• Smart Structures and Systems
• /
• 제9권3호
• /
• pp.231-251
• /
• 2012

The stochastic optimal control for a piezoelectric spherically symmetric shell subjected to stochastic boundary perturbations is constructed, analyzed and evaluated. The stochastic optimal control problem on the boundary stress output reduction of the piezoelectric shell subjected to stochastic boundary displacement perturbations is presented. The electric potential integral as a function of displacement is obtained to convert the differential equations for the piezoelectric shell with electrical and mechanical coupling into the equation only for displacement. The displacement transformation is constructed to convert the stochastic boundary conditions into homogeneous ones, and the transformed displacement is expanded in space to convert further the partial differential equation for displacement into ordinary differential equations by using the Galerkin method. Then the stochastic optimal control problem of the piezoelectric shell in partial differential equations is transformed into that of the multi-degree-of-freedom system. The optimal control law for electric potential is determined according to the stochastic dynamical programming principle. The frequency-response function matrix, power spectral density matrix and correlation function matrix of the controlled system response are derived based on the theory of random vibration. The expressions of mean-square stress, displacement and electric potential of the controlled piezoelectric shell are finally obtained to evaluate the control effectiveness. Numerical results are given to illustrate the high relative reduction in the root-mean-square boundary stress of the piezoelectric shell subjected to stochastic boundary displacement perturbations by the optimal electric potential control.

• 한국환경과학회지
• /
• 제17권2호
• /
• pp.257-267
• /
• 2008

Groundwater recharge characteristics in a fractured granite area, Mt. Geumjeong, Korea. was interpreted using bedrock groundwater and wet-land water data. Time series analysis using autocorreclation, cross-correlation and spectral density was conducted for characterizing water level variation and recharge rate in low water and high water seasons. Autocorrelation analysis using water levels resulted in short delay time with weak linearity and memory. Cross-correlation function from cross-correlation analysis was lower in the low water season than the high water season for the bedrock groundwater. The result of water level decline analysis identified groundwater recharge rate of about 11% in the study area.

• 한국음향학회지
• /
• 제13권4호
• /
• pp.20-31
• /
• 1994

다중 정현파의 주파수를 추정하는 최우도(ML) 방법은 주파수 추정에 정밀도를 보여주고 있으나, 최우도 함수가 주파수 추정에 쓰이는 경우 고도의 비선형성 때문에 추정에 많은 희생을 요구하고 있다. 본 논문에서는 최우도 방법의 비선형성을 개선하기 위해, 신호속에 포함된 정현 주파수의 추정을 용이하게 할 수 있는 단순화된 최우도 방법을 제시한다. 이 새로운 주파수 추정 방법을 백색 또는 칼라 잡음의 보기들에 적용하고, Monte-carlo 시뮬레이션을 실행하여 통계적 평균값, 평균 제곱근 및 상대 바이어스를 기존의 가장 우수한 방법인 MFBLP 방법과 비교한다. 또한 스펙트럼 파우어 밀도와 단위 원에서의 주파수 위치를 그림을 통하여 나타낸다.

• Structural Engineering and Mechanics
• /
• 제65권6호
• /
• pp.771-784
• /
• 2018

This study aims to investigate the stochastic response of isolated and non-isolated highway bridges subjected to spatially varying earthquake ground motion model. This model includes wave passage, incoherence and site response effects. The wave passage effect is examined by using various wave velocities. The incoherency effect is investigated by considering the Harichandran and Vanmarcke coherency model. The site response effect is considered by selecting homogeneous firm, medium and soft soil types where the bridge supports are constructed. The ground motion is described by power spectral density function and applied to each support point. Triple concave friction pendulum (TCFP) bearing which is more effective than other seismic isolation systems is used for seismic isolation. To implement seismic isolation procedure, TCFP bearing devices are placed at each of the support points of the deck. In the analysis, the bridge selected is a five-span featuring cast-in-place concrete box girder superstructure supported on reinforced concrete columns. Foundation supported highway bridge is regarded as three regions and compared its different situation in the stochastic analysis. The stochastic analyses results show that spatially varying ground motion has important effects on the stochastic response of the isolated and non-isolated bridges as long span structures.

• Wind and Structures
• /
• 제17권2호
• /
• pp.203-225
• /
• 2013

An analysis framework for vehicle-bridge dynamic interaction system under turbulent wind is proposed based on the relevant theory of wind engineering and dynamics. Considering the fluctuating properties of wind field, the stochastic wind velocity time history is simulated by the Auto-Regressive method in terms of power spectral density function of wind field. The bridge is represented by three-dimensional finite element model and the vehicle by a multi-rigid-body system connected by springs and dashpots. The detailed calculation formulas of unsteady aerodynamic forces on bridge and vehicle are derived. In addition, the form selection of wind barriers, which are applied as the windbreak measures of newly-built railways in northwest China, is studied based on the suggested evaluation index, and the suitable values about height and porosity rate of wind barriers are studied. By taking a multi-span simply-supported box-girder bridge as a case study, the dynamic response of the bridge and the running safety indices of the train traveling on the bridge with and without wind barriers are calculated. The limit values of train speed with respect to different wind velocities are proposed according to the allowance values in the design code.

