• 한국전산유체공학회지
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• 제11권4호
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• pp.62-66
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• 2006

High-speed flight vehicle have various cavities. The supersonic cavity flow is complicated due to vortices, flow separation, reattachment, shock waves and expansion waves. The general cavity flow phenomena includes the formation and dissipation of vortices, which induce oscillation and noise. The oscillation and noise greatly affect flow control, chemical reaction, and heat transfer processes. The supersonic cavity flow with high Reynolds number is characterized by the pressure oscillation due to turbulent shear layer, cavity geometry, and resonance phenomenon based on external flow conditions. The resonance phenomena can damage the structures around the cavity and negatively affect aerodynamic performance and stability. In the present study, we performed numerical analysis of cavities by applying the unsteady, compressible three dimensional Reynolds-Averaged Navier-Stokes(RANS) equations with the ${\kappa}-{\omega}$ turbulence model. The cavity model used for numerical calculation had a depth(D) of 15mm cavity aspect ratio (L/D) of 3, width to spanwise ratio(W/D) of 1.0 to 5.0. Based on the PSD(Power Spectral Density) and CSD(Cross Spectral Density) analysis of the pressure variation, the dominant frequency was analyzed and compared with the results of Rossiter's Eq.

• 응용통계연구
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• 제21권3호
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• pp.547-560
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• 2008

스펙트럼 밀도함수(spectral density function)는 시계열 자료가 정상성(stationarity)을 만족하는 경우에 주파수 영역(frrqllrnFr domain)에서 시계열 자료의 자기공분산함수(auto-covariance function)을 결정짓는 함수이고, 평활된 주기도(smoothed periodogram)는 스펙트럼 밀도함수의 일치 추정량(consistent estimator)이 됨이 잘 알려져 있다. 본 연구에서는 시계열 자료를 평활된 주기도를 이용하여 군집화하는 방법을 소개한다. 최근 김희영과 박만식 (2007)의 연구에 의하면 이 거리는 정상시계열들을 효율적으로 분류하고 있음을 알 수 있다. 본 연구는 시계열 자료를 분류하는데 사용된 기존의 거리들을 간략히 소개하고, 우리나라 22개 지역에서 1987년 1월부터 2007년 12월까지 측정한 월별 강수량 자료를 대상으로 평활된 주기도 거리를 이용하여 지역을 군집화한다.

• Smart Structures and Systems
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• 제15권1호
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• pp.15-40
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• 2015

This paper proposes a structural damage identification approach based on the power spectral density transmissibility (PSDT), which is developed to formulate the relationship between two sets of auto-spectral density functions of output responses. The accuracy of response reconstruction with PSDT is investigated and the damage identification in structures is conducted with measured acceleration responses from the damaged state. Numerical studies on a seven-storey plane frame structure are conducted to investigate the performance of the proposed damage identification approach. The initial finite element model of the structure and measured acceleration measurements from the damaged structure are used for the identification with a dynamic response sensitivity-based model updating method. The simulated damages can be identified accurately without and with a 5% noise effect included in the simulated responses. Experimental studies on a steel plane frame structure in the laboratory are performed to further verify the accuracy of response reconstruction with PSDT and validate the proposed damage identification approach. The locations of the introduced damage are detected accurately and the stiffness reductions in the damaged elements are identified close to the true values. The identification results demonstrated the accuracy of response reconstruction as well as the correctness and efficiency of the proposed damage identification approach.

• 대한토목학회논문집
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• 제28권1A호
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• pp.59-67
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• 2008

원전 구조물의 내진해석에서 사용하는 시간이력 함수는 적절한 하중을 보장하기 위하여 설계응답스펙트럼에 부합할 뿐만 아니라 최소 PSD함수 이상의 PSD함수를 갖도록 함으로서 각각의 진동수에서 일정 크기 이상의 에너지를 갖고 있어야 한다. 도로교 구조물에서도 구조물의 대형화와 정교화로 인한 구조물의 공공 기능의 안전성이 더욱 강조되어, 설계 목적의 PSD함수의 규정이 필요하지만 적절한 절차가 개발이 되지 않아 현실적으로 적용하기 어려운 실정이다. 본 연구에서는 다수의 인공지진을 이용하여 표준 PSD함수를 계산하는 절차를 제시하고 현재의 도로교 내진설계 스펙트럼에 부합하는 표준 PSD 함수를 제안하였다.

