• Steel and Composite Structures
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• 제20권3호
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• pp.623-649
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• 2016

Most of the early studies on plates vibration are focused on two-dimensional theories, these theories reduce the dimensions of problems from three to two by introducing some assumptions in mathematical modeling leading to simpler expressions and derivation of solutions. However, these simplifications inherently bring errors and therefore may lead to unreliable results for relatively thick plates. The main objective of this research paper is to present 3-D elasticity solution for free vibration analysis of continuously graded carbon nanotube-reinforced (CGCNTR) rectangular plates resting on two-parameter elastic foundations. The volume fractions of oriented, straight single-walled carbon nanotubes (SWCNTs) are assumed to be graded in the thickness direction. In this study, an equivalent continuum model based on the Eshelby-Mori-Tanaka approach is employed to estimate the effective constitutive law of the elastic isotropic medium (matrix) with oriented, straight carbon nanotubes (CNTs). The proposed rectangular plates have two opposite edges simply supported, while all possible combinations of free, simply supported and clamped boundary conditions are applied to the other two edges. The formulations are based on the three-dimensional elasticity theory. A semi-analytical approach composed of differential quadrature method (DQM) and series solution is adopted to solve the equations of motion. The fast rate of convergence of the method is demonstrated and comparison studies are carried out to establish its very high accuracy and versatility. The 2-D differential quadrature method as an efficient and accurate numerical tool is used to discretize the governing equations and to implement the boundary conditions. The convergence of the method is demonstrated and to validate the results, comparisons are made between the present results and results reported by well-known references for special cases treated before, have confirmed accuracy and efficiency of the present approach. The novelty of the present work is to exploit Eshelby-Mori-Tanaka approach in order to reveal the impacts of the volume fractions of oriented CNTs, different CNTs distributions, various coefficients of foundation and different combinations of free, simply supported and clamped boundary conditions on the vibrational characteristics of CGCNTR rectangular plates. The new results can be used as benchmark solutions for future researches.

• Wind and Structures
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• 제15권4호
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• pp.331-343
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• 2012

Thunderstorm downbursts are responsible for numerous structural failures around the world. The wind characteristics in thunderstorm downbursts containing vortex rings differ with those in 'traditional' boundary layer winds (BLW). This paper initially performs an unsteady-state simulation of the flow structure in a downburst (modelled as a impinging jet with its diameter being $D_{jet}$) using a computational fluid dynamics (CFD) method, and then analyses the pressure distribution on a solar updraft tower (SUT) in the downburst. The pressure field shows agreement with other previous studies. An additional pair of low-pressure region and high-pressure region is observed due to a second vortex ring, besides a foregoing pair caused by a primary vortex ring. The evolutions of pressure coefficients at five orientations of two representative heights of the SUT in the downburst with time are investigated. Results show that pressure distribution changes over a wide range when the vortices are close to the SUT. Furthermore, the fluctuations of external static pressure distribution for the SUT case 1 (i.e., radial distance from a location to jet center x=$D_{jet}$) with height are more intense due to the down striking of the vortex flow compared to those for the SUT case 2 (x=$2D_{jet}$). The static wind loads at heights z/H higher than 0.3 will be negligible when the vortex ring is far away from the SUT. The inverted wind load cases will occur when vortex is passing through the SUT except on the side faces. This can induce complex dynamic response of the SUT.

• 한국항해항만학회:학술대회논문집
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• 한국항해항만학회 2006년도 Asia Navigation Conference
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• pp.134-143
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• 2006

Out of all types of motions the critical motions leading to capsize is roll. The dynamic amplification in case of roll motion may be large for ships as roll natural frequency generally falls within the frequency range of wave energy spectrum typical used for estimation of motion spectrum. Roll motion is highly non-linear in nature. Den are various representations of non-linear damping and restoring available in literature. In this paper an uncoupled non-linear roll equations with three representation of damping and cubic restoring term is solved using a perturbation technique. Damping moment representations are linear plus quadratic velocity damping, angle dependant damping and linear plus cubic velocity dependant damping. Numerical value of linear damping coefficient is almost same for all types but non-linear damping is different. Linear and non-linear damping coefficients are obtained form free roll decay tests. External rolling moment is assumed as deterministic with sinusoidal form. Maximum roll amplitude of non-linear roll equation with various representations of damping is calculated using analytical procedure and compared with experimental results, which are obtained form forced tests in regular waves by varying frequency with three wave heights. Experiments indicate influence of non-linearity at resonance frequency. Both experiment and analytical results indicates increase in maximum roll amplitude with wave slope at resonance. Analytical results are compared with experiment results which indicate maximum roll amplitude analytically obtained with angle dependent and cubic velocity damping are equal and difference from experiments with these damping are less compared to non-linear equation with quadratic velocity damping.

