Repeatability and Two-Dimensionality of Model Scale Sloshing Impacts

Canonical test cases for sloshing wave impact problems are pre-sented and discussed. In these cases the experimental setup has been simpli?ed seeking the highest feasible repeatability; a rectangular tank subjected to harmonic roll motion has been the tested con?guration. Both lateral and roof impacts have been studied, since both cases are relevant in sloshing assessment and show speci?c dynamics. An analysis of the impact pressure of the ?rst four impact events is provided in all cases. It has been found that not in all cases a Gaussian ?tting of each individual peak is feasible. The tests have been conducted with both water and oil in order to obtain high and moderate Reynolds number data; the latter may be useful as simpler test cases to assess the capabilities of CFD codes in simulating sloshing impacts. The re-peatability of impact pressure values increases dramatically when using oil. In addition, a study of the two-dimensionality of the problem using a tank con?guration that can be adjusted to 4 di?erent thicknesses has been carried out. Though the kinemat-ics of the free surface does not change signi cantly in some of the cases, the impact pressure values of the ?rst impact events changes substantially from the small to the large aspect ratios thus meaning that attention has to be paid to this issue when reference data is used for validation of 2D and 3D CFD codes

DEVELOPMENT OF ANALYTICAL NONLINEAR MODELS FOR PARAMETRIC ROLL AND HYDROSTATIC RESTORING VARIATIONS IN REGULAR AND IRREGULAR WAVES

2004/2005Parametrically excited roll motion has become a relevant technical issue, especially in recent years, due the increasing number of accidents related to this phenomenon. For this reason, its study has attracted the interest of researchers, regulatory bodies and classification societies. The objective of this thesis is the developing of nonlinear analytical models able to provide simplified tools for the analysis of parametrically excited roll motion in longitudinal regular and irregular long crested waves. The sought models will take into account the nonlinearities of restoring and of damping, in order to try filling the gap with the analytical modelling in beam sea. In addition, semi-empirical methodologies will be provided to try extending the usual static approach to ship stability based on the analysis of GZ curve, in a probabilistic framework where the propensity of the ship to exhibit restoring variations in waves is rationally accounted for. The thesis addresses three main topics: the modelling of parametric roll in regular sea (Chapter 2 to Chapter 5), the modelling of parametric roll motion in irregular long crested sea (Chapter 6 and Chapter 7) and the extension of deterministic stability criteria based on the analysis of geometrical GZ curve properties to a probabilistic framework (Chapter 8). Chapter 1 gives an introduction, whereas Chapter 9 reports a series of final remarks. For the regular sea case an analytical model is developed and analysed both in time domain and in frequency domain. In this latter case an approximate analytical solution for the nonlinear response curve in the first parametric resonance region is provided by using the approximate method of averaging. Prediction are compared with experimental results for four ships, and the analytical model is investigated with particular attention to the presence of multiple stable steady states and the inception of chaotic motions. The influence of harmonic components higher than the first one in the fluctuation of the restoring is also investigated. In the case of irregular sea, the Grim's effective wave concept is used to develop an analytical model for the long crested longitudinal sea condition, that allows for an approximate analytical determination of the stochastic stability threshold in the first parametric resonance region. Experimental results are compared with Monte Carlo simulations on a single ship, showing the necessity of a tuning factor reducing the hydrostatically predicted magnitude of parametric excitation. The non-Gaussianity of parametrically excited roll motion is also discussed. Finally, on the basis of the analytical modelling of the restoring term in irregular waves, an extension of the classical deterministic approach to ship static stability in calm water is proposed, to take into account, although is a semi-empirical form, restoring variations in waves. Classical calm water GZ curve is then extended from a deterministic quantity to a stochastic process. By limiting the discussion to the instantaneous ensemble properties of this process, it is shown how it is possible to extend any static stability criterion based on the geometrical properties of the GZ curve, in a rational probabilistic framework taking into account the actual operational area of the ship and the propensity of the ship to show restoring variations in waves. General measures of restoring variations are also discussed, such as the coefficient of variation of metacentric height, restoring lever and area under GZ. Both the short-term and long-term point of view are considered, and the method is applied to three different ships in different geographical areas

Influence of ship dynamics modelling on the prediction of fishing vessels roll response in beam and longitudinal waves

Fishing is one of the most risky occupations worldwide. Many accidents are due to stability failures, and dynamic stability phenomena in waves are among the possible causes. Due to time, cost and regulatory lacks, these phenomena are not addressed during design of fishing vessels. High fidelity six degrees of freedom (6-DOF) codes can cope with these phenomena, but their complexity, cost and time of application, limit their use to high-end vessels. The use of less demanding approaches appears more suited for the fishing sector. The scope of this paper is to compare a 6-DOF blended code, 1-DOF nonlinear roll models and simplified analytical formulae, in order to assess the level of dispersion of the obtained roll motion predictions among tools with different levels of complexity. The roll response for a medium-sized stern trawler is investigated in regular beam and longitudinal waves, at zero speed. Results from the simplified models are in fair agreement with those from the 6-DOF code, at least for moderate wave steepnesses. Simplified models could provide a valuable tool for the assessment of fishing vessels behaviour at the early design stage, considering also their ease of implementation within typical existing naval architecture software

A set of canonical problems in sloshing. Part 2: Influence of tank width on impact pressure statistics in regular forced angular motion

Continuing along the path of previous papers in this series, the present paper addresses, experimentally, sloshing in a rectangular tank under harmonic angular forcing. Since lateral and roof impacts are relevant in sloshing assessment and present distinct dynamics, these have been studied. Water and oil have been used in order to obtain data for high and moderate Reynolds numbers. With the aim of addressing effects induced by the three-dimensionality of the flow, a study of the influence of the tank width on pressure statistics has also been conducted. For this purpose, a tank which can be adjusted to 4 different widths by replacing the side plates was built. For each combination of fluid, tank width and filling level, a total of 120 experiments, each comprising 120 peaks, have been conducted, allowing an analysis in time and ensemble domains to be performed. Impact pressure statistics are presented including mean values, percentiles, and exceedance probability graphs

