• 한국가시화정보학회지
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• 제15권2호
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• pp.59-67
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• 2017

To verify floatability of ABB (Air bubble barrier), we compared bubble swarm behavior with and without the air-driven ejector. Experiment was conducted using the fabricated air-driven ejector with 5 mm nozzle on the bottom of 1 m3 water tank. Reynolds number of air in the nozzle was ranged 1766-13248. We analyzed data with statistical method using image processing, particle mage velocimetry (PIV) and proper orthogonal decomposition (POD) analysis. As a result of POD analysis, there was no significant eigenmode in bubbly flow generated from the ejector. It means that more complex turbulent flows were formed by the ejector, thereby (1) making bubbles finer, (2) promoting three-dimensional energy transfer between bubble and water, and (3) making evenly distributed velocity profile of water. It is concluded that the air-driven ejector could enhance the performance of ABB.

• 한국전자파학회논문지
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• 제13권4호
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• pp.351-358
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• 2002

본 논문에서는 구형 도파관내에 경사진 두 금속봉을 근사화한 구조에 모드 매칭법과 일반 산란계수법을 적용하여 이들의 경사 각도에 따른 상호 작용에 의한 산란 특성을 구할 수 있었다. 여기서 경사진 두 금속봉 구조에 모드 매칭법을 적용하기 위하여 적당한 개수의 모드를 택하여 경사각도에 대한 산란특성을 구하였다. 그리고, 이들의 경사진 두 금속봉의 계산 과정을 도파관과 동축선로의 접합부에 적용하여서 산란 특성을 얻을 수 있었다. 이런 결과에 따라서 경사 각도가 50$^{\circ}$ 일 경우에 동축 선로와 도파관 출력 포트로의 전달 특성이 3 ㏈가 되는 것을 알 수 있었다. 그리고, 입력단의 반사 손실은 경사 각도에 따라서 선형적으로 감소하는 특성을 얻을 수 있었다.

• 한국항공우주학회지
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• 제39권10호
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• pp.919-926
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• 2011

고속 비행체의 날개 구조물은 스킨과 튼튼한 골격으로 구성되어 있다. 플러터 해석시 날개 구조물의 고유진동모드를 이용하여 비정상공기력을 계산하고 모달접근법을 이용하여 시간영역이나 주파수 영역에서 구조물의 진동안정성을 분석하게 되므로, 사용되는 고유 진동 모드는 신중하게 선정되어야 한다. 이를 위해 날개 구조물과 같이 고차에서 스킨의 국부진동모드가 있는 경우 이러한 모드가 비정상공기력 및 플러터 특성에 미치는 영향을 분석하였다.

• 한국군사과학기술학회지
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• 제14권5호
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• pp.768-773
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• 2011

The prediction method of future events using the time histories of velocity or pressure, etc., is a useful way for controlling various air vehicles. For example, the sensors of velocity or pressure can be used to extract the time mode coefficients of eigenmode of flow field, and then the result is applied to suppress wake or drag. The velocity information is mapped to the entire flow field, so this mapping function can be used to predict the future events based on the current information. The mapping function is composed of the huge amount of weight parameters, so the efficient way of finding these parameters is needed. Here, the neural network algorithm is studied to draw a mapping function using the number and location of velocity sensors.

• 한국정밀공학회지
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• 제28권7호
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• pp.796-804
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• 2011

Railway vehicles are capable of indicating several types of instability. This phenomenon, which is called hunting motion, is a self excited lateral oscillation that is caused by the running velocity of the vehicle and wheel frail interactive forces. The interactive forces act to change effectively the damping characteristics of railway vehicle systems. This paper will show the impact of instability on the transfer function behavior using damping characteristics of secondary suspension. The vehicle dynamics are modeled using a 17 degree of freedom considering linear wheel/rail contact. The paper deals with certain condition of the damper characteristics that one is about various damping coefficient and another is equipped damper direction.