• Earthquakes and Structures
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• 제4권4호
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• pp.383-396
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• 2013

In earthquake engineering area, the critical excitation method is an approach to find the most severe earthquake subjected to the structure. However, given some earthquake constraints, such as intensity and power, the critical excitations have spectral density functions that often resonate with the first modes of the structure. This paper presents a non-stationary critical excitation that is capable of exciting the main modes of the structure using a non-uniform power spectral density (PSD) that is similar to natural earthquakes. Thus, this paper proposes a new method to estimate the power and intensity of earthquakes. Finally, a new method for the linear seismic design of structures using a modified non-stationary critical excitation is proposed.

• Journal of the Korean Statistical Society
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• 제23권1호
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• pp.79-88
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• 1994

This paper concerns the problem of estimating the spectral density function in the analysis of stationary time series data. A kernel type estimate is considered, which entails choice of bandwidth. A data-driven bandwidth choice is proposed, and it is obtained by plugging some suitable estimates into the unknown parts of a theoretically optimal choice. A theoretical justification is give for this choice in terms of how far it is from the theoretical optimum. Furthermore, an empirical investigation is done. It shows that the data-driven choice yields a reliable spectrum estimate.

• Journal of applied mathematics & informatics
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• 제28권1_2호
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• pp.515-521
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• 2010

In multivariate analysis, the inversion formula of the Stieltjes transform is used to find the density of a spectral distribution of random matrices of sample covariance type. Let $B_n\;=\;\frac{1}{N}Y_nY_n^TT_n$ where $Y_n\;=\;[Y_{ij}]_{n\;{\times}\;N}$ is with independent, identically distributed entries and $T_n$ is an $n\;{\times}\;n$ symmetric non-negative definite random matrix independent of the $Y_{ij}$'s. In the present paper, using the inversion formula of the Stieltjes transform, we will find that the limiting distribution of $B_n$ has a continuous density function away from zero.

• Structural Engineering and Mechanics
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• 제68권1호
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• pp.39-51
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• 2018

In this study, a frequency domain model updating method is presented using power spectral density (PSD) data. It uses the sensitivity of PSD function with respect to the unknown structural parameters through a decomposed form of transfer function. The stiffness parameters are captured with high accuracy through solving the sensitivity equations utilizing the least square approach. Using numerically noise polluted data, the model updating results of a truss model prove robustness of the method against measurement and mass modelling errors. Results prove the capabilities of the method for parameter estimation using highly noise polluted data of low ranges of excitation frequency.

• 한국통신학회논문지
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• 제31권6C호
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• pp.569-574
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• 2006

PPM(Pulse Position Modulation)을 기반으로 하는 초광대역(ultra-wide bandwidth) 무선전송 시스템의 일반적인 전력 밀도 함수(power spectral density function)를 유도한다. 또한 개선된 선 스펙트럼(line spectrum) 특성과 우수한 BER(Bit Error Rate) 성능을 갖는 초광대역 무선전송 시스템을 제안한다.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2003년도 추계학술대회논문집
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• pp.298-303
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• 2003

The eccentric rotor system has various problems by vibration of unbalance mass. Specially, it is difficult to analyze vibration problem for the drum washing machine due to correlation of transmission paths. We analyze the mechanical structure by multidimensional spectral density to identify the maximum vibration magnitude at the frequency domain. It estimates the coherence function of the signals to pass the transmission paths. In case of the drum washing machine, because the transmission paths are correlated partially it needs to determinate the priority ranking. And the correlated parts are eliminated using conditioned spectral density function. Finally the shielding effect method confirms the reasonability of the modeling.