Do-Chun Hong

Numerical Study of the Radiation Potential of a Ship Using the 3D Time- Domain Forward-Speed Free-Surface Green Function and a Second- Order BEM

The radiation potential of a ship advancing in waves is studied using the 3D time- domain forward-speed free-surface Green function and the Green integral equation. Numerical solutions are obtained by making use of the 2nd order BEM(Boundary Element Method) which make it possible to take account of the line integral along the waterline in a rigorous manner. The 6 degree of freedom motion memory functions of a hemisphere and the Wigley seakeeping model obtained by direct integration of the time-domain 3D potentials over the wetted surface are presented for various Froude numbers.

Waveload Analysis for Heeled Barges with Flooded Compartments

A ship may suffer sinkage and heel due to flood in a compartment caused by damage on a deck. The motion and waveloads of the heeled ship floating in waves have been analyzed by making use of a three dimensional potential theory taking account of the hydrodynamic pressure in the flooded compartments. The shear forces and bending moments due to radiation-diffraction waves have been calculated by the direct integration of the 3-d hydrodynamic pressure on the outer and inner hulls of floating barges. The motion responses and the relative flow rate across the mean free surface of the water in the flooded compartments are also presented.

Estimation of Large Amplitude Motions and Wave Loads of a Ship Advancing in Transient Waves by Using a Three Dimensional Time-domain Approximate Body-exact Nonlinear 2 nd -order BEM

Abstract A three-dimensional time-domain calculation method is of crucial importance in prediction of the motions and wave loads of a ship advancing in a severe irregular sea. The exact solution of the free surface wave-ship interaction problem is very complicated because of the essentially nonlinear boundary conditions. In this paper, an approximate body nonlinear approach based on the three-dimensional time-domain forward-speed free-surface Green function has been presented. The Froude-Krylov force and the hydrostatic restoring force are calculated over the instantaneous wetted surface of the ship while the forces due to the radiation and scattering potentials over the mean wetted surface. The time-domain radiation and scattering potentials have been obtained from a time invariant kernel of integral equations for the potentials which are discretized according to the second-order boundary element method (Hong and Hong 2008). 접수일: 2009년 12월 3일, 승인일: 2010년3월 22일 g교신저자: sayhong@moeri.re.kr, 042-866-3930

Performance Prediction of an OWC Wave Power Plant with 3-D Characteristics in Regular Waves

The primary wave energy conversion by a three-dimensional bottom-mounted oscillating water column (OWC) wave power device in regular waves has been studied. The linear potential boundary value problem has been solved following the boundary matching method. The optimum shape parameters such as the chamber length and the depth of the front skirt of the OWC chamber obtained through two-dimensional numerical tests in the frequency domain have been applied in the design of the present OWC chamber. Time-mean wave power converted by the OWC device and the time-mean second-order wave forces on the OWC chamber structure have been presented for different wave incidence angles in the frequency-domain.

Hydrodynamic Analysis of Two Side-by-Side Moored Floating Bodies

The hydrodynamic interaction of two bodies floating in waves is studied. The two-body hydrodynamic coefficients of added mass, wave damping and exciting forces and moments are calculated using the irregular frequency free radiation-diffraction potential solution of the improved Green integral equation associated with the free surface Green function (Hong 1987) according to the conventional two-body analysis. It is well known that the conventional two-body potential solution with usual grid fineness largely overestimates the hydrodynamic coefficients at and near the resonance frequency of the free surface in the gap between two floating bodies moored side-by-side in close proximity (Huijsmans et al. 2001, Hong et al. 2005). The two-body diffraction problem has been solved by both the conventional two-body analysis without damped free surface condition and a boundary matching method with and without damped free surface condition. Numerical results of the wave exciting force coefficients of two identical caissons floating side by side obtained by the two methods have been presented and the discrepancies between them have been discussed. Particular attention is paid to the wave elevation in the gap at the resonance frequencies. Amplitudes and phases of the scattering wave elevations in the gap at the first three free surface resonance frequencies computed by the boundary matching method without damped free surface condition have been presented. It has also been shown that the unrealistic wave elevation due to the resonance of the free surface in the gap can be reduced by imposing the damped free surface condition upon the flow in the gap as used in the oscillating water column hydrodynamics (Hong et al. 2004).Copyright

