Kwang-Leol Jeong

A Fundamental Study for the Numerical Simulation Method of Green Water Occurrence on Bow Deck

AbstractGreen water load is an important parameter to be considered in designing a modern ship or offshore structures like FPSO and FSRU. In this research, a numerical simulation method for green water phenomenon is introduced. The Navier-Stokes equ ations and the continuity equation are used as governing equations. The equations are calculated using Finite Difference Method(FDM) in rectangular staggered grid system. To increase the numerical accuracy near the body, the Cartesian cut cell method is employ ed. The nonlinear free-surface during green water incident is defined by Marker-d ensity method. The green waters on a box in regular waves are simulated. The simulation results are compared with other experimental and computational results for verification. To check the applicability to moving ship, the green water of the ship which is towed by uniform force in regular wave, is simulated. The ship is set free to heave and to surge. ※Keywords: Green water(갑판침입수), Non-linear wave(비선형 파도), Marker-density method(밀도함수법), Cartesian cut cell(잘린 직사각형 격자계)접수일: 2009년 12월 21일, 승인일: 2010년 3월 11일✝교신저자: younglee@inha.ac.kr, 032-860-7340

A Study on the Resistance Performance of the Goose Neck Bulbous Bow by Numerical Simulation Method

Bulbous bow is one of the important design factors on the design of fore-body hull form. Using the interference technique of ship waves, the bulbous bow can decrease the wave resistance of ship. Recently, the goose neck bulb is applied mainly for high speed vessels like passenger ships and ferries etc.. Also, the goose neck bulb is applied for relatively high speed merchant vessels like container ships and LNG carriers. However, existing research papers about the goose neck bulb are not enough as reference data for the design of bow hull form. In this study, numerical calculations are carried out to investigate the bow wave characteristics of a high speed ferry with a normal high nose bulb or a goose neck bulb. By comparing the pressure distributions on the hull surface and the wave systems near the bow, the features of wave resistance reduction are discussed. Also, Numerical calculations were carried out for a series of goose neck bulbs to figure out the optimum bulb size. The maximum reduction rate of pressure resistance for the fore-body is achievable up to 8% by adopting the goose neck bulb in the present calculation.

Reduction of Added Resistance by Internal Flow Control in the Moonpool of a Drillship

The internal flows of moonpool usually causes huge added resistance on drillships, and those are very complex to analyze. Therefore, not only experimental approaches but also numerical simulations are required for better investigations when dealing with the hydrodynamic problems of moonpool. In the present research, numerical simulations are used to find out why the resistance increases by moonpool on a running drillship. That is, the three-dimensional numerical simulations and model tests are carried out to examine the characteristics of internal flow and added resistance by changing the section of the moonpool in both longitudinal and transverse directions. Finally, based on the present studies, an optimized shape of the moonpool is suggested, which effectively reduces added resistance, and that is confirmed with three-dimensional numerical simulations and model tests.

Numerical Simulation of Two-dimensional Sloshing Phenomena Using Marker-density Method

AbstractTwo dimensional sloshing phenomena in regularly excited liquid cargo tank are numerically simulated with finite difference method. Navier-Sto kes equations and continuity equation are computed for this study. The free-surface is deter mined every time step satisfying kinematic boundary condition using marker-density me thod. And the exciting force is treated by adding the acceleration of the tank to source term. The results are compared with other existing experiment results. And the comparison resu lts show a good agreement. The sloshing phenomena in the tank of the 138K LNG carrier in s way motion is simulated with present calculation methods in low filling level. To find the relations between impact pressure and excitation condition, the calculations are performed in various amplitudes and periods. The averaged maximum pressures are compared each other . ※Keywords: Sloshing(슬로싱), LNG carrier(천연가스 운반선), Marker-density method(밀도함수법), Free-surface(자유수면), Liquid cargo tank(액체화물창)

A Study on the Improvement of Resistance Performance for G/T 4.99ton Class Korean Coastal Fishing Boats

Korean fishing boats have had appropriate hull forms for the safety, stability and convenience of fishing ability. However, Korean fishermen are recently concerned about the resistance performance and speed of Korean fishing boats, because the prices of fuel oil are gradually risen, also the exhausting of fish resources and the demand of high speed fishing boats are increased. Therefore, the necessity of the study on the improvement of resistance performance for Korean small coastal fishing boats is gradually increased. This study compares the hull form characteristics of Korean fishing boats with those of Japanese fishing boats, and the hull form of a representative Korean fishing boat is modified. From the modification of the hull form parameters for the Korean fishing boat, the improvement of resistance performances is evaluated. Moreover, the increase of resistance performances is also achieved from the modification of local characteristics for the hull form of the Korean fishing boat. A computational method and ship model tests in towing tank are used for the conformations of the improvement of resistance performance.

Numerical simulation of the free surface around a circular column in regular waves using modified marker-density method

Abstract In this paper the wave run-up around a circular column in regular waves is numerically calculated to investigate the applicability of the Modified Marker-Density (MMD) method to prediction of wave run-up around an offshore platform. The MMD method is one of the methods to define the highly nonlinear free surface. The governing equations are the Navier-Stokes equations and the continuity equation which are computed in Cartesian grid system. To validate incident waves generated by numerical simulation, those are compared with the solutions of the Stokes 5th order wave theory. The wave run-up simulations are performed varying the steepness and period of incident waves as referred experimental data. The numerical results are compared to the experimental data and the results show good agreements.

