Hyeok-Jun Koh

Power Estimation and Optimum Design of a Buoy for the Resonant Type Wave Energy Converter Using Approximation Scheme

This paper deals with the resonant type of a WEC (wave energy converter) and the determination method of its geometric parameters which were obtained to construct the robust and optimal structure, respectively. In detail, the optimization problem is formulated with the constraints composed of the response surfaces which stand for the resonance period(heave, pitch) and the meta center height of the buoy. Use of a signal-to-noise ratio calculated from normalized multi-objective results with the weight factor can help to select the robust design level. In order to get the sample data set, the motion responses of the power buoy were analyzed using the BEM (boundary element method)-based commercial code. Also, the optimization result is compared with a robust design for a feasibility study. Finally, the power efficiency of the WEC with the optimum design variables is estimated as the captured wave ratio resulting from absorbed power which mainly related to PTO (power take off) damping. It could be said that the resultant of the WEC design is the economical optimal design which satisfy the given constraints.

Model Test of Dual-Buoy Wave Energy Converter using Multi-resonance

In this study, we proposed a new type of dual-buoy wave energy converter (WEC) exploiting multi-resonance and analyzed the experimental results from a model test in a 2-D wave flume. A dual-buoy WEC using multi-resonance has two advantages: high efficiency at the resonant frequencies and the potential to extend the frequency range available to extract wave power from the WEC. The suggested WEC was composed of an outer buoy and an inner buoy sliding vertically inside the outer buoy. As the power take-off device, a linear electric generator (LEG) consisting of permanent magnets and coils fixed at each buoy was adopted. Electricity was produced by the relative heave motion between the two buoys. To search for the optimal shape of a dual-buoy WEC, we conducted experiments on the heave motion of a two-body system in regular waves without an LEG installed. Model tests with six combinations of experimental models were conducted in order to find the motion characteristics of a dual-buoy WEC. It was found that model 2, which included a ring-shaped appendage to move the resonant frequency of the outer buoy toward a high value, showed a higher relative heave response amplitude operator (RAO) curve than model 1. In addition, the double-peak shape of the heave RAO curve shown for model 2 indicated the extension of the frequency range for extracting wave power in irregular waves.

Model Test for Heave Motion Reduction of a Circular Cylinder by a Damping Plate

Motion reduction of an offshore structure at resonant frequency is essential for avoiding critical damage to the topside and mooring system. A damping plate has a distinct advantage in reducing the motion of a floating structure by increasing the added mass and the damping coefficient. In this study, the heave motion responses of a circular cylinder with an impermeable and a permeable damping plate attached at the bottom of the cylinder were investigated thru a model test. The viscous damping coefficients for various combinations of porosity were obtained from a free-decay test by determining the ratio between any pair of successive amplitudes. Maximum energy dissipation occurred at a porous plate with a porosity P = 0.1008. Experimental results for regular and irregular waves were compared with an analytical solution by Cho (2011). The measured heave RAO and spectrum reasonably followed the trends of the predicted values. A significant motion reduction at resonant frequency was pronounced and the heaving-motion energy calculated by the integration of the area under the heave motion spectrum was reduced by more than 75% by the damping plate. However, additional energy dissi pation by eddies of strong vorticity and flow separation inside a porous damping plate was not found in the present experiments.

A Study on Experiment for Countermeasures to Overtopping at Tapdong Revetment in Jeju

Abstract − In this paper, model test on the Tapdongs revetment at Jeju-si has been conducted to figure out thecauses of frequent occurrence of wave overtopping and to suggest the action plan. In the model test, the reflec-tion coefficients were measured according to the change of wave periods for with and without armor stones.Also, the wave overtopping rate and the wave pressures inside revetment structure for 4 types of upper blockwere estimated for various wave heights and wave periods, which are chosen based on the NE design wave with50 year return period. It is found that the increase of the upper structure’s height and the modification of thecurved protruding shape are effective in reducing the overtopping rate.Keywords: Revetment(호안), Model test(모형실험), Reflection Coefficient(반사율), Overtopping(월파),Wave Pressure(파압) 1. 서론 호안(revetment)은 파랑으로부터 해안선의 후퇴와 상부의 도로,건물 등을 보호하기 위하여 해안선을 따라 건설되는 구조물이다.호안은 돌망태, 호안블럭, 옹벽, 석축 등의 형태로 축조되며, 호안의 형태에 따라 직립형, 경사형, 그리고 계단형 등으로 분류된다.직립형은 다른 형태에 비해 적은 비용으로 건설할 수 있다는 장점이 있다. 그러나 파랑의 처오름이나 월파에 대한 대응 효과가 적으며 높은 반사파로 인하여 호안 전면에서 심각한 세굴을 발생시킬수 있어 구조물의 안정성에도 문제가 될 뿐 아니라 해빈 변화에도나쁜 영향이 초래될 수 있다. 이를 방지하기 위해 구조물 전면에 적당한 크기의 피복석(armour stone)을 설치하는데 이는 파랑에너지를 감쇄시켜 호안을 보호할 뿐만 아니라 사석 공극 사이에 토사를저장하여 구조물의 이동을 방지하는데 도움을 준다. 한편 경사형과계단형 호안은 주로 1:3의 경사각으로 건설된다. 이 방식은 연안의이용성 및 친수환경 조성이 가능하며 전면의 세굴을 감소시키는 데효과가 있지만 직립형에 비하여 건설비가 비싸다는 단점이 있다. 월파는 수심, 파랑조건 및 호안 형상 등에 따라 그 거동 특성이결정되고, 생성된 월파는 호안 배후면에 직접적으로 영향을 주기때문에 파랑조건과 월파, 그리고 호안 배후면의 안정성은 서로 상관관계를 갖는다(Schttrumpf et al.[2002]). Owen[1980], Ahrensand Heimbaugh[1988] 등은 직립형 및 경사형 호안의 월파량에 대

DYNAMIC MODELING AND STRUCTURAL ANALYSIS OF MANTA-TYPE UUV

This paper describes the dynamic modeling, structural analysis, implementation and experimental test of a Manta-type Unmanned Underwater Vehicle (MUUV). Various controllers such as PID, Sliding mode, and Fuzzy and H∞ controllers are designed for depth and heading control in order to compare the performance of each controller based on simulation. In addition, experimental tests are carried out in a towing tank for depth keeping and heading angle tracking.