Challenges in Hydrodynamics of Ships and Ocean Structures

Violent fluid motions, high speed marine vehicles and Computational Fluid Dynamics (CFD) are selected as main topics. Violent fluid motions deal with green water on deck, sloshing and slamming. Slamming involves many physical effects. When analyzing slamming, one must always have the structural reaction in mind. This necessitates that hydroelastic effects are considered. Many hydrodynamic phenomena matter for the three main categories of highspeed vessels, i.e., vessels supported by the hull, foils and air cushions. Dynamic instabilities, cavitation and ventilation are limiting factors for their performance. The coupling with automatic control is discussed. A brief overview of the many different CFD methods is given and advantages and disadvantages are discussed

Gap Resonance Analyzed by a Domain-Decomposition Method

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Mooring Loads of a Circular Net Cage with Elastic Floater in Waves and Current

The mooring loads on an aquaculture net cage in current and waves are investigated by dedicated model tests and numerical simulations. The main purpose is to investigate which physical effects are dominant for mooring loads, and in this respect, to investigate the validity of different rational hydrodynamic load models. Also structural and numerical aspects are investigated. The model tests are performed to provide benchmark data, while the numerical model is used to study the effect and sensitivity of different load models and parameters. Compared to a realistic aquaculture plant, the total system is simplified to reduce the complexity. The system does, however, include all the four main components of an aquaculture plant: net cage, floater, sinker weights and moorings. The net cage is bottomless, flexible and circular. It is attached to a circular, elastic floater at the top and has 16 sinker weights at the bottom. The system is nearly linearly moored with four crow feet mooring lines. The loads are measured in the four mooring lines. A systematic variation of current only, wave only as well as combined current and wave conditions is carried out. The numerical simulation results are first benchmarked towards the experimental data. The mean loads in general dominate over the dynamic part of the loads in combined current and waves, and they significantly increase in long and steep waves, relative to current only. Next, a sensitivity study is carried out. A rigid floater significantly alters the loads in the mooring lines compared to a realistic, elastic floater. The theoretical model for the wave matters. The mooring loads are rather insensitive to a majority of the parameters and models, in particular: frequency dependent added mass of the floater and nonlinear restoring loads. It seems not to be necessary to represent the net cage with a very fine numerical mesh.acceptedVersio

Comparison and Sensitivity Investigations of a CALM and SALM Type Mooring System for Wave Energy Converters

A quasi-static analysis and sensitivity investigation of two different mooring configurations—a single anchor leg mooring (SALM) and a three-legged catenary anchor leg system (CALM)—is presented. The analysis aims to indicate what can be expected in terms of requirements for the mooring system size and stiffness. The two mooring systems were designed for the same reference load case, corresponding to a horizontal design load at the wave energy converter (WEC) of 2000 kN and a water depth of 30 m. This reference scenario seems to be representative for large WECs operating in intermediate water depths, such as Weptos, Wave Dragon and many others, including reasonable design safety factors. Around this reference scenario, the main influential parameters were modified in order to investigate their impact on the specifications of the mooring system, e.g. the water depth, the horizontal design load, and a mooring design parameter

GREEN WATER LOADING ON A DECK STRUCTURE

At the previous Workshop, a numerical investigation of water-on-deck phenomena was presented by the same au- thors. A two-dimensional problem was considered. The ef- fect of main wave- and body-parameters was studied. The fully nonlinear problem was solved by boundary-integral e- quations. Here, we discuss a continuation of that activity. Results from an on-going experimental investigation are p- resented, together with the analysis of the interaction of the fluid on the deck with superstructures

Numerical Simulation of Heavy Water Shipping

Rough-sea conditions can result in shipping of water on the deck of vessels. In particular, our ongoing investigation is focused on the bow-deck wetness in head-sea conditions for a moored ship, i.e. without forward speed. Though in practice three-dimensional effects matter, two-dimensional investigations are undertaken to gain basic insights, before developing more realistic three-dimensional approaches. In previous Workshops, a combined numerical-experimental analysis has been presented. In particular, a potential flow model has been assumed and a Mixed Eulerian-Lagrangian method has been adopted to solve the unsteady interaction between the body and the free surface. The Boundary Element Method (BEM) has been used as numerical solver. In the experiments, a two-dimensional nearly-rectangular ship model has been placed in a narrow wave flume, and the first water-on-deck event due to incoming waves generated by a flap wavemaker has been investigated. The model is fixed and resembles the centerplane of a ship. Comparisons confirmed the validity of the adopted flow model and the efficiency of the BEM in capturing the water shipping from a global point of view as well as in predicting some local features, such as the initial pressure along a superstructure under the impact of the shipped water