Numerical Simulation of Spilling Breaker using the Modified Marker-density Method

선박 및 해양구조물의 유체역학적 성능을 평가하거나, 그것들 을 설계하기 위해서는 자유수면을 포함하는 유동을 해석하는 것 이 필수적이다. 그러나 물과 공기의 큰 밀도와 점성계수의 차이 로 인하여 자유수면을 수치적으로 해석하는 것은 아직까지 그리 쉽지 않은 문제로 남아있다. 이러한 유체영역 사이에 물리적 값 들의 차이(공간적 불연속성)로 인하여 발생하는 해의 불안정성을 해결 또는 회피하기 위한 가장 일반적인 방법이 유체의 성질이 연속적으로 변하는 가상의 천이영역을 설정하는 방법이다. 현재 공학적으로 가장 널리 사용되고 있는 Volume of Fluids(VOF)법 (Hirt & Nichols, 1981)은 체적함수의 수송방정식을 계산하여 얻 은 체적함수를 이용하여 자유수면의 위치를 결정하고 체적함수를 이용하여 물과 공기의 밀도와 점성계수를 가중평균 하여 지배방 정식의 밀도와 점성계수로 사용하는 방법이다. VOF법은 많은 공 학적 문제에 유용한 결과를 주지만 격렬한 유동이 지속되면, 체 적함수가 넓게 퍼지게 되어 천이영역이 너무 넓어져 해의 정도가 떨어지는 문제를 가지고 있다. 체적함수가 넓게 퍼지는 것은 수 송방정식이 대류방정식으로서 수치적으로 확산되기 때문이다. 그 렇기 때문에 VOF법의 정도를 향상시키기 위한 연구들은 이러한 수치확산을 감소시키기 위한 방법이 주로 연구되어 왔다 (Zhao & Hu, 2011; Hu & Kashiwagi, 2004; Yu & Chen, 2009; Lopez, et al., 2005). VOF법 이외에 공학적으로 많이 활용되는 또 다른 방법으로 Level-Set(LS)법 (Osher & Sethian, 1988)이 있다. LS법은 VOF pISSN:1225-1143, Vol. 51, No. 1, pp. 58-66, February 2014

Numerical Simulation of the Flow around Advancing Ships in Regular Waves using a Fixed Rectilinear Grid System

This paper presents a numerical simulation method for the flow around advancing ships in regular waves by using a rectilinear grid system. Because the grid lines do not consist with body surface in the rectilinear grid system, the body geometries are defined by the interaction points of those grid lines and the body surface. For the satisfaction of body boundary conditions, no-slip and divergence free conditions are imposed on the body surface and body boundary cells, respectively. Meanwhile, free surface is defined with the modified marker density method. The pressure on the free surface is determined to make the pressure gradient terms of the governing equations continuous, and the velocity around the free surface is calculated with the pressure on the free surface. To validate the present numerical method, a vortex induced vibration (VIV) phenomenon and flows around an advancing Wigley III ship model in various regular waves are simulated, and the results are compared with existing and corresponding research data. Also, to check the applicability to practical ship model, flows around KRISO Container Ship (KCS) model advancing in calm water are numerically simulated. On the simulations, the trim and the sinkage are set free to compare the running attitude with some other experimental data. Moreover, flows around the KCS model in regular waves are also simulated.This paper presents a numerical simulation method for the flow around advancing ships in regular waves by using a rectilinear grid system. Because the grid lines do not consist with body surface in the rectilinear grid system, the body geometries are defined by the interaction points of those grid lines and the body surface. For the satisfaction of body boundary conditions, no-slip and divergence free conditions are imposed on the body surface and body boundary cells, respectively. Meanwhile, free surface is defined with the modified marker density method. The pressure on the free surface is determined to make the pressure gradient terms of the governing equations continuous, and the velocity around the free surface is calculated with the pressure on the free surface. To validate the present numerical method, a vortex induced vibration (VIV) phenomenon and flows around an advancing Wigley III ship model in various regular waves are simulated, and the results are compared with existing and corresponding research data. Also, to check the applicability to practical ship model, flows around KRISO Container Ship (KCS) model advancing in calm water are numerically simulated. On the simulations, the trim and the sinkage are set free to compare the running attitude with some other experimental data. Moreover, flows around the KCS model in regular waves are also simulated.

Experimental and Numerical Study on the Effects of Bow Deck Shape on the Green Water

In this paper, the effects of bow deck shape on the green water are studied by numerical and experimental method. Varying the deck shapes to triangular, elliptic and circular, the thickness and advancing velocity of green water leading edge are compared using numerical method. Also the motion, the pressure on the vertical wall and the height on the deck of green water are compared among the three bow deck shapes in the heave and pitch motion free condition by experimental method. To remove the effects of the difference of motions among the deck shapes, numerical simulations are performed varying the deck shape with the same motion. In the same motion condition, smallest impulsive pressure occurred in the condition of elliptic deck shape.

Numerical Simulation of Two-dimensional Nonlinear Waves on Beaches Using a Smoothed Particle Hydrodynamics Method

In this paper, wave breakers which occur in two dimensional coasts are simulated using a SPH(Smoothed Particle Hydrodynamics) method which represents the movement of fluidic physical volume with particles. As continuative fluid is approximated to the particles, the simulations are performed using fully Lagrangian method without any grid system. Two-dimensional Navier-Stokes equations and continuity equation are used for the numerical simulations. To generate incident waves, a piston type wavemaker is employed. The accuracy of the wave which is numerically generated by the wavemaker is verified by comparing with analytical results. The computations are carried out with various wave heights and slopes. The wave patterns generated through the numerical simulations are compared with several existing experimental and computational results. Agreement between the experimental data and the computation results is comparatively good. Also, the breaker depth index and the breaker height index from the present calculations are compared with the existing experimental results, and the tendency is very similar.