• Smart Structures and Systems
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• 제17권3호
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• pp.491-522
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• 2016

A benchmark problem for modal identification of a cable-stayed bridge was proposed by a research team at Hong Kong Polytechnic University. By taking an instrumented cable-stayed bridge as a test bed, nineteen sets of vibration records with known/unknown excitations were provided to invited researchers. In this paper, the vibration responses of the bridge under a series of excitation conditions are examined to estimate the wind and traffic-induced variations of its dynamic characteristics. Firstly, two output-only experimental modal identification methods are selected. Secondly, the bridge and its monitoring system are described and the nineteen sets of vibration records are analyzed in time-domain and frequency-domain. Excitations sources of blind datasets are predicted based on the analysis of excitation conditions of known datasets. Thirdly, modal parameters are extracted by using the two selected output-only modal identification methods. The identified modal parameters are examined with respect to at least two different conditions such as traffic- and typhoon-induced loadings. Finally, the typhoon-induced effects on dynamic characteristics of the bridge are estimated by analyzing the relationship between the wind velocity and the modal parameters.

• Structural Monitoring and Maintenance
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• 제3권4호
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• pp.395-406
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• 2016

Modal parameter identification has received much attention recently for their usefulness in earthquake engineering, damage detection and structural health monitoring. The identification method based on Matrix Pencil technique is adopted in this paper to identify structural modal parameters, such as natural frequencies, damping ratios and modal shapes using impulse vibration responses. This method can also be applied to dynamic responses induced by stationary and white-noise inputs since the auto- and cross-correlation function of the two outputs has the same form as the impulse response dynamic functions. Matrix Pencil method is very robust to noise contained in the measurement data. It has a lower variance of estimates of the parameters of interest than the Polynomial Method, and is also computationally more efficient. The numerical simulation results show that this technique can identify modal parameters accurately even if the noise level is high.

• 산업기술연구
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• 제28권B호
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• pp.135-141
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• 2008

In this paper, the Proper Orthogonal Decomposition (POD) method, which is a statistical analysis technique to find the modal characteristics of a structure, is adapted to identify the modal parameters of a tall chimney structure. A wind force time history, which is applied to the structure, is obtained by a wind tunnel test of a scale down model. The POD method is applied on the wind force induced responses of the structure, and the true normal modes of the structure can be obtained. The modal parameters including, natural frequency, mode shape, damping ratio and kinetic energy of the structure can be estimated accurately. With these results, it may be concluded that the POD method can be applied to obtain accurate modal parameters from the wind-induced building responses.

• International Journal of Fluid Machinery and Systems
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• 제2권4호
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• pp.303-314
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• 2009

The present paper shows the results of numerical and experimental modal analyses of Francis runners, which were executed in air and in still water. In its first part this paper is focused on the numerical prediction of the model parameters by means of FEM and the validation of the FEM method. Influences of different geometries on modal parameters and frequency reduction ratio (FRR), which is the ratio of the natural frequencies in water and the corresponding natural frequencies in air, are investigated for two different runners, one prototype and one model runner. The results of the analyses indicate very good agreement between experiment and simulation. Particularly the frequency reduction ratios derived from simulation are found to agree very well with the values derived from experiment. In order to identify sensitivity of the structural properties several parameters such as material properties, different model scale and different hub geometries are numerically investigated. In its second part, a harmonic response analysis is shown for a Francis runner by applying the time dependent pressure distribution resulting from an unsteady CFD simulation to the mechanical structure. Thus, the data gained by modern CFD simulation are being fully utilized for the structural design based on life time analysis. With this new approach a more precise prediction of turbine loading and its effect on turbine life cycle is possible allowing better turbine designs to be developed.

• 한국정밀공학회지
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• 제33권8호
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• pp.623-628
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• 2016

This article describes the determination of the dynamic stability for a heavy press, particularly considering rotational speed. A finite element model of the driving parts for the heavy press was generated. We also applied boundary conditions and dynamic loads considering the driving conditions. Modal analysis was conducted using the finite element construction model. Therefore, no resonance was identified with the comparison between the results of the modal analysis and vibration excitation frequency by the gear tooth. In addition, the stress distribution of the driving parts for press was determined using transient analysis. As compared to the yield strength of the material, the dynamic stability the heavy press was confirmed.

