• International Journal of Naval Architecture and Ocean Engineering
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• 제5권2호
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• pp.227-245
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• 2013

Reliable strength assessment of the Liquefied Natural Gas (LNG) cargo containment system under the sloshing impact load is very difficult task due to the complexity of the physics involved in, both in terms of the hydrodynamics and structural mechanics. Out of all those complexities, the proper selection of the design sloshing load which is applied to the structural model of the LNG cargo containment system, is one of the most challenging one due to its inherent randomness as well as the statistical analysis which is tightly linked to the design sloshing load selection. In this study, the response based strength assessment procedure of LNG cargo containment system has been developed and proposed as an alternative design methodology. Sloshing pressure time history, measured from the model test, is decomposed into wavelet basis function targeting the minimization of the number of the basis function together with the maximization of the numerical efficiency. Then the response of the structure is obtained using the finite element method under each wavelet basis function of different scale. Finally, the response of the structure under entire sloshing impact time history is rapidly calculated by synthesizing the structural response under wavelet basis function. Through this analysis, more realistic response of the system under sloshing impact pressure can be obtained without missing the details of pressure time history such as rising pattern, oscillation due to air entrapment and decay pattern and so on. The strength assessment of the cargo containment system is then performed based on the statistical analysis of the stress peaks selected out of the obtained stress time history.

• Nuclear Engineering and Technology
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• 제44권4호
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• pp.369-378
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• 2012

A load-following operation in APR+ nuclear plants is necessary to reduce the need to adjust the boric acid concentration and to efficiently control the control rods for flexible operation. In particular, a disproportion in the axial flux distribution, which is normally caused by a load-following operation in a reactor core, causes xenon oscillation because the absorption cross-section of xenon is extremely large and its effects in a reactor are delayed by the iodine precursor. A model predictive control (MPC) method was used to design an automatic load-following controller for the integrated thermal power level and axial shape index (ASI) control for APR+ nuclear plants. Some tracking controllers employ the current tracking command only. On the other hand, the MPC can achieve better tracking performance because it considers future commands in addition to the current tracking command. The basic concept of the MPC is to solve an optimization problem for generating finite future control inputs at the current time and to implement as the current control input only the first control input among the solutions of the finite time steps. At the next time step, the procedure to solve the optimization problem is then repeated. The support vector regression (SVR) model that is used widely for function approximation problems is used to predict the future outputs based on previous inputs and outputs. In addition, a genetic algorithm is employed to minimize the objective function of a MPC control algorithm with multiple constraints. The power level and ASI are controlled by regulating the control banks and part-strength control banks together with an automatic adjustment of the boric acid concentration. The 3-dimensional MASTER code, which models APR+ nuclear plants, is interfaced to the proposed controller to confirm the performance of the controlling reactor power level and ASI. Numerical simulations showed that the proposed controller exhibits very fast tracking responses.

• 대한기계학회논문집
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• 제16권6호
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• pp.1179-1194
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• 1992

본 연구에서는 이 기존의 방법들을 개선하여 비균일격자계에서도 사용할 수 있는 비선형 함수관계를 제시하며, 이 방법과 기존의 해법들을 비교할 때 실제 유동장 에서의 해의 정확도 차이, 전산비용의 경제성등은 어느정도 인지를 밝히고, 또 개선된 해법이 앞서 제시한 좀 더 복잡한 유동장에서도 성공적으로 적용 가능한지의 여부를 판단하는 것을 그 목적으로 한다.

• 한국지진공학회논문집
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• 제9권2호통권42호
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• pp.71-80
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• 2005

유체자유수면의 동적거동을 합리적으로 예측하기 위해서는 비선형 특성을 보이는 자유수면의 동역학적 경계조건을 고려해야할 뿐만 아니라 시간에 따라 변화하는 자유수면의 위치변화에 따른 운동학적 경계조건을 고려하여야 한다. 이러한 문제는 대상구조물이 3차원이 될 경우 더욱 복잡해지므로 3차원 비선형 유체자유수면의 해석은 이론해의 도출이 어려우며 수치해석 방법을 이용하는 것이 효과적이다. 본 연구에서는 수치해석 안정성이 높고 3차원 문제에서도 하나의 변수로 유체거동을 모사할 수 있는 arbitrary Lagrangian-Eulerian approach 를 경계요소에 적용하여 효율적이며 안정적인 유체 대변형 해석기법을 개발하였다. 개발된 기법은 향후 자유수면의 비선형 효과를 고려한 유체-구조물 상호작용 해석에 효과적으로 적용할 수 있을 것으로 판단된다.