Application of a 2D BEM-Level Set Domain Decomposition to the Green-Water Problem

A Domain-Decomposition strategy for the study of nonlinear air-water interface problems has been developed and presented to the last workshop (see Colicchio et al. 2004b). The method is based on the use of a Boundary Element Method (BEM) and a Navier-Stokes solver combined with a Level-Set technique to capture the interface evolution (NS-LS). Both solvers are accurate to the second order both in space and in time. The two solution techniques are applied to simulate the air-water evolution in different portions of the fluid domain. In particular, the NS-LS analyzes the regions that can be characterized by breaking and fragmentation of the interface, vortex shedding and air entrainment phenomena. The BEM is adopted in the rest of the domain of interest. Many practical problems in ship hydrodynamics exist where such kind of zonal approach can be adopted. An example is the water on deck caused by the vessel interaction with pre-existing waves. In this case the field solver is needed to describe all the stages of the water evolution onto the deck and near the vessel

Influence of gaseous cavities in ship-hydrodynamic problems: a simplified study

Several phenomena of interest in ship hydrodynamics involve the presence of time-varying air cavities in the water flow. Two of the most well known, with important practical consequences, are the erosion of propeller blades and the noise. The former is connected with propellers working near the air-water interface and, in particular, to the formation of air cavities during cavitation events. It is caused by the propeller interaction with the air-water two-phase fluid and represents a danger for the success of the vessel operations. The latter is connected with entrapment of air cavities during breaking and fragmentation of the air-water interface. It is a consequence of the collapse of such bubbles evolving in the surrounding water and its major relevance is in military context. Cavitating bubbles are characterized by a uniform interior pressure equal to the vapor pressure. Instead, entrapped cavities have in principle a non uniform pressure. However, in many practical cases their inner pressure can be approximated as uniform with value generally time dependent, in particular compressibility may matter. Assuming a uniform pressure in the cavity means that the effect of the interior air flow on the surrounding water and structures is neglected

An investigation of water on deck phenomena

Shipping of water on deck represents a danger both for moored (as FPSO) and for advancing ships. When a compact mass of water (\u27green water\u27) is shipped onto the deck, the fluid moves with high speed, being able to damage superstructures and equipments and to be a risk for human lives. Also, the shipped water can significantly alter the dynamics of smaller vessels. Such events make the green water loads an important parameter to take into account even at design stage and call for predictive tools. However, knowledge about the physics involved is still limited and motivates increasing research effort

Nonlinear air-water interface problems through a BEM-Level set domain decomposition

Several water flow problems of practical interest are characterized by fluid regions where the flow evolution is efficiently de- scribed by the potential theory, and other regions where this model is not valid. For instance, confined fluid areas can experience large air-water interface deformations followed by wave breaking and fragmentation phenomena. Limited water portions can be characterized by substantial vorticity generation due to water-water or water-structure interaction. In these cases the surrounding fluid domains can be slightly affected by such events. The ship hydrodynamic field is full of similar circumstances. The water-on-deck problem represents an example. In this case, compact masses of water enter the ship deck and the subsequent motion can result in important loads for the deck superstructures. On a long time scale, water breaking, air entrainement and vorticity generation are expected to occur. The later water-off-deck phase will cause the re-entering of water in the sea surrounding the vessel. As a result, near the vessel the free surface cannot be modeled as a smooth surface. Both the water shipping event and the final water-entry phase can involve substantial induced water loads on the vessel and large movements of the ship. Therefore related phenomena are of great interest for ship hydrodynamics, both from the operability and safety points of view. Due to the large free-surface deformations involved, a nonlinear analysis is needed. Before breaking and/or vortex shedding events, potential flow theory can capture accurately and with computational efficiency the involved flow evolution and predict connected loads and motions. After that, in the water regions where such phenomena occur and develop, this model has to be substituted by more general methods suitable to track the free surface deformations after the breaking, to handle the flow vorticity introduced in the fluid domain and to model the entrapped air. The present research activity is aimed to develop a numerical method able to simulate such ship flows and to adapt itself to the specific analyzed problem for an efficient and suitable solution. This has been done by considering a domain-decomposition strategy (see i.e. Quarteroni and Valli 1999, Campana and Iafrati 2001)