Key-Pyo Rhee

A Study on the Effects of Weight and Center of Gravity of a Planing Craft on Running Attitude

AbstractMotion characteristics of a planing craft are sensitively chang ed according to its weight and longitudinal center of gravity. In this paper, planing craft model tests were performed in calm water for various test conditions and Froude numbers. Sinkage and trim were measured to analyze the relations between the attitudes of a planing craft and the weight and center of gravity of it. Theoretical formula for the prediction of the attitudes of a prismatic planing hull was modified so that it can be applied t o the prediction of the attitudes of a non-prismatic planing hull, and the calculation results by the modified formula were in good agreements with the experimental data. ※Keywords: Planing craft(활주선), High-speed towing test(고속 예인 시험), Weight and Center of Gravity(하중 및 무게중심), Running attitude(항주 자세) 1. 서론 활주형 고속선은 선박의 중량을 부력으로 지지하는 통상의 배수량형 선박과는 달리 일정 속도 이상이 되면 주로 선저의 양력에 의해 자중의 대부분을 지지하게 되므로, 선체가 수면 위로 들어 올려져서 고속으로 항주할 수 있다. 이러한 활주 접수일: 2009년 3월 4일, 승인일: 2009년 5월 11일✝교신저자: kdj8277@snu.ac.kr, 010-3994-4007

Experimental Study on the Six Degree-of-Freedom Motions of a Damaged Ship Floating in Regular Waves

One of the most critical issues for ship owners, shipbuilders, and insurance companies is the operational safety. In particular, keeping damaged ships stable in waves is of great interest, because more nonconventional hull forms are being introduced for military and passenger vessels while international rules and regulations are becoming stricter. However, ship stability for damaged ships is quite different from that for intact ships as the assessment is very complicated and difficult due to the highly nonlinear behavior. Computational fluid dynamics (CFD) methods that solve the Navier-Stokes equations are acknowledged as the only viable approach to simulate and analyze these complex physical phenomena. Although there have been a number of research activities reported on damaged ship stability recently, most of them are not designed to validate CFD studies. For a data set to be valuable for CFD validation and development, model tests should eliminate unclear factors as much as possible. The main objective of this study is to establish an experimental database for CFD validation by collecting data from towing tank tests of a ship hulls six degree-of-freedom (6DOF) motion responses in regular waves for both intact and damaged conditions. A mooring system was designed to prevent drift motions of the ship model. Parametric roll was not observed when the ship was damaged, although it was observed for the intact ship in the same wave conditions. The mooring force acting on the ship model due to spring tension was also calculated.

Depth Control of a Submerged Body Near the Free Surface by LQR Control Method

AbstractThe submerged body near the free surface is disturbed by the 1s t and 2nd order wave forces, which results in unstable movements when no control is applied. In this paper, the vertical motions of the submerged body are analyzed, and the time-variant nonlinear system for the vertical motions of the submerged body is transformed to the time-invariant linear system in state space. Next, depth controller of the submerged body is designed by using LQR control, one of the modern optimal control technique. Numerical simulation shows that effective depth controls can be achieved by LQR control. ※Keywords: Submerged body(수중운동체), Vertical motion(연직면 운동), 1st and 2nd order Wave forces and moments(1,2차 파 강제력 및 파 강제 모멘트), Free surface effects(자유표면 효과), LQR control(LQR 제어) 1. 서론 수중운동체가 수면 근처에서 운항할 경우 파도에 의한 외력을 받게 되는데, 파 강제력을 파 진폭에 비례하는 1차항 힘과 2차항 힘인 시간 평균 표류력으로 나타내었다. 이 중에서 2차항 힘과 모멘트는 운동체를 수면으로 끌어올리는 역할을 한 접수일: 2009년 3월 24일, 승인일: 2009년 7월 20일✝교신저자: kdj8277@snu.ac.kr, 010-3994-4007 다. 따라서 수중운동체가 잠망경 심도로 운항할 때 원하는 심도를 유지 및 제어하기 위해서는 이러한 2차항 힘과 모멘트를 고려하여 선수와 선미의 수평타를 적절히 제어할 필요가 있다.수면 근처에서 파 강제력 및 파 강제 모멘트를 고려하여 수중운동체의 심도를 제어하는 연구는 이전부터 진행되어 왔다. Richards and Stoten (1982)은 수중운동체의 형상을 고려한 파강제력 및 파 강제 모멘트 수학 모형을 정립하였다. Dumlu and Istefanopulos(1995)는 수중운동체 심

