• 한국항공운항학회지
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• 제13권3호
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• pp.9-21
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• 2005

The stable motion has been judged by mathematical modeling of the conditions that a rocket flies flexibly to take an active part in atmosphere. In this paper, the rocket conditions consist of the air speed, thrust and automatic attitude control. Aerodynamic force, a critical trust and a critical air speed are determined by comparing mathematical knowledges with eigenfrequencies of vibration equation. And then rocket object model is designed. Parameters and eigenfrequencies are used in dimensionless forms for in general applications by eliminating restrictions such as dimension, weight and select of materials.

• 대한기계학회논문집A
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• 제40권4호
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• pp.363-372
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• 2016

강체와 유연체가 혼합된 다종 구조 시스템의 동특성을 개선을 위한 최적화를 수행하는 경우, 일반적으로 그 시스템의 고유 진동수를 높이게 된다. 강체와 유연체의 시스템을 동시에 다루는 위상 최적화 정식화가 있으나, 그 시스템의 고유 진동수를 다룬 연구는 드물며, 특히 목적하는 진동수가 중복 고유 진동수 의 하나로 되는 경우를 다룬 연구는 보고된 바 없다. 본 연구에서는 중복 고유 진동수를 다루어야 하는 경우에 나타나는 수치적 문제를 해결하였으며 그 방법을 활용한 위상최적설계 정식화와 민감도 해석을 제시하였다. 그 다음, 몇 가지 수치 예제를 통해 제안된 정식화의 타당성을 검증해 보았다.

• Structural Engineering and Mechanics
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• 제25권3호
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• pp.311-330
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• 2007

Model-based predictions of structural behavior are negatively affected by uncertainties of various type and in various stages of the structural analysis. The present paper focusses on dynamic analysis and addresses the effects of uncertainties concerning material and geometric parameters, mainly in the context of modal analysis of large-scale structures. Given the large number of uncertain parameters arising in this case, highly scalable simulation-based methods are adopted, which can deal with possibly thousands of uncertain parameters. In order to solve the reliability problem, i.e., the estimation of very small exceedance probabilities, an advanced simulation method called Line Sampling is used. In combination with an efficient algorithm for the estimation of the most important uncertain parameters, the method provides good estimates of the failure probability and enables one to quantify the error in the estimate. Another aspect here considered is the uncertainty quantification for closely-spaced eigenfrequencies. The solution here adopted represents each eigenfrequency as a weighted superposition of the full set of eigenfrequencies. In a case study performed with the FE model of a satellite it is shown that the effects of uncertain parameters can be very different in magnitude, depending on the considered response quantity. In particular, the uncertainty in the quantities of interest (eigenfrequencies) turns out to be mainly caused by very few of the uncertain parameters, which results in sharp estimates of the failure probabilities at low computational cost.

• 한국음향학회지
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• 제35권1호
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• pp.16-23
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• 2016

본 논문은 무한 원통형 실린더의 자유진동에서 발생하는 고유진동수(eigenfrequency)를 구하는 3가지 방법에 대해 다루었다. 일반적인 경우에 적용될 수 있는 탄성 이론을 적용한 방법과 얇은 두께의 실린더에 효율적으로 적용될 수 있는 얇은 원통형 쉘 이론을 적용한 방법, 유한요소법(FEM: Finite Element Method)을 통한 수치 해석 방법을 통해 구해진 결과에 대한 비교 및 검증을 수행하였다. 주어진 실린더의 외반경에 두께를 서로 달리하여 1 kHz 이하에 존재하는 원통형 쉘의 고유진동수를 구하였고 모드수와 두께의 변화에 따른 이들 결과에 대해 관찰하였다.

• Structural Engineering and Mechanics
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• 제63권1호
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• pp.103-113
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• 2017

Free vibration analysis of plates is necessary for the field of structural engineering because of its wide applications in practical life. Free vibration of plates is largely dependent on its thickness, aspect ratios, and boundary conditions. Here we investigate the natural frequencies of simply supported tapered isotropic rectangular plates with internal cutouts using a nine node isoparametric element. The effect of rotary inertia on Eigenfrequencies was demonstrated by calculating with- and without rotary inertia. We found that rotary inertia has a significant effect on thick plates, while rotary inertia term can be ignored in thin plates. Based on comparison with literature data, we propose that the present formulation is capable of yielding highly accurate results. Internal cutouts at various positions in tapered rectangular simply supported plates were also studied. Novel data are also reported for skew taper plates.