• 한국수자원학회논문집
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• 제43권3호
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• pp.283-294
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• 2010

본 연구의 목적은 저유량에서 발산하는 문제를 해결하여 다양한 흐름과 불규칙한 하도단면을 반영하는 안정적이고 정교한 동적수질해석모형을 개발하는데 있다. 이에 본 연구에서는 기존 모형들의 저유량에서 발산하는 문제를 해결하고자 확산파 운동방정식 기법을 이용하여 수리해석모형인 DyHYD를 개발하였으며, 수질해석모형은 자생 BOD와 수온, DO, TN, TP 등 12가지 수질농도를 계산할 수 있는 DyQUAL을 개발하였다. 개발된 모형의 적용성을 검토하기 위하여 가상하도 및 실제 낙동강유역에서 모의를 수행한 후 실측치와 비교 검토함으로써 모형의 적용성과 신뢰성을 검토하였다. 모형의 수행결과를 관측치와의 비교를 통해 Nash-Sutcliffe 계수를 산정하였으며, 보정과정에서는 유량과 BOD, TN, TP의 Nash-Sutcliffe 계수는 0.391에서 0.591의 범위를 나타내며, 검증과정에서는 0.704에서 0.902의 범위를 나타내었다.

• 한국항해학회지
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• 제5권2호
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• pp.49-58
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• 1981

One of the main purposes of the marine gyrocompass follow-up system is to preserve the sensitive part from the wandering error due to the frictional or torsional torque around the vertical axis. This error can be diminished through the rapid follow-up action, which minimizes the relative azimuthal angular displacement between the sensitive and follow-up parts and shortens the duration of the same displacement. But an excessive rapidity of the follow-up action would result in a sustained oscillation to the system. Therefore, to design a new type of the follow-up system, the theoretical annlysis of the problems concerned should be studied systematically by introducing the control theory. This paper suggest a concrete procedure for the optimal adjustment of the gyrocompass follow-up system, utilizing the mathematic model and the stability informations formerly investiaged by the author. For theoptimal determination of the adjustable paramfter K, the performance index(P.I.), ITSE(Intergral of the Time multiplied by the Squared Error) is proposed, namely, P.I. = $\int_{0}^{\infty} t \cdot e^{2}(t)dt$ where t is time and e(t) means control error. Then, the optimal parameter minimizing the performance index is calculated by means of Parseval's theorem and numerical computation, and the validity of the obtained optimal value of the parameter Ka is examined and confirmed through the simulations and experiments. By using, the proposed method, the optimal adjustment can be performed deterministically. But, this can not be expected in the conventional frequency domain analysis. While the Mps of the original system vary to the extent of from 0.98 to 46.27, Mp of the optimal system is evaluated as 1.1 which satisfies the generally accepted frequency domain specification.

• 한국전산구조공학회논문집
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• 제31권6호
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• pp.331-337
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• 2018

본 논문은 MLS(moving least squares) 차분법의 1차 미분 근사함수를 바탕으로 시간에 따른 수치해석이 가능한 해석기법을 제시한다. 오직 1차 미분 근사함수로만 지배방정식을 이산화했으며, 근사함수를 조립하는 형태로 전체 시스템 방정식을 구성하여 차분법으로 이산화된 운동방정식이 유한요소법(finite element method)과 유사한 모습을 갖게 되었다. 운동방정식을 시간적분하기 위해서 중앙차분법(central difference method)을 사용하였다. 유한요소 알고리즘을 통해서 MLS 차분법과 유한요소법의 고유진동 해석을 수행하였으며, 두 해석결과를 비교하였다. 또한, 동적해석 결과를 기존의 2차 미분 근사함수를 활용한 해석결과와 함께 도시함으로써 제안된 수치기법의 정확성을 검증하였다. 1차 미분 근사함수를 조립하는 과정에서 해석결과의 떨림현상이 억제되었으며 상대적으로 균일한 응력분포를 구할 수 있었다.

