• 한국소음진동공학회논문집
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• 제13권3호
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• pp.210-216
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• 2003

A nonlinear dynamic modeling method for cantilever beams undergoing axially oscillating motion is presented in this paper. Hybrid deformation variables are employed for the modeling method with which frequency response characteristics of axially oscillating cantilever beams are investigated. It is shown that the geometric nonlinear effects of stretching and curvature play important roles to accurately predict the frequency response characteristics. The effects of the amplitude and the damping constant on the frequency characteristics are also exhibited.

• Nuclear Engineering and Technology
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• 제38권6호
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• pp.551-560
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• 2006

In this study, a controller for a nuclear reactor power is designed. The reactor is modeled using the three dimensional reactor design code MASTER. From the relationship of the input and output of the reactor code, a reactor dynamic model is derived by the system identification method. This model is more realistic than the one based on mathematical theories. With this model, a robust controller is designed by the extended frequency response method. As this method has the same theoretical background as the classical method, all of the existing design techniques of the classical method can be used directly. Furthermore, by introducing the real part of a Laplacian operator into the frequency response, the control design specification can be considered at the initial stage of design. The designed controller is simple, and gives a sufficient robustness with good performance.

• 한국전산구조공학회논문집
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• 제23권2호
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• pp.153-163
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• 2010

Krylov 부공간 모델차수축소법은 초기 유한요소모델과 축소모델의 전달함수의 계수인 모멘트를 일치시키는 방법을 이용하는 축소기법으로 이미 대형 유한요소모델의 주파수응답함수의 효율적인 계산에 많이 사용되고 있는 방법 중의 하나이다. 본 논문에서는 Krylov 부공간 축소기법을 이용한 관심 주파수영역에 대한 주파수응답 해석과 이를 통하여 계산된 주파수응답의 여러 가지 설계변수에 대한 설계민감도 해석 방법을 제안하였다. 일반적으로 기계시스템의 주파수응답을 고려한 최적설계를 위해서는 설계변수에 대한 관심 주파수영역에서의 주파수응답 및 그의 민감도 정보가 요구되므로, 고려하는 유한요소모델이 대형일 경우에는 관심 주파수영역에서의 반복적인 해석으로 인한 계산비용의 문제가 심각하게 대두된다. 본 논문에서는 축소모델을 이용하여 주파수응답과 주파수응답의 설계민감도 해석을 수행하여 계산의 효율성을 극대화하였다. 계산상 시스템행렬의 민감도 계산에는 시간측면과 구현의 용이성 측면에서 장점이 있는 준해석적 방법을 이용하였다. 수치 예제를 통하여 축소기법을 이용한 주파수응답의 설계민감도 해석 결과를 초기 유한요소모델의 민감도 결과와 비교하여 우수한 정확성 및 효율성을 확인하였다. 본 논문에서 제안된 방법을 주파수응답을 고려하는 최적설계에 이용하는 경우, 결과의 정확성 및 계산비용 측면에서 매우 효과적인 방법이 될 수 있을 것으로 판단된다.

• 대한기계학회논문집
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• 제19권1호
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• pp.45-60
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• 1995

It is attempted to estimate excitation force of a linear vibratory system using measured vibration responses. The excitation force is estimated from the relationship between the vibration response and system characteristic matrices which are extracted from both the mathematical model of the system and actual response in contrast to the usual approach of inverting the frequency response matrices. This extraction scheme is based on the fact that the vibration response can be expressed in term of linear combination of frequency domain modal vectors defined as mutually orthonormal basis vectors in frequency domain. The extracted frequency domain basis vectors are very stable in computational manipulation. It is found that the estimated excitation force is in good agreement with actually measured force except at the natural frequencies the structure, which is the common feature still to be overcome by the research efforts in this area. From the results of this paper, this disagreement is considered to come from the discrepancy between the model and actual value of the mass, damping and stiffness of the structure.

• International Journal of Air-Conditioning and Refrigeration
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• 제8권1호
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• pp.14-28
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• 2000

The frequency response characteristics of a condenser were numerically studied for the control of refrigeration and air conditioning systems. The important parameters, such as the refrigerant flow rate, refrigerant temperature, air velocity, and air temperature at the condenser inlet, were analyzed. Superheated vapor, two phase, and subcooled liquid domain in condenser can be described by using the energy balance equation and the mass balance equation in refrigerant and tube wall, the basic equation for describing the dynamic characteristics of condenser can be derived. The transfer function for describing dynamic response of the condenser to disturbances can be obtained from using linearizations and Laplace transformations of the equation. From this transfer function, analytical investigation which affects the frequency responses of condenser has been made. Block diagrams were made based on the analytic transfer function; dynamic responses were evaluated in Bode diagrams on the frequency response. Through this study, it became possible that the information about the dynamic characteristics of air-cooled condenser is offered. The results may be used for determining the optimum design parameters in actual components and entire systems. Also, the mathematical models, frequency response may be used to help understanding, evaluate optimum design parameters, design control systems and determine on setting the best controller for the refrigeration and air-conditioning systems.