• Structural Engineering and Mechanics
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• 제64권3호
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• pp.279-285
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• 2017

Modal expansion technique (MET) is a method to estimate the vibration fields of flexible structures by using eigenmodes of the structure and the signals of sensors. It is the useful method to estimate the vibration fields but has the truncation error since it only uses the limit number of the eigenmodes in the frequency of interest. Even though block-wise MET performed frequency block by block with different valid eigenmodes was developed, it still has the truncation error due to the absence of other eigenmodes. Thus, this paper suggested an improved block-wise modal expansion technique. The technique recovers the truncation errors in one frequency block by utilizing other eigenmodes existed in the other frequency blocks. It was applied for estimating the vibration fields of a cylindrical shell. The estimated results were compared to the vibration fields of the forced vibration analysis by using two indices: the root mean square error and parallelism between two vectors. These indices showed that the estimated vibration fields of the improved block-wise MET more accurately than those of the established METs. Especially, this method was outstanding for frequencies near the natural frequency of the highest eigenmode of each block. In other words, the suggested technique can estimate vibration fields more accurately by recovering the truncation errors of the established METs.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집A
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• pp.854-859
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• 2001

An eigenvalue analysis of a tunable micro-mechanical actuator is presented. The actuator is modeled as a continuum structure. The eigenvalue modified by the tuning voltage is computed through the linearization of the relation between the electrostatic force and the displacement at the equilibrium. A staggered algorithm is employed to perform the coupled analysis of the electrostatic and elastic fields. The stiffness matrix of the actuator is modified at this equilibrium state. The displacement field is perturbed using an eigenmode profile of the actuator. The configuration change of the actuator due to perturbation modifies the electrostatic field and thus the electrostatic force. The equivalent stiffness matrix corresponding to the perturbation and the change in the electrostatic force is then added to stiffness matrix in order to explain natural frequency shifting. The numerical examples are presented and compared with the experiments in the literatures.

• 한국소음진동공학회논문집
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• 제21권10호
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• pp.960-966
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• 2011

An improved NDIF method is introduced to efficiently extract eigenvalues and eigenmodes of two-dimensional, arbitrarily shaped acoustic cavities. The NDIF method, which was developed by the authors for the eigen-mode analysis of arbitrarily shaped acoustic cavities, membranes, and plates, has the feature that it yields highly accurate eigenvalues compared with other analytical methods or numerical methods(FEM and BEM). However, the NDIF method has the weak point that the system matrix of the NDIF method depends on the frequency parameter and, as a result, a final system equation doesn's take the form of an algebra eigenvalue problem. The system matrix of the improved NDIF method developed in the paper is independent of the frequency parameter and eigenvalues and mode shapes can be efficiently obtained by solving a typical algebraic eigenvalue problem. Finally, the validity and accuracy of the proposed method is verified in two case studies, which indicate that eigenvalues and mode shapes obtained by the proposed method are very accurate compared to the exact method, the NDIF method or FEM(ANSYS).

• 한국지구과학회지
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• 제35권5호
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• pp.333-341
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• 2014

A new set of basis functions was constructed using the Rossby-Haurwitz waves, which are the eigenfunctions of nondivergent barotropic vorticity equations on the sphere. The basis functions were designed to be non-separable, that is, not factored into functions of either the longitude or the latitude. Due to this property, the nodal lines of the functions are aligned neither along with the meridian nor the parallel. The basis functions can be categorized into groups of which members have the same degree or the total wavenumber-like index on the sphere. The orthonormality of the basis functions were found to be close to the machine roundoffs, giving the error of $O(10^{-15})$ or $O(10^{-16})$ for double-precision computation (64 bit arithmetic). It was demonstrated through time-stepping procedure that the basis functions were also the eigenfunctions of the non-divergent barotropic vorticity equations. The projection of the basis functions was carried out onto the low-resolution geopotential field of Gaussian bell, and compared with the theory. The same projections were performed for the observed atmospheric-geopotential height field of 500 hPa surface to demonstrate decomposition into the fields that contain disturbance of certain range of horizontal scales. The usefulness of the new basis functions was thus addressed for application to the eigenmode analysis of the atmospheric motions on the global domain.

• 한국지진공학회논문집
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• 제27권1호
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• pp.1-12
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• 2023

Structures of high-rise buildings are less prone to earthquake damage. This is because the response acceleration of high-rise buildings appears to be small by generally occurring short-period ground motions. However, due to the increased construction volume of high-rise buildings and concerns about large earthquakes, long-period ground motions have begun to be recognized as a risk factor for high-rise buildings. Ground motion observed on each floor of the building is affected by the eigenmode of the building because the ground motion input to the building is amplified in the frequency range corresponding to the building's natural frequency. In addition, long-period components of ground motion are more easily transmitted to the floor or attached components of the building than short-period components. As such, high-rise buildings and non-structural components pose concerns about long-period ground motion. However, the criteria (ASCE 7-22) underestimate the acceleration response of buildings and non-structural components caused by long-period ground motion. Therefore, the characteristics of buildings' acceleration response amplification ratio and non-structural components were reviewed in this study through shake table tests considering long-period ground motions.