• 한국해양공학회지
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• 제35권1호
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• pp.38-49
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• 2021

Slug flow is the most common multi-phase flow encountered in oil and gas industry. In this study, the hydrodynamic features of flow in pipes investigated numerically using computational fluid dynamic (CFD) simulations for the effect of slug flow on the vertical and bent pipeline. The compressible Reynold averaged Navier-Stokes (RANS) equation was used as the governing equation, with the volume of fluid (VOF) method to capture the outline of the bubble in a pipeline. The simulations were tested for the grid and time step convergence, and validated with the experimental and theoretical results for the main hydrodynamic characteristics of the Taylor bubble, i.e., bubble shape, terminal velocity of bubble, and the liquid film velocity. The slug flow was simulated with various air and water injection velocities in the pipeline. The simulations revealed the effect of slug flow as the pressure occurring in the wall of the pipeline. The peak pressure and pressure oscillations were observed, and those magnitudes and trends were compared with the change in air and water injection velocities. The mechanism of the peak pressures was studied in relation with the change in bubble length, and the maximum peak pressures were investigated for the different positions and velocities of the air and water in the pipeline. The pressure oscillations were investigated in comparison with the bubble length in the pipe and the oscillation was provided with the application of damping. The pressures were compared with the case of a bent pipe, and a 1.5 times higher pressures was observed due to the compression of the bubbles at the corner of the bent. These findings can be used as a basic data for further studies and designs on pipeline systems with multi-phase flow.

• 전기학회논문지
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• 제60권8호
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• pp.1606-1616
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• 2011

The blood pressure measuring equipment, which is being supplied and used most widely by being recognized convenience and accuracy now generally, is oscillometric blood pressure monitor. However, a change in blood pressure is basically influenced by diverse elements such as each individual's physiological status and physical condition. Thus, the measurement of blood pressure, which used single element called oscillation in blood pressure of being conveyed to cuff, is not considered on physiological elements such as cardiovascular system status and blood vessel stiffness index, and on external elements, thereby being quite in error. Accordingly, this study detected diverse bio-signals and body informations in each individual as the measurement subject such as ECG, PPG, and Korotkoff Sound in order to enhance convenience and accuracy of measuring blood pressure in the complex measurement equipment, thereby having extracted regression method for compensation in error of oscillometric blood pressure measurement on the wrist, and having improved accuracy of measuring blood pressure. To verify a method of improving accuracy, the blood pressure value in each of SBP, DBP, MAP was acquired through 4-stage experimental procedure targeting totally 51 subjects. Prior to experiment, the subjects were divided into two groups such as the experimental group for extracting regression method and the control group for verifying regression method. Its error was analyzed by comparing the reference blood pressure value, which was obtained through the auscultatory method, and the oscillometric blood pressure value on the wrist. To reduce the detected error, the blood pressure compensation regression method was calculated through multiple linear regression analysis on elements of blood pressure, individual body information, PTT, HR, K-Sound PSD change. Verification was carried out on improving significance and accuracy by applying the regression method to the data of control group. In the experimental results, as a result of confirming error on the reference blood pressure value in SBP, DBP, and MAP, which were acquired through applying regression method, the results of $-0.47{\pm}7.45$ mmHg, $-0.23{\pm}7.13$ mmHg, $0.06{\pm}6.39$ mmHg could be obtained. This is not only the numerical value of satisfying the sphygmomanometer reference of AAMI, but also shows the lower result than the numerical value in SBP : $-2.5{\pm}12.2$ mmHg, DBP : $-7.5{\pm}8.4$ mmHg, which is the mean error in the experimental results of Brram's research for verifying accuracy of Omron RX-M, which shows relatively high accuracy among wrist sphygmomanometers. Thus, the blood pressure compensation could be confirmed to be made within significant level.

• 한국항공우주학회지
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• 제46권3호
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• pp.197-209
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• 2018

본 연구에서는 전진 비행하는 쿼드 틸트 로터(Quad Tilt Rotor, QTR)에 대한 수치적 연구를 수행하여, 각 구성 요소에 의한 간섭효과와 위상차 및 로터 회전 방향 등 로터 운용조건에 의한 영향을 분석하였다. 효율적인 로터 유동장 계산을 위해 오픈소스 CFD 코드인 OpenFOAM에 Actuator Surface Method(ASM) 기법을 결합한 해석자를 사용하였다. 전 후방 날개의 양력은 날개 끝단에 부착된 로터의 회전 방향에 따라 증감하며 특히 후방 날개에서는 전 후방 로터에 의한 간섭효과가 복합적으로 나타난다. 전 후방 로터의 위상차에 의한 날개의 공력 성능 변화는 상대적으로 미미함을 확인하였다. 로터에서는 날개로 인한 폐쇄효과 발생에 따라 국소적으로 높은 추력이 발생한다. 특히, 후방 로터에서는 전방 나셀에서 발생한 후류의 간섭효과로 인해 전방 로터 대비 높은 국소적 추력이 발생한다. 또한 로터 간의 위상차에 따라 추력 요동의 진폭이 감소할 수 있음을 확인하였으며 운용 조건에 따른 전 후방 로터의 성능과 전체 비행체의 공력 성능을 비교, 분석하였다.