• Structural Engineering and Mechanics
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• 제25권2호
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• pp.147-160
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• 2007

In this paper, a dynamic analysis based on the fuzzy set theory is presented as a possible complementary tool to the classical stochastic methods for dynamic analyses. Material parameters of a structure are influenced by uncertainties and therefore they are considered to be fuzzy quantities with given distributions, that means fuzzy numbers with given membership functions. The fuzzy dynamic analysis is conducted with help of fuzzy arithmetic defined on the so-called ${\alpha}$-cuts. The results of the analysis are also obtained in the form of fuzzy numbers, which compared to the stochastic methods is less computationaly expensive while at the same time they still provide information about the distribution of a quantity. This method is demonstrated on an analysis of a two-dimensional frame subjected to possible seismic load, where the uncertain eigenmodes and eigenfrequencies are used in the modal analysis.

• Structural Engineering and Mechanics
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• 제10권3호
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• pp.277-288
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• 2000

The present paper, deals with the dynamic analysis of a thin-walled tower with varying cross-section and additional masses. It, especially, deals with the effect of the rotary inertia of those masses, which have been neglected up to now. Using Galerkin's method, we can find the spectrum of the eigenfrequencies and, also, the shape functions. Finally, we can solve the equations of the problem of the forced vibrations, by using Carson-Laplace's transformation. Applying this method on a tall mast with 2 concentrated masses, we can examine the effect of the rotary inertia and the diaphragmatic operation of the above masses, on the 3 first eigenfrequencies.

• 제어로봇시스템학회:학술대회논문집
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• 제어로봇시스템학회 1992년도 한국자동제어학술회의논문집(국제학술편); KOEX, Seoul; 19-21 Oct. 1992
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• pp.349-354
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• 1992

In this paper, a modal analysis is applied for a hung Euler-Bernoulli beam with a lumped mass. We first derive the equations of motion using the Hamilton's principle. Then regarding the tension of beam as constant, we characterize the eigenfrequencies and the feature of eigenfunctions. The approximation employed here is corresponding that the lumped mass is sufficiently large than that of beam. Finally we compare the eigenfrequencies derived here with those obtained based on the Southwell's method.

• Smart Structures and Systems
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• 제8권2호
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• pp.191-204
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• 2011

A technique for damage localization and assessment based on measurements of both eigenvectors curvatures and eigenfrequencies is proposed. The procedure is based on two successive steps: a model independent localization, based on changes of modal curvatures, and the solution of a one-dimensional minimization problem to evaluate damage intensity. The observability properties of damage parameters is discussed and, accordingly, a suitable change of coordinates is introduced. The proposed technique is illustrated with reference to a cantilever Euler beam endowed with a set of piezoelectric transducers. To assess the robustness of the algorithm, a parametric study of the identification errors with respect to the number of transducers and to the number of considered modal quantities is carried out with both clean and noise-corrupted data.

• 대한기계학회논문집A
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• 제24권1호
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• pp.190-195
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• 2000

It has been established that a crack has an important effect on the dynamic behavior of a structure. This effect depends mainly on the location and depth of the crack. To identify the location and depth of a crack in a structure. Nikolakopoulos et. al. used the intersection point of the superposed contours that correspond to the eigenfrequency caused by the crack presence. However the intersecting point of the superposed contours is not only difficult to find but also incorrect to calculate. A method is presented in this paper which uses optimization technique for the location and depth of the crack. The basic idea is to find parameters which use the structural eigenfrequencies on crack depth and location and optimization algorithm. With finite element model of the structure to calculate eigenfrequencies, it is possible to formulate the inverse problem in optimization format. Method of optimization is augmented lagrange multiplier method and search direction method is BFGS variable metric method and one dimensional search method is polynomial interpolation.