• 한국지반공학회논문집
• /
• 제16권1호
• /
• pp.203-210
• /
• 2000

최근 대중교통 및 물류 운송 수단으로써 철도의 중요성이 부각되면서 부수적으로 발생되는 열차하중에 의한 환경진동은 국제표준화기구(ISO)의 기준에 반영될 정도로 관심의 대상이 되었다. 하지만 도심지의 시설물 또는 진동원 주위의 주거지역 조성 시, 이에 대한 체계적이고 합리적인 대책은 미흡한 실정이다. 따라서 현재까지 설계 및 시공 시에 주요 인자로 취급되지 않았던 동적 열차하중에 대한 지반진동의 예측과 저감대책의 마련이 절실히 요구되고 있다. 본 연구에서는 열차하중에 의한 철도연변의 지반진동을 분석/예측하기 위한 열차주행에 의한 하중산정 모델과 이에 의한 토공구간에서의 진동의 지중 전달과정 해석기술 및 전산프로그램을 개발하고 그에 따른 해석치와 실측치를 비교 검증하였다.

• 대한기계학회논문집
• /
• 제18권4호
• /
• pp.920-932
• /
• 1994

The combustion phenomena of a reciprocating engine is one of the most important processes affecting performance and emissions. One effective way to improve the engine combustion is to control the motion of the charge inside a cylinder by means of optimum induction system design, because the flame speed is mainly determined by the turbulence at compression(TDC) process in S.I. engine. It is believed that the tumble and swirl motion generated during intake breaks down into small-scale turbulence in the compression stroke of the cycle. However, the exact nature of their relationship is not well known. This paper describes cycle resolved LDV measurement of turbulent flow inside the cylinder of a 4-valve engine under motoring(non-firing) conditions, and studies the effect of intake port configurations on the turbulence characteristics using following parameters ; Eulerian temporal autocorrelation coefficient, turbulence energy spectral density function, Taylor micro time scale, integral time scale, and integral length scale.

• 한국의학물리학회지:의학물리
• /
• 제9권1호
• /
• pp.23-28
• /
• 1998

최근에 당뇨병이 말초혈관 혈류흐름에 치명적인 영향을 준다는 의학보고에 따라 기존의 혈액체취를 통한 혈액 속의 당치를 측정하는 방식 대신에 고분해능을 가진 LDF(Laser Dopller Flowmeter)를 이용하여 말초혈관 혈류를 측정하여 정상인과 당뇨병환자와의 차이점에 대하여 연구하였다. 여러 가지 분석 방법중, 본 연구에서는 주파수영역에서의 ASD방법을 통하여 그 차이를 분석하였다. 실험대상은 음성 신경병증, 양성 신경병증, 갑상선 기능 항진증, 갑상선 기능 저하증 등 4개의 당뇨병환자군과 당뇨병에 대한 정상군에 대하여 LDF를 측정하였다. 그 결과 모든 당뇨환자군은 정상인에 대하여 모든 진동수영역에서 낮은 ASD를 가져 당뇨병이 말초혈관 혈류운동에 손실을 가져옴을 확인하였고, 신경병증을 가진 환자와 갑상선기능저항증 당뇨환자군은 거의 모든 진동수 영역에서 낮은 ASD를 가졌다.

• Wind and Structures
• /
• 제31권2호
• /
• pp.165-183
• /
• 2020

The first step of performance-based design for transmission lines is the determination of wind fields as well as wind loads, which are largely depending on local wind climate and the surrounding terrain. Wind fields in a mountainous area are very different with that in a flat terrain. This paper firstly investigated both mean and fluctuating wind characteristics of a typical mountainous wind field by wind tunnel tests and computational fluid dynamics (CFD). The speedup effects of mean wind and specific turbulence properties, i.e., turbulence intensity, power spectral density (PSD) and coherence function, are highlighted. Then a hybrid simulation framework for generating three dimensional (3D) wind velocity field in the mountainous area was proposed by combining the CFD and proper orthogonal decomposition (POD) method given the properties of the target turbulence field. Finally, a practical 220 kV transmission line was employed to demonstrate the effectiveness of the proposed wind field generation framework and its role in the performance-based design. It was found that the terrain-induce turbulence effects dominate the performance-based structural design of transmission lines running through the mountainous area.

• 한국지진공학회:학술대회논문집
• /
• 한국지진공학회 2001년도 추계 학술발표회 논문집 Proceedings of EESK Conference-Fall 2001
• /
• pp.67-74
• /
• 2001

This paper presents the Safe Shutdown Earthquake(SSE) input motion for the seismic design of the Advanced Power Reactor 1400(APR1400). The Design Ground Response Spectra(DGRS) far the SSE is based on the design spectrum specified in regulatory Guide(RG) 1.60 of U.S. Nuclear Regulatory Commission(US NRC), anchored to a Peak Ground Acceleration(PGA) of 0.3g and enriched in the high frequency range. This SSE seismic input motion is to be applied to the seismic analysis as the free-field seismic motion at the ground surface of both the rock and generic soil sites fur APRI1400. The enrichment for APR1400 seismic input motion is performed considering the current US NRC regulations, the seismic hazard studies performed by the Lawrence Livermore National Laboratory (LINL) and Electric Power Research Institute(EPRI) for the Central and Eastern United States nuclear power plant sites, and the seismic input motions used in the design certifications of the three existing U.S. advanced standard plants. It is represented by a set of DGRS and the accompanying Target Power Spectral Density(PSD) Function in both the horizontal and vertical directions.