Numerical Study of Large Amplitude Ship Motion With Forward Speed in Severe Seas

A three-dimensional time-domain calculation method is of crucial importance in prediction of ship motion with forward speed in a severe irregular sea. The exact solution of the free surface wave–ship interaction problem is very complicated because of the extremely nonlinear boundary conditions. In this paper, an approximate body nonlinear approach based on the three-dimensional time-domain forward-speed free-surface Green function has been presented. It is a simplified version of the method known as LAMP (Lin and Yue 1990) where the exact body boundary condition is applied on the instantaneous wetted surface of the ship while free-surface condition is linearized. In the present study, the Froude-Krylov force and the hydrostatic restoring force are calculated on the instantaneous wetted surface of the ship while the forces due to the radiation and scattering potentials on the mean wetted surface. The time-domain radiation and scattering potentials have been obtained from a time invariant kernel of integral equations for the potentials. The integral equation for the radiation potential is discretized according to the second-order boundary element method (Hong and Hong. 2008). The diffraction impulse response functions of the Wigley seakeeping model are presented for various Froude numbers. A simulation of coupled heave-pitch motion of the Wigley model advancing in regular head waves of large amplitude has been carried out. Comparisons between the fully linear and the present approximate body nonlinear computations have been made at various Froude numbers.Copyright

Numerical Study of the Ship Motion in Waves Using the Three-Dimensional Time-Domain Forward-Speed Free-Surface Green Function and a Second-Order Boundary Element Method

The radiation and diffraction potentials of a ship advancing in waves are calculated in the time-domain using the three-dimensional time-domain forward-speed free-surface Green function and the Green integral equation on the basis of the Neumann-Kelvin linear wave hypothesis. The Green function approximated by Newman for large time is used together with the Green function by Lamb for small time. The time-domain diffraction problem is solved for the time derivative of the potential by using the time derivative of the impulsive incident wave potential represented by using the complementary complex error function. The integral equation for the potential is discretized according to a second-order boundary element method where the collocation points are located inside the panel. It makes it possible to take account of the line integral along the waterline in a rigorous manner. The six-degree-of-freedom motion and memory functions as well as the diffraction impulse response functions of a hemisphere and the Wigley seakeeping model are presented for various Froude numbers. Comparisons of the wave damping and exciting force and moment coefficients for zero forward speed, calculated by using the Fourier transforms of the time-domain results and the frequency-domain coefficients calculated by using the improved Green integral equation which is free of the irregular frequencies, have been shown to be satisfactory. The wave damping coefficients for non-zero forward speed, calculated by using Fourier transforming of the present time-domain results have also been compared to the experimental results and agreement between them has been shown to be good. A simulation of coupled heave-pitch motion of the Wigley seakeeping model advancing in regular head waves of unit amplitude has been carried out.Copyright

A B-Spline Based Higher-Order Panel Method Applied to Marine Hydrodynamic Problems

A B-spline based higher order panel method (hereinafter, HiPan) is developed for the motion of bodies in ideal fluid, either of infinite extent or with free boundary surface. In this method, both the geometry and the potential are represented by B-splines, and it guarantees more accurate results than most potential based panel methods. In the present work, we apply the HiPan, which differs with the works at MIT in evaluating the induction integrals, to two major marine hydrodynamic problems: analysis of propulsive performance of the marine propellers and the motion of the floating bodies on the free surface. The present HiPan is shown superior to the constant panel method (hereinafter, CoPan) in predicting flow quantities in the area of the thin trailing edge and blade tip of the propeller. Numerical results are validated by comparison with experimental measurements.Copyright

Experimental Study on Wave Loads of Flooded Ship

This paper investigates wave loads of a flooded ship by model test. Model tests are performed in ocean engineering basin of MOERI (Maritime and Ocean Engineering Research Institute). Ship motions are measured by RODYM6D. Wave loads such as shear forces, bending moments and torsion moments are measured by ATI load cell mounted on segmented parts of the ship model. A 300 m-long barge ship with two flooded compartments is considered in model test. Responses of intact and flooded cases are compared. The test results are also compared with numerical analyses using boundary element method.Copyright

Numerical Study of Transmisson and Reflection Coefficients of a BBDB-Type Floating Breakwater

The transmission and reflection coefficiencies of a BBDB-type floating breakwater in water of finite depth are studied taking account of fluctuating air pressure in the air chamber. The wave potential is calculated by a hybrid integral equation consisting of a Green integral equations associated with the Rankiue Green function inside the BBDB and the Kelvin Green function outside. The transmission and reflection coefficients of the breakwater are obtained directly from the potential solution in the outer region.