Il-Hyoung Cho

Wave absorbing system using inclined perforated plates

The interaction of oblique monochromatic incident waves with horizontal/inclined/ dual porous plates is investigated in the context of two-dimensional linear potential theory and Darcys law (the normal velocity of fluid passing through a thin porous plate is linearly proportional to the pressure difference across it). The developed theory is verified by both small-scale and full-scale experiments. First, matched eigenfunction expansion (MEE) solutions for a horizontal porous plate are obtained. The relationship between the plate porosity and the porous parameter is obtained from systematic model tests by using six porous plates with different sizes and spacing of circular holes. Secondly, a multi-domain boundary-element method (BEM) using simple-sources (second-kind modified Bessel function) is developed to confirm the MEE solutions and to apply to more general cases including inclined or multiple porous plates. The BEM-based inner solutions are matched to the eigenfunction-based outer solutions to satisfy the outgoing radiation condition in the far field. Both analytical and BEM solutions with the developed empirical porous parameter agree with each other and correlate well with both small-scale data from a two-dimensional wave-tank test and full-scale measurement in a large wave basin. Using the developed predictive tools, wave-absorption efficiency is assessed for various combinations of porosity, water depth, submergence depth, wave heading, and plate/wave characteristics. In particular, it is found that the performance can be improved by imposing the proper inclination angle near the free surface. The optimal porosity is near porosity P=0.1 and the optimal inclination angle is around 10° as long as the plate length is greater than 20 % of the wavelength. Based on the selected optimal parameters (porosity = 0.1 and inclination angle=11.3°), the effective wave-absorption system for MOERIs square basin is designed.

Extraction of Wave Energy Using the Coupled Heaving Motion of a Circular Cylinder and Linear Electric Generator

*Department of Ocean System Engineering, Jeju National Universi ty, Jeju, Korea**Department of Civil Engineering, Kyongju University, Kyongju, KoreaKEY WORDS: Heave motion 수직운동, Linear electric generator 선형발전기, Power absorbtion 에너지 흡수, Matched eigenfunction expansion method 고유함수전개법, Double resonance 이중 공진, Circular cylinder 원기둥ABSTRACT: The feasibility of wave energy extraction from a heaving trunca ted cylinder and the corresponding response of the linear elect ric generator (LEG) composed of spring, magnet, and coil has been i nvestigated in the frame of three-dimensional linear potential theory. The heaving motion of a circular cylinder is calculated by means of the matched eigenfunction expansion method. Further, the analytical results are validated by numerical results using the ANSYS AQWA commercial code. By the action of a heaving circular cylinder, the magnet suspended by a spring can slide vertically inside the heaving cylinder. The mechanical po wer is extracted from the magnet motion relative to the coil/st ator which is attached to the cylinder. The coupled ODE of a heaving cylinder and LEG sys tem in waves is derived to obtain the magnet motion relative to a cylinder. To maximize the relative motion of the magnet, both the buoy draft and the LEG system parameters (spring stiffness, damping) should be selected properly for generating the double resonance considering the pe ak frequency of the target spectrum.교신저자 조일형: 제주시 제주대학로 66, 064-754-3482, cho0904@jejunu.ac.kr

Design of the dual-buoy wave energy converter based on actual wave data of East Sea

Abstract A new conceptual dual-buoy Wave Energy Converter (WEC) for the enhancement of energy extraction efficiency is suggested. Based on actual wave data, the design process for the suggested WEC is conducted in such a way as to ensure that it is suitable in real sea. Actual wave data measured in Korea’s East Sea (position: 36.404 N° and 129.274 E°) from May 1, 2002 to March 29, 2005 were used as the input wave spectrum for the performance estimation of the dual-buoy WEC. The suggested WEC, a point absorber type, consists of two concentric floating circular cylinders (an inner and a hollow outer buoy). Multiple resonant frequencies in proposed WEC affect the Power Ttake-off (PTO) performance of the WEC. Based on the numerical results, several design strategies are proposed to further enhance the extraction efficiency, including intentional mismatching among the heave natural frequencies of dual buoys, the natural frequency of the internal fluid, and the peak frequency of the input wave spectrum.

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.

Survey of Seafloor at Chagwi-do of Jeju Island to Select 60-m-class Sea Test Bed of Wave Energy Converter

The purpose of this study was to investigate the sea floor using a seismic profiler in the northern part of Chagwi-do of Jeju Island in order to select the optimal location for the 60-m-class berth of a sea test bed for wave energy converters and provide basic environmental data for designing a suction anchor. The echo types of the seismic profiles were classified based on the study of Kim et al. (2016a), and the location for installing the suction anchor was selected based on a sediment thickness of more than 10 m. The physical properties of the surface sediments were determined by analyzing the sediment samples obtained from 16 grab sample points. Based on the investigation and analysis, we proposed a survey area in the North-Eastern sea as an optimum location for the 60-m-class berth where the suction anchor could be installed. Received 13 March 2017, revised 14 March 2017, accepted 4 August 2017 Corresponding author Il-Hyoung Cho: +82-10-9232-1253, cho0904@jejunu.ac.kr c 2017, The Korean Society of Ocean Engineers This is an open access article distributed under the terms of the creative commons attribution non-commercial license (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Design and analysis of tubular permanent magnet linear generator for small-scale wave energy converter

This paper reports the design and analysis of a tubular permanent magnet linear generator (TPMLG) for a small-scale wave-energy converter. The analytical field computation is performed by applying a magnetic vector potential and a 2-D analytical model to determine design parameters. Based on analytical solutions, parametric analysis is performed to meet the design specifications of a wave-energy converter (WEC). Then, 2-D FEA is employed to validate the analytical method. Finally, the experimental result confirms the predictions of the analytical and finite element analysis (FEA) methods under regular and irregular wave conditions.

Survey of Sedimentary Environment and Sediment at the West-Northern Site of Chagwi-do nearby Jeju Island

The sedimentary environment and sediment were surveyed at the West-Northern site of Chagwi-do nearby Jeju Island for the design of the embedded suction anchor system of 10 MW-class floating wave-offshore wind hybrid power generation system. According to the classification scheme of Chough et al.[2002], the echo type of the seismic profiles using the chirp III was classified. As a results, the center and west-northern area of survey site were proved to be type I-3 where subbottom layer with thickness 5~15 m exists under the flat seafloor. On the other hands, the east-southern area were regarded to be type I-1, I-2 and III-1 where seafloor reflection is much stronger than type I-3. Also, the physical tests (unit weight, moisture content, grain size, liquid limit, specific gravity) were performed with samples taken from 8 fixed locations. It is found that the sand (SP), the sand blended with silt (SM) and the mixture of SP-SM are distributed uniformly on the survey area.

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