• Transactions of the Korean Society of Mechanical Engineers A
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• v.21 no.7
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• pp.1156-1165
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• 1997

Complex modal testing is employed for the in-situ parameter identification of a four-axis active magnetic bearing system while the system is in operation. In the test, magnetic bearings are used as exciters as well as actuators for feedback control. The experimental results show that the directional frequency response function, which is properly defined in the complex domain, is a powerful tool for identification of bearing as well as modal parameters. It is also shown that the position and current stiffnesses can be accurately estimated using the relations between the measured forces, displacements, and currents.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.16 no.7 s.112
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• pp.769-776
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• 2006

A measured frequency response function can be represented as a ratio of two polynomials. A curve-fitting of frequency responses with Legendre polynomialis suggested in the paper. And the suggested curve-fitting algorithm is based on the least-square error method. Since the Legendre polynomials satisfy the orthogonality condition, the curve-fitting with the polynomials results to more reliable curve-fitting than ordinary polynomial method. Though the proposed curve-fitting with Legendre polynomials cannot cover all frequency range of interest, example shows that the suggested method is quite applicable in a limited frequency band.

• Journal of KSNVE
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• v.9 no.3
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• pp.570-576
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• 1999

This paper presents numerical and experimental results of modal parameter identification in a generator stator frame for 500 MW fossil power plants. A commercial finite element analysis S/W was employed for modal analysis. The generator is excited by alternating electromagnetic forces, mainly of 120 Hz in 60 Hz machines, due to magnetic field and electric current in windings. It is necessary to verify that the stator frame has adequate frequency margin from the excitation frequency to avoid possible resonance when operating. Thus, frequency margin required for the stator frame is established using the numerical and experimental results. The results show that the stator frame meets the frequency-margin requirements. Also, results of modal analysis for design modification in order to reduce weights of the stator frame without deteriorating vibration characteristics are presented.

• Journal of KSNVE
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• v.7 no.1
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• pp.6-12
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• 1997

물리계에서 일어나는 동적 현상들은 선형해석 만으로 설명하기에는 불충분한 점이 많이 있다. 이는 기계구조물과 같은 실제 계의 진동이 기하학적 비선형성, 강성 의 비선형성 또는 경계조건의 비선형성 등의 영향으로 비선형적인 거동을 하기 때문 이다. 비선형 진동을 하는 기계 계는 우리 주변에서 쉽게 찾아 볼 수 있는데, 그 예로써 진자운동을 포함하여 동흡진기, 회전체계, 공작기계의 절삭운동, 건마찰 (dry friction) 관련 기계장치, 치차 및 기차의 바퀴와 레일 간의 접촉에서 볼수 있는 구분적 선형(piecewise linear) 진동계, 충격 진동계 등을 들 수 있다. 비선형 진동 연구는 limit cycle, 준주기운동(quasiperiodic motion), 점프현상(jump phenomena) 등의 인식에서 시작되어, 과거에는 설명이 안되어 회피되 왔던 랜덤(random) 형태의 비주기운동에 대한 연구로 까지 발전하고 있다. 비선형 진동을 다루는데 있어서 정규모드(normal mode)를 이용하는 방법이 있다. 일반적으로 선형계는 선형 정규모드 (linear normal mode)가 존재하는 것과 같이 비선형계에도 이와 유사한 정규모드가 존재한다는 사실이 연구 보고된 바 있다. 비선형계에 존재하는 정규모드는 계의 매개 변수(system parameters)에 따라 그 안정성이 바뀔 수 있으며, 만일 안정한 정규모드 가 어떤 매개변수에서 그 안정성이 바뀐다면 선형이론으로는 설명될 수 없는 새로운 운동이 일어나고 이러한 운동을 분기모드(bifurcation mode)라고 한다. 안정한 정규 모드 및 분기모드를 포함하여 비선형계를 다류는 것을 "정규모드 동역학(normal mode dynamics)"이라고 한다. 정규모드 동역학은 앞에서 언급된 비선형 현상들의 원인규명, 예측, 안정성해석 및 강제진동 해석을 가능하게 한다. 또한 최근에 활발히 연구되고 있는 혼돈운동(chaotic motion)의 해석도 가능하다. 이 글에서는 비선형 진동해석을 위한 정규모드 동역학에 대한 연구동향 및 기본 이론을 살펴 보았고, 그 적용 예를 통하여 실험결과와 비교 고찰 함으로써 정규모드 동역학의 적용성을 서술하여 보았다. 선형이론으로 이해하기 어려운 현상들에 대하여는 비선형의 관점에서 새롭게 접근하 려는 노력이 필요하며 비선형 이론에 대한 연구가 지속적으로 진행되어야 한다. 진행되어야 한다.