• 한국해안해양공학회지
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• 제14권1호
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• pp.19-33
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• 2002

경사식 방파제의 설계에 필요한 정보를 제공할 목적으로 수리특성들의 계산을 비교적 쉽게 수행하기 위한 수치해석기법의 수립을 시도하였다. 수치해석은 방파제 외부의 파동장과 내부 공극매체에서의 파동장을 결합시킴으로써 반사율, 전달율(월파전달율과 투과전달율) 그리고 개체내부의 간극수압을 계산하는 것이다. 외부 파동장은 장파가정의 근사 이론을, 내부 파동장은 경계요소법을 이용하여 해석하였다. 수치해석결과를 수리실험결과와 비교한 결과, 반사율과 전달율은 허용오차 범위내에서 비교적 잘 일치하는 것으로 나타났다. 또한, 간극수압은 정량적으로는 다소의 차이를 보이나 정성적으로는 대체적으로 유사한 성향을 보이는 것으로 나타났다. 정량적 차이를 극복하기 위하여 전사면에서의 비선형 파동장에 의한 동수압을 직접 계측하여 수치해석에 입력하는 방안을 제시하였다

• 정보처리학회논문지D
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• 제11D권2호
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• pp.281-292
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• 2004

공간 질의에 대한 선택율 추정은 가장 효율적인 실행 계획을 찾는데 이용되는 매우 중요한 과정이다. 공간 도메인이 큰 경우, 기존 연구의 요약정보는 상대적으로 적은 정보로 선택율을 추정하기 때문에 좋은 선택율을 유지하기 어렵다. 따라서, 이 논문에서는 작은 저장공간에 공간요약정보를 압축하는 새로운 기법인 MW 히스토그램을 제안한다. 이 히스토그램은 MinSkew 분할 알고리즘과 웨이블릿 변환이 결합되어 적은 저장공간에서도 타당한 선택율과 압축효과를 얻을 수 있고, 동적 갱신에 대해 효율적으로 대처할 수 있는 구조를 가진다. 실험 결과를 통하여, 버켓 수가 0.3M/6인 MW 히스토그램이 5%-20% 질의에서 평균적으로 좋은 성능을 보이고 있어, MW 히스토그램이 적은 저장공간에서 더 좋은 선택율을 얻을 수 있음을 확인시켜주었다.

• Journal of Power Electronics
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• 제15권4호
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• pp.1047-1053
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• 2015

A time-domain simulation tool to predict the dynamic power output of wind turbines in an offshore wind farm was developed in this study. A wind turbine model consisting of first or second order transfer functions of various wind turbine elements was combined with the Ainslie's eddy viscosity wake model to construct the simulation tool. The wind turbine model also includes an aerodynamic model that is a look up table of power and thrust coefficients with respect to the tip speed ratio and pitch angle of the wind turbine obtained by a commercial multi-body dynamics simulation tool. The wake model includes algorithms of superposition of multiple wakes and propagation based on Taylor's frozen turbulence assumption. Torque and pitch control algorithms were implemented in the simulation tool to perform max-Cp and power regulation control of the wind turbines. The simulation tool calculates wind speeds in the two-dimensional domain of the wind farm at the hub height of the wind turbines and yields power outputs from individual wind turbines. The NREL 5MW reference wind turbine was targeted as a wind turbine to obtain parameters for the simulation. To validate the simulation tool, a Danish offshore wind farm with 80 wind turbines was modelled and used to predict the power from the wind farm. A comparison of the prediction with the measured values available in literature showed that the results from the simulation program were fairly close to the measured results in literature except when the wind turbines are congruent with the wind direction.

• 한국멀티미디어학회논문지
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• 제11권4호
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• pp.516-524
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• 2008

본 논문은 기존에 연구되지 않았던 옷의 독특한 특징들을 묵시적 제한방법을 사용하여 강력한 제한력으로 구현하는 새로운 모델링 방법을 제안한다. 기존의 명시적 제한방법인 Baumgarte 안정화 방법은 해에 빠르게 수렴하도록 하기 위해 사용자가 시뮬레이션에 따라서 값이 달라지는 안정화 변수 값을 선택해야 하고, 시뮬레이션의 시간 간격 사용에 있어서 안정화에 한계가 있는 단점들이 있다. 본 논문은 큰 시간 간격에도 안정적이고, 안정화 변수 값을 요구하지 않고, 물리적으로도 적합한 물체의 움직임을 보장하는 묵시적 제한방법을 사용한다. 또한 묵시적 제한 방법의 계산 복잡도는 Baumgarte 안정화 방법과 같다. 본 논문은 묵시적 제한방법의 수식과 제한의 오차 분석을 설명하였고 옷의 솔기, 단추, 옷 주름, 옷의 구김, 과잉 늘어짐 방지 등의 복잡한 옷의 요소들에 대한 모델링 방법을 제시하였다. 본 논문에서 제안된 방법은 외부 상황에 의해 각종 제한들이 자동적으로 설정되고 제거되어 계산 비용을 절약함과 동시에 옷의 독특한 특징들의 구현을 통해 현실감 있는 옷 시뮬레이션의 결과들을 얻었다.

