensional numerical method based on potential-flow theory for water waves... more A fully nonlinear two-dimensional numerical method based on potential-flow theory for water waves and their interaction with surface-piercing rigid bodies is presented. The harmonic polynomial cell (HPC) method is used to solve the Laplace equation for the velocity potential and its time derivative. The HPC method, which is a high-order method using analytical expressions (harmonic polynomials) to represent the solution inside overlapping cells, has previously been shown accurate and efficient. Supplementary research has shown that, in order to maximally benefit from the method's accuracy, it is a requirement that square or close-to-square cells are used. Here, we use an immersed boundary method to model non-stationary boundaries such as the free surface or the surface of a rigid body, and overlapping, body-fixed grids that are locally Cartesian to refine the solution near moving bodies. Combining these two modelling concepts with the HPC method represents the main novel contribution in the present work. With this combined method, denoted as an immersed-boundary overlapping grid method (IBOGM), the challenge of generating boundary-fitted grids for complex boundaries is avoided. Moreover, square cells can be used throughout the domain and the solution can be refined locally without increasing the number of unknowns unnecessarily. The method is systematically validated and verified against analytical, experimental and numerical reference results. The cases studied include propagation of steep waves, forced heave motions of a semi-submerged circular cylinder and a fixed and freely floating ship section in beam-sea waves. For the freely floating ship section, the present method is compared in detail with results from a dedicated study performed with a fully nonlinear boundary element method for cases with roll motions up to 30 •. The present results are generally in good agreement with reference results, even for the most challenging wave-body interaction cases considered. Based on this, we later intend to use the method to examine in depth the importance of nonlinear effects in the interaction between waves and rigid bodies.

Journal of Ship Research, 2003

The water on deck caused on a restrained ship model without forward speed in head waves is studie... more The water on deck caused on a restrained ship model without forward speed in head waves is studied experimentally by using a transient test technique. A single water-shipping event is induced by the wave packet, and the severity of the interaction is controlled by the wave-packet steepness. Three different bow geometries are considered. Two of them are analytical hull forms, and the last is the ESSO-Osaka tanker. The models are equipped with a transparent-material deck to study the flow-field evolution by image analysis. A vertical wall is placed at a certain distance from the forward perpendicular to mimic the presence of deck structures. Velocity of the shipped water along the deck, pressure field on the deck, and horizontal impact force on the wall are measured. The main fluid-dynamic aspects of the green-water phenomenon are highlighted. For the tested cases, water shipping starts always with the free surface exceeding the freeboard, plunging onto the deck, and forming complex c...

Marine Structures, 2019

Water impact (slamming) is a strongly nonlinear phenomenon including significant fluid structure ... more Water impact (slamming) is a strongly nonlinear phenomenon including significant fluid structure interactions. In the case of slamming with a small impact angle between the structure and water, the coupling between hydrodynamic pressure and the elastic responses of structures, known as hydroelasticity, matters. This has been studied extensively. However, when structures are subjected to violent water slamming in extreme sea states, large stresses may occur that exceed the material yield stress, causing large plastic flow and permanent damage. In such cases, the plastic responses of a structure will be strongly coupled with the hydrodynamic pressure, termed as hydro-plasticity. Hydro-plastic slamming has rarely been studied before. This is Part I of a two-part companion paper. The paper advances the state-of-the-art of hydroplastic slamming by formulating, for the first time, an analytical model coupling the hydrodynamic forces and the plastic response of rectangular beams and one-way stiffened panels. The studied scenarios are flat or nearly flat water impacts, which are critical for hydro-plasticity excitation. The impact angle between the water free surface and the structure should preferably be no larger than 5°. Based on the proposed model, the governing non-dimensional parameters for hydro-plastic slamming are identified and discussed. Design curves for plate strips and stiffened panels against extreme slamming are developed. Part II-Numerical verification and analysis presents numerical verification and discussion of the analytical model by comparing with results from the multi-material Arbitrary Lagrangian Eulerian (ALE) simulations. The proposed analytical model does not require the challenging estimation of pressure history that is normally used in the design against slamming. Only the initial impact velocity is needed as the main input. The resulting non-dimensional curves may be utilized in rules and standards for the design of ships and offshore structures against extreme slamming loads.