• Journal of Korean Association for Spatial Structures
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• v.4 no.3 s.13
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• pp.103-109
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• 2004

The lowest load when the equilibrium condition becomes to be unstable is defined as the buckling load. The primary objective of this paper is to be analyse stability boundaries for star dome under combined loads and is to investigate the iteration diagram under the independent loading parameter. In numerical procedure of the geometrically nonlinear problems, Arc Length Method and Newton-Raphson iteration method is used to find accurate critical point(bifurcation point and limit point). In this paper independent loading vector is combined as proportional value and star dome was used as numerical analysis model to find stability boundary among load parameters and many other models as multi-star dome and arch were studied. Through this study we can find the type of buckling mode and the value of buckling load.

• Computational Structural Engineering
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• v.10 no.3
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• pp.195-202
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• 1997

The charateristics of stabilization for stabilized truss unit-structures with cable and truss are investigated in this paper. This unit system is composed of a central post and eight cables, and is connected by hinge joints, and stabilized by self-equilibrated stress field. As this unit structure itself is a statically closed and stabilized system individually, it can be employed to assemble structures with various configurations. In this study, for stabilization of truss structures stabilized by cable tension, the structural concept of unit structures, the range of various geometrical parameters and the relationship of governing parameters about unit systems are explained.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2009.05a
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• pp.141-144
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• 2009

An injector plays an important role in the process of an efficient combustion in liquid-rocket engines (LRE). This paper is focused on the injection performance of a small LRE-injector by employing the spray characteristic parameters made up of the velocity, Sauter mean diameter, and turbulence intensity. An experimental investigation is carried out with the aid of a dual-mode phase Doppler anemometry (DPDA) according to the injection pressure variation and along transverse axis, spatially. The Weber number and Reynolds number are used to characterize the atomization and turbulence nature of injector spray.

• Journal of KSNVE
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• v.8 no.6
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• pp.1103-1112
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• 1998

A new time series method, directional ARMAX (dARMAX) model-based approach. is proposed for rotor dynamics identification. The dARMAX processes complex-valued signals, utilizing the complex modal testing theory which enables the separation of the backward and forward modes in the two-sided frequency domain and makes effective modal parameter identification possible, to account for the dynamic characteristics inherent in rotating machinery. This paper is divided into two parts : The dARMAX modeling, analysis. and fitting strategy are presented in the first part. whereas a evaluation of its performance characteristics based on both simulated and experimental data is presented in the second.

• Journal of the Korean Geotechnical Society
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• v.26 no.2
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• pp.45-54
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• 2010

The SCW numerical model described in the companion paper was used to carry out a comprehensive parametric study to evaluate the performance of the SCW. The five ground related parameters, which are porosity, hydraulic conductivity, thermal conductivity, specific heat, geothermal gradient, and five SCW design parameters, which are pumping rate, well depth, well diameter, dip tube diameter, bleeding rate, were used in the study. Two types of numerical simulations were performed. The first type was used to perform short-term (24-hour) simulation, while the second type 14 day simulation. The study results indicate that the parameters that have important influence on the performance of SCW were hydraulic conductivity, thermal conductivity, geothermal gradient, pumping rate, and bleeding rate. The thermal conductivity had the most important influence on the performance of the SCW. With the increase in the geothermal gradient, the performance increased in the heat mode, but decreased in the cooling mode. The hydraulic conductivity influenced the performance when the value was larger than $10^{-4}m/s$. The depth of the well increased the performance, but at the cost of increased cost of boring. The bleeding had an important influence on SCW, greatly enhancing the performance at a limited increased cost of operation. Overall, this study showed that various factors had a cumulative influence on the performance of the SCW, and a numerical simulation can be used to accurately predict the performance of the SCW.

• Journal of the Korea institute for structural maintenance and inspection
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• v.19 no.3
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• pp.55-63
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• 2015

Post-installed concrete set anchors are installed after the concrete hardened. These anchors increasing usage in development of construction equipment and flexible construction. The anchor loaded in shearing exhibits various failure modes such as steel failure, concrete failure, concrete pryout, depending on the shear strength of steel, the strength of concrete, edge distance and anchor interval, etc,. In this study, the objective is to investigate the effects of the variations like anchor embedment depth, edge distance and concrete strength on experimental and finite element analysis of shear failure behavior of post-installed concrete set anchor for light load embedded in concrete. The results of embedment depth experiments show that concrete strength has much affection on the shallow embedment depth. Concrete strength has no much affection with anchor interval and edge distance parameter and both experimental results occurred same failure mode. By comparing the experimental results that occurred steel failure mode show that as strong as concrete strength are the displacement results are small.