• Proceedings of the Korean Institute of Navigation and Port Research Conference
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• 2005.10a
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• pp.153-159
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• 2005

Recently, various novel numerical models based on Navier-Stokes equation rave been developed for calculating wave motions in the sea with coastal or ocean structures. Among those models, Volume Of Fluid (VOF) method might be the most popular one, and it has been used for numerical simulations of wave motions including complicated phenomena of wave breakings. VOF method, however, needs enormous computation time and large computational storage memories in general, thus it is practically difficult to use VOF method for calculations in the case of random waves because long and stable computation ( e.g. for more than 100 significant wave periods) is required to obtain statistically meaningful results. On the other hand of the wave motion is potential motion, Boundary Element Method (BEM), which is a much faster and more accurate method than VOF method, am be effectively used. The aim of this study is to develop a new efficient model applicable to calculations of wave motion and/or wave-structure interactions under random waves. To achieve this, a strictly combined BEM-VOF model has been developed by making the best use of both methods' merits; VOF method is used in a restricted fluid domain around a structure where complicated phenomena of wave breakings may exist, and BEM is used in the other domains far from the disturbance where the wave motion may be assumed to be potential. The verification of the model was performed with numerical results for Stokes'5th order wave propagation and a random wave propagation.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.29 no.2
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• pp.92-108
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• 2017

Present work shows a numerical method to analysis of interaction analysis between solitary wave and onshore bridge. Numerical simulation is carried out by TWOPM-3D (three-dimensional one-field model for immiscible two-phase flows), which is based on Navier-Stokes solver. To do this, the solitary wave is generated numerically in numerical wave channel, and numerical results and experimental results were compared and analyzed in order to verify the applicability of force acting on an onshore bridge. From this, we discussed precisely the characteristics of horizontal and vertical forces (uplift and downward forces) changes including water level and velocity changes due to the variation of solitary wave height, water depth, onshore bridge's location and type, and number of girder. Furthermore, It is revealed that the maximum horizontal and vertical forces acting on the girder bridge show different varying properties according to the number of girder, although each maximum force acting on the girder bridge is proportional to the increasement of incident solitary wave height, and the entrained air in the fluid flow affects the vertical force highly.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.32 no.1
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• pp.55-68
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• 2020

In order to investigate the hydraulic characteristics of a boundary layer streaming over the beach cusps appeared in swells prevailing mild seas, we numerically simulated the shoaling process of Edge waves over the beach cusp. Synchronous Edge waves known to sustain the beach cusps could successfully be duplicated by generating two obliquely colliding Edge waves in front of beach cusps. The amplitude A_B and length L_B of Beach Cusp were elected to be 1.25 m and 18 m, respectively based on the measured data along the Mang-Bang beach. Numerical results show that boundary layer streaming was formed at every phase of shoaling process without exception, and the maximum boundary layer streaming was observed to occur at the crest of sand bar. In RUN 1 where the shortest waves were deployed, the maximum boundary layer streaming was observed to be around 0.32 m/s, which far exceeds the amplitude of free stream by two times. It is also noted that the maximum boundary layer streaming mentioned above greatly differs from the analytical solution by Longuet-Higgins (1957) based on wave Reynolds stress. In doing so, we also identify the recovery procedure of natural beaches in swells prevailing mild seas, which can be summarized such as: as the infra-gravity waves formed in swells by the resonance wave-wave interaction arrives near the breaking line, the sediments ascending near the free surface by the Phase II waves orbital motion were carried toward the pinnacle of foreshore by the shoreward flow commenced at the steep front of breaking waves, and were deposited near the pinnacle of foreshore due to the infiltration.

• KOREAN JOURNAL OF CROP SCIENCE
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• v.26 no.2
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• pp.125-136
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• 1981

This experiment was conducted to study the influence of different cultivation periods on some morphological and agronomic characteristics of rice plant at Iri, the Honam plain, in 1979. Early maturing varieties showed little delay in heading with late transplanting, but intermediate and late maturing varieties showed more than 7 day delay in heading. The number of days required to reach heading was over 55 and 67 days after transplanting in early and intermediate-late maturing varieties, respectively. There was significantly postitive correlation between the length of vegetative stage and the grain yield. The mean temperature during ripening period was significantly correlated with the grain yield and the percentage of filled grains. The length, width, and thickness of leaf increased with earlier transplanting and the elongation degree between upper and lower leaves showed the same tendency. The productivity of dry matter and the translocation rate of photosynthate to spikelet decreased with late transplanting mainly due to low temperature. LAI was also influenced by transplanting date showing close correlation with yield. Early transplanting was desirable for assimilation and dry matter accumulation.

• Journal of the Korean Society for Marine Environment & Energy
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• v.10 no.4
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• pp.187-192
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• 2007

Impact on cylindrical surface caused by plunging breaking waves is investigated experimentally. The breaking waves are generated in a wave flume by decreasing the wave maker frequencies linearly and focusing the generated wave components at one specific location. The breaking wave packets are based on constant wave steepness spectrum. Three inclination angles of cylinder are applied to examine the effect of contact angle between cylinder and front surface of breaking waves. Also, the effect of cylinder diameter on pressure distribution and its peak value is investigated by adopting three cylinders with different diameters. The longitudinal location of cylinder is slightly moved in eight different points to find out a probable maximum value of impact pressure. The pressures and total force on cylinder surface are measured by piezo-electric pressure sensors and 3-components load cell with 30kHz sampling rate. The variation of peak impact pressures and forces is analyzed in terms of cylinder diameter, inclination angle and location. Also, the pressure distribution on cylindrical surface is examined. The cylinder location and surface position are more important parameters that govern the magnitude and shape of peak pressures, while the cylinder diameter and inclined angle are relatively insignificant. In a certain conditions, the impact phenomenon becomes very unstable which results in a large variation of measured valves in repeated runs.