• 한국산학기술학회논문지
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• 제6권2호
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• pp.210-214
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• 2005

본 논문에서는 폐목재의 재활용을 위한 파쇄기의 경계조건과 하중조건을 적용하여 ANSYS로 구조해석과 모달해석을 수행하여 변위(displacement), 응력(stress), 변형률(strain)을 구하고 구조적 타당성을 검토하였다. 이는 안전성을 고려한 설계의 기초자료가 되며 고유진동수(natural frequency)를 구하여 모터의 회전을 제어함으로서 공진(resonance)현상을 피하는데 있다.

• 대한기계학회논문집A
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• 제40권12호
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• pp.1061-1066
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• 2016

주름 강판의 주름은 강판의 중량당 굽힘 강성을 획기적으로 증가시킨다. 그러나 주름 강판은 같은 중량을 갖는 평판보다 낮은 차음 성능을 보인다. 특히, 특정한 주파수대역에서는 투과손실이 크게 감소한다. 이 문제의 주 요인은 주름단위에서의 국부적 공진으로 알려져 있다. 특정 주파수대역에서 다수의 국부적 공진모드가 발생하고 모드밀도가 급속히 증가한다. 본 연구에서는 음 투과손실과 모드 밀도 사이의 관계를 검토한다. 최종적으로, 중량의 증가 없이 주름 강판의 차음 성능을 향상시키기 위한 모드 밀도의 측면에서의 설계전략을 제시한다.

• 한국정밀공학회:학술대회논문집
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• 한국정밀공학회 2011년도 춘계학술대회 논문집
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• pp.333-334
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• 2011

• 한국항해항만학회지
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• 제26권3호
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• pp.281-288
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• 2002

진동해석과 기술에 있어서 실험과 해석분야 모두 다 같이 발전한 것은 사실이지만, 실험해석법은 유한요소법과 같은 이론해석법만큼 발달한 것이라 할 수 없을 것이다. 그 이유 중에 하나로 비선형 방정식을 푸는데 있어 초기치의 선택에 따라 계산시간이 길어지고 해가 종종 발산하기도 하기 때문이다. 일반적으로 실험모드해석에 있어 방정식은 고유진동수와 감쇠비의 비선형 항과 등가강성의 선형 항으로 구성된다. 본 연구에서는 비선형 항을 먼저 구하고, 다음에 선형 항을 구하였다. 개발된 실험모드해석법(곡선맞춤법)을 선체상부구조 모델에 적용하여, 모드 특성치들을 계산하였다. 또한 개발된 프로그램은 그래픽기법을 적용하여 간단한 마우스 조작으로 채용 모드수와 감쇠비를 결정할 수 있도록 하였다.

• 전기학회논문지
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• 제61권12호
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• pp.1948-1952
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• 2012

The paper presents modal testing and analysis in order to obtain the dynamic characteristics of a middle spacer grids of a nuclear fuel rod. A spacer grid is one of the important structural elements supporting nuclear fuel rods. Such a fuel rod can be oscillated by its thermal expansion, neutron irradiation and etc. due to cooling water flow under the operation of a nuclear power plant. When the fuel rod vibrates, fretting wear due to repeated friction motion between the fuel rods and spacer grids can be occurred, and so the fuel rod is damaged. In this paper, through modal analysis and testing, natural frequencies and modes of a middle spacer grid were calculated, and the following conclusions were obtained. Firstly the numerical first-seven natural frequencies for spacer grids of a fuel rod having complicated structures have a small difference within 3.8% with experimental natural frequencies, and so the suitability of simulation results was verified. Secondly, experimental mode shapes for a middle spacer grid of a nuclear fuel rod were verified by obtaining lower non-diagonal terms through MAC(Modal Assurance Criteria), and were confirmed by the simulation modes.