• Journal of Advanced Marine Engineering and Technology
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• v.39 no.4
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• pp.393-398
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• 2015

A very important part in vibration analysis is to calculate the frequency response function (FRF). In general, a large sized or/and complicated structure has many thousands to millions of degrees. Therefore, the FRF cannot be calculated by the traditional analysis method using an inverse matrix. This paper presents a new FRF calculation method of a superstructure by synthesizing sub-structure modes, of which the DOF can be deduced by partitioning into some sub-structures. To confirm its analysis results, the method was applied to an assembled plate ($B300{\times}L900{\times}t5mm$) with three diagonal sub-plates($B300{\times}L300{\times}t5mm$) in series and compared with the measured data. The test results have were comparable those of the calculated ones with an error less than 5%.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 1994.10a
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• pp.40-45
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• 1994

기계시스템의 비선형특성 해석을 위하여 여러가지 방법이 활용되고 있는데, 이들은 Nyquist 선도의 찌그러짐(distortion), Hilbert 변환, 복원력면(restoring force surface), NARMAX, 고차 주파수응답함수(higher order frequency response function), DPE(direct parameter estimation)를 이용한 방법등이다. 이들중 고차 FRF(frequency response function)는 그 개념이 선형시스템의 FRF와 유사하여 비선형시스템의 해석방법으로서 주목을 받고 있으나 아직은 고차 FRF의 특성에 대한 이론적 연구 단계이고, 고차 FRF로부터 비선형특성을 정량적으로 해석하는 연구는 거의 이루어지지 않고 있다. 다항식으로 표시되는 비선형성을 갖는 시스템이 정현파가진을 받을 때 그 응답의 가진주파수 성분은 가진력진폭과 고차 FRF의 무한급수로 나타낼 수 있다. 가진력의 진폭을 변화시켜가며 응답을 측정하고, 고차항을 무시하면 고차 FRF의 값을 근사적으로 구할 수 있다. 고차 FRF는 비선형 시스템의 매개변수의 식으로 나타낼 수 있으므로 이로부터 비선형 매개변수를 추정할 수 있다. 본 논문에서는 비선형강성과 비선형감쇠를 갖는 1자유도 시뮬레이션 시스템에 이 매개변수 추정법을 각각 적용함으로써 이 방법의 가능성을 고찰하였다.

• Journal of Convergence for Information Technology
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• v.10 no.7
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• pp.131-137
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• 2020

The characteristics of forced vibration with excited sinusoidal force was introduced. Also, numerical analyses and FRF in frequency domain were performed in detail. In this regard, the responses of displacement, velocity and acceleration were investigated in a forced vibration model. The FRF characteristics in real and imaginary part around natural frequency are also discussed. This response approach of forced vibration in time domain is used for the identification and monitoring of sinusoidal forced vibration. For acquiring a displacement, velocity and acceleration, a numerical technique of Runge-Kutta-Gill method was performed. For the FRF(frequency response function), These responses are used. Also, the FRF can represent the intrinsic characteristics of the forced vibration. These performed results and analysis are successful in each damped condition for the forced vibration model. After numerical analysis of the different mass, damping and stiffness, the forced vibration response characteristics with sinusoidal force was discriminated considering its amplitude and frequency simultaneously.

• Journal of the Korean Institute of Intelligent Systems
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• v.12 no.1
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• pp.39-43
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• 2002

In the frequency response analysis using a modal method, it is very important to determine the number of modes involved with the formulation of a frequency response function. Most engineers are inclined to determine mode truncation with their experience. But it is difficult for non-experts to decide the mode truncation reasonably in many problems of dynamic analyses. In this study, fuzzy theory is used to standardize the empirical determination of mode truncation so that not only the experts but also non-experts can decide a Proper mode truncation easily. Fuzzy rule base is based on the simulation results using finite element method. Numerical simulations show that the developed mode truncation method is a very effective method to choose the number of the considered modes.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.30 no.5
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• pp.79-87
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• 2002

Acoustic responses to pressure oscillations in axisymmetric combustion chamber are numerically investigated to examine the qualitative trend of acoustic instability in liquid rocket engine. Chamber operating condition and excitation frequency of oscillating pressure are selected as exciting parameters of acoustic instability. Artificial perturbation is simulated by total-pressure oscillation with sine wave at chamber inlet. Many approximations and simplifications are introduced without losing the essence of acoustic pressure response. First, steady-state solution for each operating condition is obtained and next, transient analysis is conducted. Depending on operating condition and excitation frequency, the distinct response characteristics are brought. Weak-strength flames and high-frequency excitation tend to cause sensitive acoustic pressure response leading to unstable pressure field. These results are analyzed based on the correlation with acoustic pressure responses from the previous works adopting laminar flamelet model.