• 대한원격탐사학회지
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• 제21권3호
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• pp.173-188
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• 2005

The present study aims to derive and compare narrow and broad bandwidths of ocean color sensor’s algorithms for the study of monitoring highly dynamic coastal oceanic environmental parameters using high-resolution imagery acquired from Multi-spectral Camera (MSC) on KOMPSAT-2. These algorithms are derived based on a large data set of remote sensing reflectances ($R_{rs}$) generated by using numerical model that relates $b_b/(a + b_b)$ to $R_{rs}$ as functions of inherent optical properties, such as absorption and backscattering coefficients of six water components including water, phytoplankton (chl), dissolved organic matter (DOM), suspended sediment (SS) concentration, heterotropic organism (he) and an unknown component, possibly represented by bubbles or other particulates unrelated to the first five components. The modeled $R_{rs}$ spectra appear to be consistent with in-situ spectra collected from Korean waters. As Kompsat-2 MSC has similar spectral characteristics with Landsat-5 Thematic Mapper (TM), the model generated $R_{rs}$ values at 2 ㎚ interval are converted to the equivalent remote sensing reflectances at MSC and TM bands. The empirical relationships between the spectral ratios of modeled $R_{rs}$ and chlorophyll concentrations are established in order to derive algorithms for both TM and MSC. Similarly, algorithms are obtained by relating a single band reflectance (band 2) to the suspended sediment concentrations. These algorithms derived by taking into account the narrow and broad spectral bandwidths are compared and assessed. Findings suggest that there was less difference between the broad and narrow band relationships, and the determination coefficient $(r^2)$ for log-transformed data [ N = 500] was interestingly found to be $(r^2)$ = 0.90 for both TM and MSC. Similarly, the determination coefficient for log-transformed data [ N = 500] was 0.93 and 0.92 for TM and MSC respectively. The algorithms presented here are expected to make significant contribution to the enhanced understanding of coastal oceanic environmental parameters using Multi-spectral Camera.

• 디지털융복합연구
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• 제17권10호
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• pp.169-176
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• 2019

최근 한국경제는 일본의 경제보복조치와 미국과 중국 간 무역전쟁, 한국은행의 0.25% 기준금리 인하, 한국의 경제성장률 전망치 수정으로 혼잡한 상황이다. 본 연구의 목적은 한국종합주가지수와 소비자물가지수, 국고채(3년) 금리, 전산업의 매출실적을 분석하여 각 지수별로 종합주가지수에 미치는 영향을 살펴보는데 있다. 분석기간은 2003년 1월부터 2019년 6월까지이고, 각 지수별로 종합주가지수에 미치는 정도를 분석하였다. 본 연구는 수치분석에서 상관계수와 회귀분석을, 모형분석에서 분포도, 사분위도, Scatter, Box-Plot, Impulse Response를 살펴보았다. 이를 통해 각 지수별 변동성과 동행성을 찾아보았다. 분석결과 종합주가지수는 매출실적과 국고채금리 순으로 높은 동행성과 변동률을 보인 반면 소비자물가지수와는 매우 낮은 상관관계를 보여 주었다. 앞으로 종합주가지수는 매출실적과 금리의 영향을 받을 것으로 보여 꾸준한 관심이 필요해 보인다.

• 대한조선학회논문집
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• 제30권3호
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• pp.75-89
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• 1993

본 논문은 기본적으로 미국 기계학회의 성능시험 코드에 준하여 운동 모형시험에 불확실성 해석을 적용하였다. 총 불확실성은 시험준비 과정, 계기의 검증, 데이타 취득 과정 및 분석과정에서 발생하는 오차요인들과 오차 전파 방정식에 의해 구하여진 민감도와 결합하여 구해진다. 저항추진 시험과 같은 정적시험과 운동시험과 같은 동적시험의 차이들이 불확실성 해석의 모든 과정동안 분명하게 구별되며 또한 비대칭 고정오차항목이 고려되었다. 본 불확실성 해석이 수행된 운동 응답 항목은 무차원화된 주파수의 함수로 나타낸 상하동요와 종동요의 RAO이다. 운동시험에 대한 불확실성 해석은 측정 정도를 향상시키고 오차들을 정량화하는데 유용할 뿐만아니라 여러가지 운동 해석 프로그램에 대한 검증의 기준으로 제시될 수 있다.