Estimation of damping through internally excited roll tests

Roll damping represents a key factor for a proper estimation of the ship behaviour in a seaway. However, due to the typical dominance of viscous effects, accurately estimating roll damping is a challenging task. The most common experimental approach for determining roll damping parameters is based on the analysis of roll decays, although forced and excited roll tests in calm water or in waves have been used as well. This paper proposes a technique for estimating roll damping from internally excited roll tests in calm water. Tests are performed by exciting roll motion through an internal shifting mass. Roll damping parameters can then be determined from the analysis of the obtained roll response curves. The paper describes the experimental technique and a nonlinear mathematical model for representing the system dynamics. A procedure is proposed for determining roll damping coefficients, using, as a basis for the analysis, the developed mathematical model. A case study is reported where damping coefficients are determined for a trawler fishing vessel using the proposed technique. Obtained results are compared with those from standard roll decays analysis. For model validation purposes, the experimental roll response curves are also compared with those simulated through the developed mathematical model

Smoothed particle hydrodynamics (SPH) simulation of a tuned liquid damper(TLD) with angular motion.

The roll motion response of a single degree of freedom (SDOF) structural system to which a rigid rectangular partially filled liquid tank has been attached is considered. The SDOF structural system with the empty tank is first described with a mathematical model and this model is validated by performing decay experiments as well as experiments in which periodic excitations are applied to the system. The responses are accurately predicted by the model. The accuracy of these predictions allows us to study both experimentally and numerically, with weakly compressible SPH, the performance of the partially filled tank as a tuned liquid damper (TLD). The sloshing flows inside the tank comprise the onset of breaking waves which make the TLDs devices extremely difficult to model, especially for the potential flow multimodal approaches commonly used to simulate these sorts of coupled systems. In order to characterise the wave breaking effects on the response curves, tests have been performed with liquids of different viscosity, the increasing viscosity preventing the onset of breaking waves. The capabilities of SPH to treat this coupling problem are assessed and the results show that SPH is able to capture a substantial part of the physics involved in the addressed phenomena but further work remains still to be done relating to a more accurate treatment of the laminar viscosity and turbulence effects

Estabilidad y efectos dinámicos del agua sobre cubierta en la supervivencia de barcos pesqueros pequeños

Starting from the analysis of casualties at sea involving small fishing vessels (Lbp<24 m), the paper presents the results of an experimental study on the capsizing resistance in beam or following steep or high waves of scale models in the towing tank of University of Trieste. The physical mechanisms leading to capsizing are highlighted and the experimental result are compared with existing theoretical approaches. The results stress the importance of deck wetness and of the presence of fishing nets as wave trappers on the probability of capsizing. The importance of complying with the existing regulations for dimensioning the freeing ports, although only voluntary, appears also an element of paramount importance.En la Universidad de Trieste se ha desarrollado un plan de investigación en seguridad y estabilidad del buque. La investigación se dirige principalmente a mejorar el conocimiento de los movimientos de grande amplitud del buque, con particular atención al movimiento de balanceo en diversas condiciones de la ola, y al efecto del chapoteo del agua a bordo.La importancia del agua en la cubierta con la capacidad de volcar pequeños pesqueros, de puntal bajo y francobordo, cubierta grande, en olas de grande altura y/o inclinación, se ha confirmado en base de los experimentos conducidos después de accidentes en el mar. Particularmente, la degradación posible de la estabilidad, debida al agua atrapada en las redes, es relevante a los estudios de la seguridad del buque. Finalmente,la altura de ola crítica propuesta por Dahle y otros, se confirma para los buques muy pequeños en olas de través. Sería interesante extender en el caso de olas de popa, considerando que actualmente el criterio más acreditado de la estabilidad refiere a un mecanismo que vuelca en viento y mar de través

Three SPH Novel Benchmark Test Cases for free surface flows.

Benchmark Test Cases have been used by SPHERIC interest group members for the validation of SPH models and their corresponding computer implementations. Since the use of SPHERIC benchmark test cases as validation reference for SPH implementations has slightly declined in the most recent editions, we think it might be interesting to document three novel test cases with the aim of enriching the database with complementary validation data. The first proposed test case is a wave impact problem in a rectangular tank. The time history of the motion of the tank and the pressure of the first instances of lateral and roof impacts for both water and oil are provided. An analysis of the two-dimensionality and repeatability of the pressure peaks is provided. The second proposed test case treats the coupling of the angular motion of a sloshing tank and a single degree off reedom structural system. Finally, the third proposed test case, is a canonical fluid structure interaction problem consisting in the interaction between a free surface sloshing flow and an elastic body. As both SPH practitioners and experimentalists, regard less of the discussion provided in this paper, we are committed to improving these test cases for future use. We hope to increase our experimental skills and capabilities not only in light of experience from our own simulations but mainly by receiving a feedback from the SPH communit

Natural roll period of river-sea ships

The natural roll period is an important component of stability assessment. The need for development of a proper estimation method is particularly important for river-sea ships, which represent inland vessels intended for operations in maritime environment. The overall goal of this study is to contribute to the development of a suitable method for the natural roll period estimation for river-sea ships. For this purpose, a method based on the linear potential hydrodynamics is applied on a database consisted of 31 river-sea ships. The obtained results are analysed and compared with those calculated using semi-empirical methods, and a practical natural roll period estimation method for river-sea ships is proposed