• Korean Journal of Weed Science
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• v.18 no.2
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• pp.116-121
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• 1998

Field experiment were carried out to elucidate the changes of weed occurrence according to the sowing time and to establish the efficient weed control system in dry seeded rice cultivation. E crus-galli and D. sanguinalis are tow major weeds at the earlier sowings, but C. difformis was dominating at the later sowing. In the case of E. crus-galli, there was a tendency of gaining its dominance values as the rice growth stage advances. The later the sowing time was, the more weeds occurred at the early rice growth stages: However, when measured at heading, total dry weight was greater at the earlier sowings. Yield loss due to weeds were also greater at the earlier sowings. Community diversity of weeds was greater at the later sowings, while specific weed was gradually dominating as rice growth stage advances, regardless of sowing time. When herbicides were applied before 15 DAS, weeds were effectively controlled only upto 35 DAS. So systemic herbicide application, one on 0 to 10 DAS and the other on just after flooding, appeared to be recommendable in direct seeded rice cultivation.

• Journal of Navigation and Port Research
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• v.42 no.6
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• pp.378-387
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• 2018

In this paper, the towing force for the LNG bunkering barge was investigated. Currently, LNG bunkering barge is being developed as an infrastructure for the bunkering of LNG (Liquefied Natural Gas), an eco-friendly energy source. In the case of the LNG bunkering barge, self-propulsion is considered through retrofit from an operating point. Therefore, the LNG bunkering barge's shape is similar to that of the ship as compared to a towed barge, so a rule of the towed barge overestimates the towing force. In order to improve accuracy, the calm water resistance was calculated using ITTC 1978 method which considers wave resistance by the Rankine source method. The added resistance in waves was calculated using the modified radiated energy method which considers the shortwave correction method of NMRI. The performance of the towing resistances through the calm water resistance and the added resistance in waves was compared to rules associated with towed barges.

• Journal of the Korean Society of Marine Environment & Safety
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• v.25 no.4
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• pp.433-440
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• 2019

Offshore working environments such as ships, offshore oil and gas plants, and offshore wind turbines are isolated and directly exposed to rough seas, which pose high risks of safety accidents. Therefore, all workers in offshore plants should be able to cope with emergency situations and must be qualified according to relevant laws and regulations such as the International Convention on Standards of Training, Certification and Watchkeeping for Seafarers (STCW Convention) and Offshore Petroleum Industry Training Organization (OPITO) standards. In particular, marine workers should be able to perform cardiopulmonary resuscitation (CPR) in isolated locations or enclosed and confined spaces such as those in life-boats, life-rafts, rescue-boats, etc. Because the floor material is made of rubber, it may be difficult to perform chest compressions in life-rafts used to escape from emergency situations in ships or offshore plants. Chest compressions performed on life-rafts may reduce the accuracy of CPR and increase fatigue for those providing aid. To measure the accuracy and fatigue of those performing CPR in life-rafts, 15 experimenters with more than five years of experience as first aid instructors were exposed to different CPR environments in a marine safety training center equipped with an artificial wave generator. The results showed that the accuracy of CPR in the classroom was 99.6 %, but that in various life-raft environments was only 84 %. T-verification of the two sites confirmed the reduced accuracy of CPR performed on life-rafts. CPR on life-rafts should be performed in groups of two and with the use of automated chest compression devices.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.31 no.3
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• pp.170-179
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• 2019

In this study, the wave responses in a 'Y'shape water channel resonator for amplifying wave energy of a low density has been investigated. A water channel resonator is composed of the long channel and wave guider installed at the entrance. If the period of the incident waves coincides with the natural period of the fluid in a water channel resonator, resonance occurs and the internal fluid amplifies highly to a standing wave form. In order to analyze the wave response in a water channel resonator, we used the matched asymptotic expansion method and boundary element method. The both results were in good agreement with the results of the model test carried out in the two-dimensional wave tank of Jeju National University. Wave guider has an optimum length and installation angle according to the period of the incident wave, and especially effective in enhancing the amplification factor in a period range deviated from the resonance period. It is expected that the wave energy can be effectively extracted by placing the point absorber wave energy converter at the position of anti-node where the maximum wave height is formed by the internal fluid resonance.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.34 no.1
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• pp.59-69
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• 2021

Recently, large-scale offshore and coastal structures have been constructed owing to the increasing interest in eco-friendly energy development. To achieve this, precise simulations of waves are necessary to ensure the safe operations of marine structures. Several experiments are required in the field to understand the offshore wave; however, in terms of scale, it is difficult to control variables, and the cost is significant. In this study, numerical waves under various wave conditions are produced using a piston-type wavemaker, and the produced wave profiles are verified by comparing with the results from a numerical wave tank (NWT) modeled using the smoothed particle hydrodynamics (SPH) method and theoretical equations. To minimize the effect by the reflected wave, a mass-weighted damping zone is set at the right end of the NWT, and therefore, stable and uniform waves are simulated. The waves are generated using the linear and Stokes wave theories, and it is observed that the numerical wave profiles calculated by the Stokes wave theory yield high accuracy. When the relative depth is smaller than two, the results show good agreement irrespective of the wave steepness. However, when the relative depth and wave steepness are larger than 2 and 0.04, respectively, the errors are negligible if the measurement position is close to the excitation plate. However, the error is 10% or larger if the measurement position is away from the excitation location. Applicable target wave ranges are confirmed through various case studies.