Experimental Study on Free Roll Decay Motions of a Damaged Ship for CFD Validation Database

Among many factors to be considered for higher safety level requirements, the hull stability in intact and damaged conditions in seaways is of utmost importance. Since the assessment of a damaged ship is complicated due to the highly non-linear behavior, it is widely acknowledged that computational fluid dynamics (CFD) methods are one of the most feasible approaches. Although many research activities are being reported on the damaged ship stability recently, most of them are not designed for validation of CFD studies. In this study, well-designed model tests were performed to build a CFD validation database, which is essential in developing better CFD methods for the damage stability assessment. The geometry of the damaged compartment and test conditions were determined based on preliminary CFD simulations. Free roll decay tests in calm water with both intact and damaged ships were performed and the roll motion characteristics were compared. The damaged ship showed a larger roll damping coefficient and more rapid decrease of roll amplitude than the intact ship. The primary reason of these efforts can be explained by the movement of the flooded water.

On the Vertical Plane Dynamics Modeling and Depth Control of a Submerged Body Moving beneath Free Surface

In this paper, submerged body dynamics model in vertical plane which can include the effect of free surface and wave is suggested to simulate the motions of submerged body moving beneath free surface precisely. A controller is designed, which can maintain a constant depth below the mean sea level and minimize the pitch angle. Numerical simulations show that the designed controller is effective on depth keeping and minimizing pitch angle in regular waves and irregular waves.

An Experimental Study of the Submerged Depth Effect on the Manoeuvrability in a Horizontal Plane of an Underwater Vehicle

In this paper, horizontal manoeuvrability of an underwater vehicle near free surface was investigated. Planar Motion Mechanism(PMM) tests were performed at the shallow depth within 4.5 times of vehicle`s diameter. Hydrodynamic coefficients related to the horizontal movement were estimated from the measured data using Least SQuare(LS) method and analyzed at each submerged depth. Furthermore, horizontal dynamic stability, trajectory of turning and zigzag test were investigated for the various depths. As underwater vehicle is positioned nearer to the free surface, forces increase and moment decreases. Tested model was found to be stable only at the depth 0.5 times of vehicle`s diameter.

An Experimental Study on the Vertical Motion of a High-Speed Planing Craft in Regular Following Waves

It is well known that when a high-speed planing craft travels in following seas it experiences long-periodic motions due to low encounter frequency, and it often loses its course keeping stability. Therefore, it is necessary to study the sea-keeping performance and stability of it in the following seas. In this paper, the vertical motions of a planing craft were measured in following regular waves, and the test results were compared with the theoretical results. In the case of the same encounter frequency, non-dimensionalized motion amplitudes become larger as Froude number is higher, and non-dimensionalized motion amplitudes in head waves are larger than those in follow ing waves. The mean values of the motions in following waves are similar to the running attitudes of a craft in calm water at the same Froude number. ※Keywords: High-speed planing craft(고속 활주선), Regular following waves(선미 규칙파), High-speed towing test(고속 예인 시험), Vertical motion response(연직면 운동응답)

The Effect of Hull Appendages on Maneuverability of Naval Ship by Sensitivity Analysis

