• Journal of Ocean Engineering and Technology
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• v.35 no.4
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• pp.296-304
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• 2021

Research on the development of marine renewable energy is actively in progress. Various studies are being conducted on the development of wave energy converters. In this study, a numerical analysis of wave-energy extraction performance was performed according to the body shape and scale of the sloped oscillating water column (OWC) wave energy converter (WEC), which can be connected with the breakwater. The sloped OWC WEC was modeled in the time domain using a two-dimensional fully nonlinear numerical wave tank. The nonlinear free surface condition in the chamber was derived to represent the pneumatic pressure owing to the wave column motion and viscous energy loss at the chamber entrance. The free surface elevations in the sloped chamber were calculated at various incident wave periods. For verification, the results were compared with the 1:20 scaled model test. The maximum wave energy extraction was estimated with a pneumatic damping coefficient. To calculate the energy extraction of the actual size WEC, OWC models approximately 20 times larger than the scale model were calculated, and the viscous damping coefficient according to each size was predicted and applied. It was verified that the energy, owing to the airflow in the chamber, increased as the incident wave period increased, and the maximum efficiency of energy extraction was approximately 40% of the incident wave energy. Under the given incident wave conditions, the maximum extractable wave power at a chamber length of 5 m and a skirt draft of 2 m was approximately 4.59 kW/m.

• Ocean Systems Engineering
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• v.1 no.4
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• pp.317-336
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• 2011

A time domain finite element based method is employed to analyze wave radiation by multiple cylinders. The nonlinear free surface and body surface boundary conditions are satisfied based on the perturbation method up to the second order. The first- and second-order velocity potential problems at each time step are solved through a finite element method (FEM). The matrix equation of the FEM is solved through an iteration and the initial solution is obtained from the result at the previous time step. The three-dimensional (3D) mesh required is generated based on a two-dimensional (2D) hybrid mesh on a horizontal plane and its extension in the vertical direction. The hybrid mesh is generated by combining an unstructured grid away from cylinders and two structured grids near the cylinder and the artificial boundary, respectively. The fluid velocity on the free surface and the cylinder surface are calculated by using a differential method. Results for various configurations including two-cylinder and four-cylinder cases are provided to show the mutual influence due to cylinders on the first and second waves and forces.

• Journal of Ocean Engineering and Technology
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• v.20 no.6 s.73
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• pp.67-74
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• 2006

A high-order spectral/boundary-element method is newly adapted as an efficient numerical tool. This method is one of the most efficient numerical methods by which the nonlinear gravity waves can be simulated and hydrodynamic forces also can be calculated in time-domain. In this method, the velocity potential is expressed as the sum of surface potential and body potential. Then, surface potential is solved by using the high-order spectral method and body potential is solved by using the high-order boundary element method. By the combination of these two methods, the wave-making problems by a submerged sphere oscillating with large amplitude under the free~surface are solved in time-domain. Through the example calculations, nonlinear effects on free-surface profiles and hydrodynamic forces are shown and discussed.

• Journal of Ocean Engineering and Technology
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• v.26 no.3
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• pp.33-38
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• 2012

The aim of this experimental study was to analyze the effect of the shape parameters and chamber pressure of a land-based oscillating water column (OWC) in regular incident waves. The magnitude of the free surface elevations inside the chamber was measured in a two-dimensional wave tank for various chamber skirt drafts and bottom slope angles. The surface elevations were also measured under both open chamber and partially open chamber conditions. From these measurements, the optimum shape of the OWC device could be predicted for the maximum wave energy conversion efficiency. It was found that the resonance frequency of the OWC system associated with incident waves moved toward the long wave region with increments of the draft of the chamber skirt and bottom slope. The behavior of the free surface elevation inside the chamber was also found to be dependent on the chamber pressure.