Journal of Fluids and Structures, 2019

The dynamic response of a coupled well boat-fish farm system in irregular long-crested waves and ... more The dynamic response of a coupled well boat-fish farm system in irregular long-crested waves and current is analyzed numerically in the time domain. The main purpose is to investigate the influence of the well boat on the fish farm and then to determine the operational conditions of the well boat. The numerical study of slow-drift sway motion of the well boat is performed at first. Hydrodynamic and statistical theories are briefly introduced. The cross-flow principle is assumed valid for evaluating the transverse viscous loads and the needed cross-sectional drag coefficients are estimated empirically and validated against available experiments. The mean value and standard deviation of the slow-drift motion from time domain agree well with those from frequency domain when equivalent linearized drag damping is incorporated. The coupled system with the well boat placed at the weather side of the fish farm is then analyzed in detail. Special attention is paid to two critical response variables, i.e., maximum anchor-line loads and maximum floating collar stresses. Numerical results show that the examined two variables will increase more than 300% due to the well boat in moderate exposure sea states. A sensitivity analysis is also carried out to identify the important parameters influencing these two response variables. Cross-sectional drag coefficients for the well boat and fish-farm related parameters (pretension load in the anchor lines and anchor-line stiffness) have moderate influence on the two variables. Simplifying the modeling of the coupled system, for instance neglecting the net cage and the first-order motion, has more effect on the maximum anchor load than on the maximum floating-collar stress and reduced sensitivity is observed in current, especially for the latter variable. Lastly, the operational conditions of the well boat are determined through systematic simulations. Numerical results show that the maximum loads in the mooring lines are moderate compared with the corresponding breaking limits even in high exposure sea states, while for the maximum stress in the floating collar can be close to the yield stress when operating in moderate exposure regions.

Journal of Fluids and Structures, 2018

A realistic aquaculture fish farm system in both regular and irregular waves is investigated by n... more A realistic aquaculture fish farm system in both regular and irregular waves is investigated by numerical simulations and model tests. The main purpose is to develop a reliable numerical tool and in this respect to investigate the survival conditions of the system. The structural and hydrodynamic modelings of the system are briefly introduced. Numerical sensitivity analysis is performed to investigate which physical parameters are dominant when modeling the system. The considered fish farm comprises a floating collar with two concentric tubes, a flexible net cage including a cylindrical part and a conical part with a center point weight at the bottom, and a sinker tube attached directly to the net. The system is moored with a complex mooring system with bridle lines, frame lines and anchor lines, supported by buoys. The mooring loads in the front two anchor lines and bridle lines are investigated in detail. Numerical results are first validated by the experimental data. Both numerical and experimental results show that one of the bridle lines experiences larger load than the rest of the mooring lines, which is surprising. Then a sensitivity analysis is carried out. The mooring loads are not sensitive to the majority of the parameters. The flow reduction factor in the rear part of the net is the most important parameter for the anchor loads. Modeling the floating collar as a rigid body has a small effect on the anchor loads but not for the bridle lines as it will alter the force distribution between bridles. The mooring loads are not sensitive to the wave load model for the floating collar in both regular and irregular seas and modeling the floating collar as elastic with zero frequency hydrodynamic coefficients is enough to give reliable results. Finally, the survival conditions of the fish farms with different setups is studied. Numerical results indicate that the dominant limitation to move the conventional fish farms to more exposed sea regions is the large volume reduction of the net cage. The existing mooring system can be applied in offshore regions as long as the bridle lines are properly designed. The maximum stress in the floating collar is moderate compared with the yield stress.