• 한국재료학회지
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• 제21권7호
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• pp.403-409
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• 2011

Micro speakers are used to reproduce sound in small electric and information and communications devices, such as cellular phones, PMPs, and MP3 players. The acoustical properties and sound quality, which are changed due to the decreased size of the speaker, are often adjusted varying the type and thickness of the diaphragm. The most widely used diaphragm material is thin polymer. It was previously reported by the author of this paper that the resonance frequency of a micro speaker is changed by the type and thickness of a polymer diaphragm. In this paper, the frequency response near the resonance frequency of a micro speaker was studied as functions of the type and thickness of the polymer diaphragm. While $R_{max}$ and $R_{DC}$ were affected by the type and thickness, an analysis of the electrical impedance curve revealed that $R_o(= R_{max}/R_{DC})$ and ${\Delta}f$ were not changed. Thus, $Q_{TS}$ which was function of $R_o$, ${\Delta}f$, and the resonance frequency, is only related to the resonance frequency. The increase of the resonance frequency led to a proportional rise of $Q_{TS}$. The change of the frequency response near the resonance frequency was not dependent on the type or thickness of the polymer diaphragm, but was affected by the resonance frequency.

• Structural Engineering and Mechanics
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• 제72권1호
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• pp.71-81
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• 2019

The objective of this paper is to develop a size-dependent nonlinear model of beams for fluid-conveying nanotubes with an initial deflection. The nonlinear frequency response of the nanotube is analysed via an Euler-Bernoulli model. Size influences on the behaviour of the nanosystem are described utilising the nonlocal strain gradient theory (NSGT). Relative motions at the inner wall of the nanotube is taken into consideration via Beskok-Karniadakis model. Formulating kinetic and elastic energies and then employing Hamilton's approach, the nonlinear motion equations are derived. Furthermore, Galerkin's approach is employed for discretisation, and then a continuation scheme is developed for obtaining numerical results. It is observed that an initial deflection significantly alters the frequency response of NSGT nanotubes conveying fluid. For small initial deflections, a hardening nonlinearity is found whereas a softening-hardening nonlinearity is observed for large initial deflections.

• 한국군사과학기술학회지
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• 제8권3호
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• pp.73-82
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• 2005

This paper describes shock waveform synthesis methods for a shock response spectnlm over a short time interval with which intereference between parts within a test item is increased to perform a sufficient shock test for damage or malfunction which may be caused by the interference between parts, and a digital filter for obtaining a shock response history required for the shock waveform synthesis and a digital inverse filter for restoration by inversely using the digital filter. The time at which the maximax value occurs in the response history is detected in order to establish a delay time which is one of the parameters in the wavelet, on the condition that the natural frequency of SDOF system with a Q (quality factor) of 10 equals to the wavelet frequency of the zero delay wavelet input. A shock response spectrum over a short time interval and an abrupt change in the acceleration for an instant are illustrated as features of the synthesized waveform.

• 대한토목학회논문집
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• 제12권4_1호
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• pp.129-136
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• 1992

압력식(壓力式) 파고계(波高計)에서 측정(測定)되어진 압력파형(壓力波形)으로부터 수위변동(水位變動)을 추정(推定)하는 방법(方法)에 대해서 1) Fast Fourier Transform Method(FFTM), 2) Local Curvature Method(LCM), 3) Individual Wave Method(IWM)의 3가지 방법(方法)을 비교(比較)하여, 천해역(淺海域)에 있어서의 파랑측정(波浪測定)에 대한 압력식(壓力式) 파고계(波高計)의 적용성(適用性)에 대해 검토(檢討)했다. 그 결과(結果) 다음과 같은 결론(結論)을 얻었다. 1) 측정(測定)되어진 압력파형(壓力波形)으로부터 수위변동(水位變動)을 추정(推定)할 때에 사용(使用)되어지는 이론선형응답함수(理論線形應答函數)(Hp=coshkh/coshk(h+z)의 적용가능범위(適用可能範圍)(cut-off-frequency)는 $kh{\leq}3.0$이다. 그리고 kh > 3.0의 영역(領域)에 있어서의 이론선형응답함수(理論線形應答函數)는 일정치(一定値)를 부여함으로써 매우 정도높은 수위변동(水位變動)을 얻을 수 있다. 2) LCM에 의한 수위변동(水位變動)의 변환(變換)에 있어서는 측정(測定)된 압력파형(壓力波形)속에 포함되어 있는 단주기파(短週期波)들에 의해 크게 영향(影響)을 받는다. 그러므로, kh > 1.5인 영역(領域)의 압력변동(壓力變動)을 무시할 필요(必要)가 있다. 3) FFTM 및 IWM에 의해 추정(推定)되어진 통계량(統計量)의 재현성(再現性)은 양호하고, 특(特)히 평균주기(平均週期)의 재현성(再現性)으로부터 비선형성(非線型性)이 강한 영역(領域)에 있어서는 IWM이 유효(有效)한 수단이라고 사려되어진다.

• 한국추진공학회지
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• 제26권4호
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• pp.1-9
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• 2022

본 연구에서는 외부 음향장 가진에 따른 시스템의 동적 응답 특성을 파악하기 위하여 기존의 네트워크 모델에 스피커를 통한 외부 사인-스윕 가진 기능을 추가한 수치해석적 모델이 개발되었다. 본 모델을 통하여 대상 연소기의 물리적 치수 및 경계조건과 같은 시스템 매개변수에 따른 주파수 및 압력 진폭 변화의 민감도를 넓은 주파수 영역에서 분석하였다. 대상 연소기의 가진 응답 특성 분석 결과, 높은 동압 반응을 보이는 주파수 영역은 동일 연소기에서 계측된 불안정 범위와 유사하였으며, 특히 음향 가진 소스항의 위치에 따라 시스템의 반응이 크게 의존하는 것으로 나타났다.