JOURNAL OF MARINE ENGINEERING
Year:2005 | Volume:1 | Issue:2|Papers Count:11

Extensive experimental investigations of hydrofoil-assisted catamarans have identified the main hydrodynamic parameters affecting the performance of these vessels. Design principles of existing vessels are discussed in the light of these hydrodynamic findings. The experience gained from experimental testing has provided the basis for the development of a non-linear vortex lattice method for design purposes. The method is validated by comparison with experimental results and applied to the problem of optimizing a 40- knot, 40m hydrofoil-assisted catamaran with a hydrofoil system developed for modem high speed catamarans. It is -shown that the method can be successfully applied to optimization problems of these vessels as it successfully captures the complex hull-foil interactions. Results of the numerical investigation performed, indicate that optimizing the mutual distance between the hydrofoils can significantly improve the lift to drag ratio of such a vessel.

The motion of fluids within partially filled containers has been the subject of much study by scientists and engineers due, in large part, to its importance in many practical applications. For example, civil engineers and seismologists have actively studied the effects of earthquake-induced fluid motions on oil tanks and water towers. In recent years, aerospace engineers have been concerned with the effect of fluid sloshing within propellant tanks on the stability of aircraft, rockets, and satellites. All of these applications seek container designs which minimize the amplitude of fluid forces over again range of operating conditions. In this paper, an incompressible smoothed particle hydrodynamics (SPH) method is developed to numerically simulate viscous free surface flows in partially filled containers. The mass conservation and Navier-Stokes equations are solved as basic equations. The method uses a prediction-correction fractional step technique. In the prediction step, the temporal velocity field is integrated in time without enforcing incompressibility and in the correction step the resulting deviation of particle density is implicitly projected onto a divergence-free space to satisfy incompressibility through a pressure Poisson equation derived fr an approximate pressure projection. The proposed SPH method is used to simulate the sloshing of a omliquid wave with low amplitude under the influence of gravity. Initial shape of free surface is defined by one half of a cosine wave with low amplitude. The results of simulation are in good agreement with experimental and other modeling data.

One of the most important issues in designing coastal and offshore structures is the prediction of wave and current forces on slender cylinders. Such forces are often considered as dominate loadings. Many analytical and empirical methods such as Morison equation have been suggested for estimation of waves and current forces. Such methods, however, have shown inaccuracies in predicting hydrodynamic forces. On the other hand, Artificial Neural Networks (ANNs) have received a great deal of attention in recent years and are being touted as one of the greatest computational tools ever developed. In fact, ANNs are nonlinear systems consisting of a large number of highly interconnected processing units, nodes or artificial neurons, which have the ability of learning. In this research, ANNs have been used to estimate wave and current forces on slender cylinders. Data of 308 experimental specimens have been used for training and testing the networks. Considering the aim of this research for the application of ANNs, these data were consisted of recorded force values in different time series. The supervised learning neural networks models have been used in this research. The results indicate the success of the application of neural networks approach which can efficiently predict waves and current forces on slender cylinders after carrying out appropriate training. Furthermore, the results are within acceptable accuracy in comparison with experimental results and the results obtained from Morison equation.

The significant effect of waves on coastal and marine activities urges the precise identification of wave characteristics using field, theoretical studies, physical modeling or numerical simulations In order to study thoroughly the wave climate in the Caspian Sea, a wave modeling and hind cast project was performed by Iranian National Center for Oceanography In this study, one of the latest versions of numerical wave models (3rd generation) was employed for long-term simulation of waves in the Caspian Sea using wind data. The wind field was obtained from ECMWF global operational model after a few local modifications were made. For calibrating the model, in-situ measurements and available satellite data were used Extreme value analysis was the next stage in which for different return periods the wave characteristics were calculated. Finally, user-friendly software was developed with the aim of presenting the results of the project.

To investigate buckling/ultimate strength of ship bottom plates, a series of elastoplastic large deflection analyses is performed applying finite element method. To achieve this purpose, symmetrical conditions are imposed on the plate boundaries. Also a series of elastic large deflection analyses is done in order to assess buckling behaviour and strength of continuous plates under combined lateral pressure and inplane compression.

Liu et al. (1987) proposed a theoretical solution for scattering of waves through a permeable dissipation region. On the basis of bulk dissipation of the incident wave energy, this method is extended to evaluate the reflection and transmission coefficients of waves through a wave filter composed of rows of perforated sheets aligned normally to the direction of wave propagation The results of the theoretical model are verified by laboratory tests with regular waves.

Nowadays, various numerical methods are developed to extend computational fluid dynamics in engineering applications. One of the most useful methods in free surface modeling is Boundary Element Method (BEM). BEM is used to model inviscid fluid flow such as flow around ships. BEM solutions employ surface mesh at all of the boundaries. In order to model the linear free surface, BEM can be modified to a simple form called Green Function Method (GFM).In this paper a developed code is applied GFM to several problems including flows around ships and the results are presented.To verify the accuracy and application of the code, it is applied to flows around an analytically body which is Wigley hull and results are in good agreement with available data. The wave-making properties are also investigated. Variations of the wave pattern versus the speed are studied, and a quantitative investigation of the forces is performed. In addition, the effects of linear solution are studied. Presented results and computer code can be used to optimize the hull forms for decreasing wave making resistance.