• Journal of the Society of Naval Architects of Korea
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• v.56 no.5
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• pp.418-426
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• 2019

To derive a simplified analytic solution which can be utilized as a fundamental solution for the wave maker design, a segment of the wave board has been idealized as a submerged line segment in a two dimensional domain of a wave flume. The lower end of the line segment could be located at arbitrary depth of the wave flume and the upper end of the board could be also submerged to any depth from the free surface. The freely oscillating motion of the wave board is assumed to be defined by determining the condition of horizontal oscillation on both ends differently. The submerged wave board oscillating in horizontal direction could be specified by selecting the amplitude, frequency and the phase lag differently on lower and upper ends of the board. The simplified two dimensional wave generated by the wave board segment has been obtained by the first order perturbation method. It is found that the general solution of the freely oscillating wave board in two dimensional domain could be decomposed into the solution of flap motion with lower end hinge and swing motion with upper end hinge. The case study of the analytic solutions has been carried out to evaluate the effect on the wave height due to the difference of oscillation frequency, phase difference and variation of stroke between for the motion of both ends. It is found that the solution of the freely oscillating wave board could be utilized for the development of high performance wavemaker especially for irregular waves.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.26 no.6
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• pp.1143-1148
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• 2002

This paper analyses the transient response of a spherical elastic shell located near fee surface and impinged by spherical step-exponential acoustic shock waves. The problem is solved through extension of a method (Huang, 1969) previously formulated for the excitation in an infinite domain, which employs the classical separation of variables, series solutions, and Laplace transform technique The effect of the free surface reflection is taken into account using the image source method. The reflection of the incident wave has been treated by the same image formulation. If the reflection of the pressure field scattered and radiated by the shell is considered, the problem becomes that of multiple scattering by two spheres. However, this is in general negligible considering errors inherent from other sources and that the scattered and radiated pressure waves emanating from the shell are small. Thus, the problem is reduced to that of a structure immersed in an infinite fluid and impinged upon the origin and the image incident.

• Journal of Hydrospace Technology
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• v.2 no.1
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• pp.10-17
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• 1996

If a point impulse travels over free surface of water of finite depth, surface waves consist of divergent waves. The crestlines of those divergent waves are short and end on the cusp line if the impulse travels at a subcritical speed. But the crestlines become infinitely long and there are no cusps if the impulse travels at a supercritical speed.

• Journal of computational fluids engineering
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• v.1 no.1
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• pp.55-63
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• 1996

Numerical computations are carried out to analyze the characteristics of flow fields around Air Supported Ships. The computations are performed in a rectangular grid system based on MAC(Marker And Cell) method. The governing equations are represented in finite difference forms by forward differencing in time and centered differencing in space except for its convection terms. For the certification of this numerical analysis method, the computations of flow fields around a Catamaran, an ACV(Air Cushion Vehicle) modeled with pressure distribution on free surface and two SES(Surface Effect Ship)'s are carried out, The results of the present computations are compared with the previously presented computational and experimental results in the same condition.

• Proceedings of the Korean Institute of Navigation and Port Research Conference
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• 2022.11a
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• pp.113-114
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• 2022

This study demonstrates the performance of flexible surface wave resonators in spaces on a ship to overcome environmental limits like non-metallic walls where conventional surface wave resonators cannot installable. Although test results in plane structures show that the performance of conventional surface wave resonators are better than the flexible ones, the results are reversed in curved structures. Flexible surface wave resonators can be installed on metal-pipes that connects all spaces in a ship, and this will allow to build ultimate communication network all over the ship including the rooms like cabins or bridges that are enclosed in non-metallic walls.

• Bulletin of the Society of Naval Architects of Korea
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• v.19 no.2
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• pp.13-18
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• 1982

The method of Rankine source distribution is emerging as a powerful yet simple alternative for the solution of complicated free surface problems. But it has been uncertain whether the radiation condition could be satisfied exactly by distributing the simple sources on the free surface only. In this paper, it is proved rigorously that the Rankine sources, whose intensities are varying sinusoidally along the axis satisfying the free surface boundary condition, generate the radiation waves both in the infinite and finite-depth flows. A formula is derived to give the upper and lower bounds of the errors in the induced velocity computation that will be introduced by truncating the extent of source distribution on the free surface. Since the truncation is inevitable in the numerical analysis, this formula may be used as a criterion to limit the position of the field points, where velocity computation is made, away from the truncation boundary. A typical analysis shows that the maximum error will be 3.4 percent of the exact induced velocity when the field point is on the free surface two wave lengths away from the truncation boundary.