Physics of Fluids, 2017

The present work represents the research continuation on the semi-integrated method proposed by A... more The present work represents the research continuation on the semi-integrated method proposed by Antuono and Brocchini 1 , this being composed by a subset of depthaveraged equations (like the popular Boussinesq-like models) and by a Poisson equation that accounts for the vertical dynamics. On the theoretical side, the subset of depth-averaged equations has been reshaped in a conservative-like form with inherent advantages when applied at the discrete level. On the numerical side, the Poisson equation has been inspected in both formulations proposed in Antuono and Brocchini 1 : a Poisson equation for the vertical component of the velocity (formulation A) and a Poisson equation for a specific depth semi-averaged variable, Υ (formulation B). The studies showed that formulation A is prone to instabilities as the problem nonlinearities increase. On the contrary, formulation B allows for an accurate and robust numerical implementation. Some relevant test cases derived from the scientific literature on Boussinesq-type models-i.e. solitary and Stokes wave analytical solutions for linear dispersion and nonlinear evolution and experimental data for shoaling properties-have been used to assess the proposed solution strategy and to highlight its features and characteristics. The method proved to predict reliable results for wave solutions in shallow to intermediate waters, both in terms of semi-averaged variables and conservation properties.

Applied Ocean Research, 2015

An inconvenience in the experimental setup of a FPSO in regular waves highlighted occurrence of p... more An inconvenience in the experimental setup of a FPSO in regular waves highlighted occurrence of parametric-roll events promoted by yaw-roll coupling and motivated a combined physical and numerical analysis on the relevance of this phenomenon on the roll resonance, as well as on the water shipping. The model tests examine the ship in head-and bow-sea waves in the zone of the first parametric resonance. Numerically, it is adopted a 3D Domain-Decomposition (DD) strategy combining a weakly-nonlinear potential-flow solver based on the weak-scatterer theory with a shallowwater approximation for the shipped liquid and with a bottom-slamming solution. Detailed comparisons against these and other seakeeping experiments validated the numerical method in its different aspects with global success. At first, a 2-dof equivalent linearized yaw-roll coupled system is examined and the measurements are used to estimate hydrodynamic coefficients required to complete the mathematical model of the problem. Then the DD method is applied to verify the instability occurrence and compared against the experiments. From the analysis, the parametric-roll instability does not occur if all nonlinearities in the roll restoring load are not accounted for. However the amplitude of the resonant roll is affected by the coupling with the other degrees of freedom. Especially the coupling with yaw tends to increase the steady-state roll amplitude. It also affects the water shipping with the trend in reducing its severity for the vessel, this is opposite to the influence of the parametric roll in head-sea waves on the water on deck, as documented in Greco et al. (2014).

Volume 7: Ocean Space Utilization; Professor Emeritus J. Randolph Paulling Honoring Symposium on Ocean Technology, 2014

Numerical simulations and experiments of an elastic circular collar of a floating fish farm are r... more Numerical simulations and experiments of an elastic circular collar of a floating fish farm are reported. The floater model without netting structure is moored with nearly horizontal moorings and tested in regular deep-water waves of different steepnesses and periods without current. Local overtopping of waves were observed in steep waves. The focus here is on the vertical accelerations along the floater in the different conditions. The experiments show that higher-order harmonics of the accelerations matter. A 3D weak-scatter model with partly nonlinear effects as well as a 3D linear frequency-domain method based on potential flow are used. From their comparison against the measurements, strong 3D and frequency dependency effects as well as flexible floater motions matter. The weak-scatter model can only partly explain the nonlinearities present in the measured accelerations.

Volume 8A: Ocean Engineering, 2014

Present investigation is based on a numerical study using a time-domain Rankine panel method. The... more Present investigation is based on a numerical study using a time-domain Rankine panel method. The effort and novelty is to extend the applicability of the solver to shallower waters and to steeper waves by including additional non-linear effects, but in a way so to limit the increase in computational costs. The challenge is to assess the improvement with respect to the basic formulation and the recovery of linear theory in the limit of small waves. The wave theories included in the program are Airy, Stokes 5th order and Stream function. By their comparison the effect of the incoming-wave non-linearities can be investigated. For the free-surface boundary conditions two alternative formulations are investigated, one by Hui Sun [1] and one developed here. The two formulations combined with the above-mentioned wave theories are applied to two relevant problems. The first case is a fixed vertical cylinder in regular waves, where numerical results are compared with the model tests by Grue & Huseby [2]. The second case is a freely floating model of a LNG carrier (with zero forward speed) in regular waves, where computations are compared with the experimental results from the EC project "Extreme Seas". This comparison revealed several challenges such as how to interpret/post process the experimental data. Some of these are described in the paper. After careful handling of both computed and measured data the comparisons show reasonable agreement. It is proven that including more non-linear effects in the free-surface boundary conditions can significantly improve the results. The formulation by Hui Sun gives better results compared to the linear condition, but the present formulation is shown to provide a further improvement, which can be explained through the nonlinear terms included/retained in the two approaches.