• Journal of the Earthquake Engineering Society of Korea
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• v.11 no.2 s.54
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• pp.69-79
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• 2007

In this paper, a technique to detect structural damage and estimate its severity using peaks and zeros of frequency response functions (FRFs) is developed. The peaks in FRFs represent the natural frequencies of the structure and the zeros provide additional information. The characteristics of peaks and zeros are defined and the calculation procedure to obtain the peaks and zeros from the relationship between frequency response function and stiffness and mass matrices are clearly explained. A structural system identification theory which is utilizing the sensitivity of stiffness of a structural member to eigenvalues, i.e., peaks and zeros, is established. The proposed method can identify damage location and its severity, with natural and zero frequencies, by estimating structural stiffness of the structure in the process of making a analytical model The accuracy and feasibility is demonstrated by numerical models of a spring-mass system and a beam structure.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.16 no.1
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• pp.95-104
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• 2003

This paper deals with the analytic and FEM analyses of sloshing frequency response of incompressible, invicid and irrotational flow in two dimensional rectangular tank. We use Laplace equation based on potential theory as governing equation. For small amplitude sloshing motion, the linearized free surface condition was applied and the analytic solution as obtained by the separation of variables. To simulate the effect of the energy dissipation due to viscous damping, artificial viscous coefficient is introduced and the divergence of response at resonance frequencies may be avoided by this coefficient. This problem was solved by FEM using 9-node elements in order to predict the maximum amplitude of sloshing response. Numerical results of free surface height, fluid pressure and fluid force show good agreement with those by analytic solution. After verifying the test FEM program, we analyze the frequency response characteristics of sloshing to the fluid height.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.13 no.2
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• pp.1-10
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• 1993

Frequency response functions(FRF) are the most fundamental data for the frequency domain identifications of structural systems. In this paper, an improved method for estimating FRF's is presented. The new FRF estimator takes the weighted average of two conventional estimators, $H_1$(f) and $H_2$(f), utilizing the fact that $H_2$(f) gives more accurate estimate at resonance, while $H_1$(f) yields better results at antiresonances. Based on the estimated FRF's, the modal parameters of the structures, such as, natural frequencies, damping ratios and mode shapes, are also estimated. The effectiveness of the proposed method is investigated through numerical and experimental studies. The estimated results indicate that the proposed estimator gives more accurate results than other estimators.

• The Journal of the Acoustical Society of Korea
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• v.17 no.7
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• pp.61-73
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• 1998

라우드스피커의 비선형 음향특성을 해석하기 위하여 라우드스피커 다이아프램의 비 선형특성을 실험적으로 분석하였다. 라우드스피커를 집중질량계로 모델링하고, 출력소음이 없을 때 적용이 가능한 하모닉 발란스법과 출력소음이 있을 때도 적용할 수 있는 통계적 방 법인 라그란지 배수법을 사용하여 비선형 매개변수를 규명하였다. 70Hz 이하의 주파수 영역 에서는 라그란지 배수법이 2차 하모닉 왜곡특성에서 상대적으로 좋은 결과를 보였으며, 3차 하모닉 왜곡특성에서는 하모닉 발란스법이 실험결과와 잘 일치하였다. 전압가진 및 전류 가 진실험을 통하여, 일반화된 주파수 응답함수를 유도하였고 하모닉 왜곡 및 상호변조 특성을 고찰하였다. 고차 주파수 응답특성은 라우드스피커 비선형 매개변수의 변동에 따른 다이아 프램의 비선형특성을 해석하는데 유용한 수단이 될 수 있음도 확인하였다.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.41 no.10
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• pp.905-913
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• 2017

In this work, a method of size optimization is proposed to maximize the natural frequency of a rod that consists of a hidden shape in one part and an exposed shape in the other. The frequency response function of a rod composed of two parts is predicted by using the frequency response functions of each of the parts instead of the shapes of the parts. The mass and stiffness matrices of the rod are obtained by using the mass and stiffness matrices of the equivalent vibration systems, which are obtained by applying the experimental modal analysis method to the frequency response functions of the parts. Through several numerical examples, the frequency response function obtained by using the proposed method is compared with that of a rod to validate the prediction method based on equivalent vibration systems. A size optimization problem is formulated for maximizing the first natural frequency of a combined rod, which is replaced with an equivalent vibration system, and a rod structure is optimized by using an optimization algorithm.