선박의 조종성능 추정은 설계 단계에서 수행되는 필수적인 부 분이다. 특히나 함정은 상선에 비해 비교적 빠른 속도를 가지며, 실제 운용 중에 상선에서는 볼 수 없는 회피기동을 하기 때문에 더 좋은 조종성능이 요구되며, 그에 따라 초기 설계 단계에서 더 정확한 조종성능의 추정이 필요하다. 함정은 일반적으로 상선에 비해 더 많은 부가물들 (스케그, 스트럿, 샤프트 라인 등)이 부착 되며, 작은 방형계수를 가진 탓에 넓은 면적이 바깥으로 노출되 어 있다. 직진 안정성을 좋게 하기 위해 선미에 추가적으로 부착 하는 스케그 외에 다른 부가물들도 역시 직진 안정성에 영향을 미친다. 그러므로 함정의 설계 단계에서 각 종 선체 부가물들이 조종성능에 미치는 영향을 반드시 고려해 주어야 한다. 함정은 일반적으로 2축2타 추진 시스템을 가지는데, 이 때문 에 기존의 조종운동 수학모델은 다소 부정확한 부분들이 있다. 2 축2타 상선의 조종성능 추정에 관한 선행 연구가 몇몇 있었다 (Lee, et al., 1997; Sohn, et al., 2001; Lee, et al., 2003; Kim, et al., 2006). 실제 함정의 조종성능 추정 연구와 관련하여 대표 적으로 사용되는 함정 공시선형 DTMB 5415 모델의 조종성능 추 정에 관한 연구가 있었다 (Hess, et al., 2008; Miller, et al., 2008; Toxopeus, et al., 2008; Carrica, et al., 2013). 이에 비 해 상대적으로 선체 부가물이 조종성능에 미치는 영향에 관련된 직접적 연구는 수행된 바가 많지 않다. Biandardi (1997) 는 선체 부가물에 의한 영향을 선체의 종방향 형상 변화로 고려하여 조종 성능 추정에 반영하였으며, Jacobs (1963) 와 Ankudinov (1993) 는 선체 부가물이 조종성능에 미치는 영향을 선체와는 별개로 고 려하여 조종성능 추정에 반영하였다. Lee, et al. (1998) 은 컨테 이너선의 러더 및 부가물 그리고 선미형상을 변화시켜가며 구속 모형시험을 수행하여, 부가물이 직진 안정성 향상에 미치는 영향 을 실험적으로 입증하였다. 하지만 여기까지 연구된 내용에서 조 종성능에 영향을 미치는 선체 부가물은 오직 스케그에 한정되어 pISSN:1225-1143, Vol. 51, No. 2, pp. 154-161, April 2014

The Effect of the Turning Rate of the Pod Propeller on the Roll Control System of the Cruise Ship

Recently, the application and installation of the pod propeller to the cruise ship is dramatically increased. It is because pod propulsion system allows a lot of flexibility in design of the internal arrangement of a ship. To reflect this trend, many researches have conducted to use the pod propeller for the roll stabilization of a ship. In the paper, a roll stabilization controller is designed by using fins and pod propellers as the control actuators for cruise ships. Two kinds of control algorithms are adopted for the roll control system; LQR (Linear Quadratic Regulator) algorithm and frequency-weighted LQR algorithm. Through the numerical simulation, the effect of the turning rate of the pod propeller on the roll control system is analyzed. Analysis of the simulation results indicated that the turning rate of the pod propellers is one of the important parameters which give the significant effects on the roll stabilization.

Study on Coning Motion Test for Submerged Body

A submerged body is sensitive to changes in the roll moment because of the small restoring moment and moment of inertia. Thus, a method for predicting the roll-related hydrodynamic coefficients is important. This paper describes a deduction method for the hydrodynamic coefficients based on the results of a coning motion test. A resistance test, static drift test, and coning motion test were performed to obtain the coefficients in the towing tank of Seoul National University. The sum of the hydrodynamic force, inertial force, gravity, and buoyancy was measured in the coning motion test. The hydrodynamic force was deduced by subtracting the inertial force, gravity, and buoyancy from the measured force. The hydrodynamic coefficients were deduced using the regression method.