Journal of Offshore Mechanics and Arctic Engineering, 2002

Green Water Loading in the bow region of a Floating Production Storage and Offloading unit (FPSO)... more Green Water Loading in the bow region of a Floating Production Storage and Offloading unit (FPSO) in head sea waves is studied by numerical means. A 2-D method satisfying the exact nonlinear free-surface conditions within potential-flow theory has been developed as a step towards a fully 3-D method. The flow is assumed 2-D in a plane containing the ship’s centerplane. The method is partly validated by model tests. The importance of environmental conditions, 3-D flow effects, ship motions, and hull parameters are summarized. The wave steepness of the incident waves causes important nonlinear effects. The local flow at the bow is, in general, important to account for. It has become popular to use a dam-breaking model to study the propagation of water on the deck. However, the numerical studies show the importance of accounting for the coupled flow between the deck and outside the ship. When the water is propagating on the deck, a suitable distance from the bow can be found from where ...

Journal of Hydrodynamics, 2010

A numerical approach has been used to analyze the water shipping caused by head sea waves for a F... more A numerical approach has been used to analyze the water shipping caused by head sea waves for a FPSO ship at rest. A 3D Domain-Decomposition (DD) strategy is used, where a linear potential-flow seakeeping analysis of the vessel is coupled with a local nonlinear rotational-flow investigation for the prediction of water-ondeck phenomena. The Navier-Stokes solver is applied in the region close to the ship bow. It combines a finite-difference spatial algorithm with a predictor-corrector time scheme. The sea and ship surfaces are tracked with a Level-Set (LS) technique and a hybrid Eulerian-Lagrangian algorithm. The inner solver receives the initial and boundary conditions in terms of velocity, pressure, sea-surface location and ship motions and provides the loads due to the nonlinear waveship interaction (including green-water loads) to the seakeeping method. Here the inner solver and its implementation within the DD are described in detail. Preliminary results in terms of water-on-deck occurrence are discussed and compared against 3D water-on-deck experiments.

Journal of Fluid Mechanics, 2007

The generation and evolution of two-dimensional bores in water of uniform depth and on sloping be... more The generation and evolution of two-dimensional bores in water of uniform depth and on sloping beaches are simulated through numerical solution of the Euler equations using the smoothed particle hydrodynamics (SPH) method, wherein particles are followed in Lagrangian fashion, avoiding the need for computational grids. In water of uniform depth, a piston wavemaker produces cyclically breaking bores in the Froude number range 1.37–1.82, which were shown to move at time-averaged speeds in very good agreement with the requirements of global mass and momentum conservation. A single Strouhal number for the breaking period was discovered. Complex repetitive splashing patterns are observed and described, involving forward jet formation growth, impact and ricochet, and similarly, backward jet formation and impact. Observed consequences were the creation of vortical regions of both signs, dipole creation through pairing, large-scale transport of surface water downward and high tangential scou...

Journal of Engineering Mathematics, 2004

Practical slamming problems for ships and ocean structures are briefly described. Theoretical sta... more Practical slamming problems for ships and ocean structures are briefly described. Theoretical status and future challenges for water entry on an initially calm free surface, wetdeck slamming, green water and sloshing are presented. It is emphasized that slamming should be considered in the framework of structural dynamics response and integrated with the global flow analysis around a ship or ocean structure or with violent fluid motion inside a tank. Two-phase flow can give important loading and needs to be better understood. Slamming on a VLFS with shallow draft is dealt with in detail.