• 한국소음진동공학회논문집
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• 제18권7호
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• pp.718-724
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• 2008

A flexible structure submerged in acoustic fluid is affected by its surrounding fluid. In this case, the coupling effects between structures and surrounding fluid have an effect on the submerged structure as external force and change impedance of acoustic domain. Therefore, the coupling effects by its surrounding fluid complicatedly change the characteristics of a submerged structure such as natural frequencies and damping coefficients. In this paper, using the analytic modal equation of a spherical shell surrounded by water and air, the complex changes of damping coefficients and natural frequencies of submerged structures are studied for various external acoustic fluid and structures.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2008년도 추계학술대회A
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• pp.967-972
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• 2008

This paper presents the free vibration analysis of a circular plate with a concentric square hole. The present problem deals with the numerical calculation of the natural frequencies and mode shapes of vibration of the structure by means of Independent Coordinate Coupling Method (ICCM). In this study, the boundary condition is the edge of the square hole is free and the outer circular plate is simply supported. Due to the geometric abnormality, this analysis does not permit an exact solution. Since the ICCM employs coordinate systems corresponding to each domain independently, the kinetic and potential energy expressions necessary for the Rayleigh-Ritz method can be easily obtained. Lastly, the kinematic relation is imposed. In this way, the eigenvalue problem can be easily set up. The numerical results show the efficacy of the ICCM and changes in natural frequencies and modes due to the square hole size.

• 대한기계학회논문집
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• 제16권10호
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• pp.1900-1907
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• 1992

본 연구에서는 원형평판의 경계조건을 일부분은 고정단으로 일부분은 자유단 으로 구성하여 자유단각 .alpha.(Fig.4 참조)를 0˚, 30˚, 60˚, 90˚, 120˚, 150˚, 180˚로 변화시킴에 따른 진동모드를 시간평균 홀로그래픽 간섭계(time-average holographic interferometry)를 이용해 분석하고 자유단의 변화에 따른 고유진동수의 비를 비교하였고, 레이저 도플러 유속계(LDV)를 변형한 레이저 도플러 진동계(laser doppler vibrometer)로 원형평판의 진동변위와 시간평균 홀로그래픽 간섭계로 구한 진 동변위를 비교하였다.

• 한국소음진동공학회논문집
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• 제13권8호
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• pp.591-597
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• 2003

The oscillation of the fluid caused by external motions is called sloshing, which occurs in moving vehicles with contained fluid masses, such as the oil tankers, railroad cars, aircraft and rockets. Natural frequencies of fluid are much lower than that of solid structures, and the deformation caused by the excitation that is less than 1st natural frequency of fluid is very large. For the reason of that, sloshing characteristics under the ekcitation that is less than the 1st natural frequency must be studied prior to the consideration of natural frequencies of fluid. The experimental devices are constructed to simulate the translation motion. The rectangular tanks are made to study the sloshing characteristics under external excitation. The changes of water height are measured using an analogue camcorder and MPEG board, and those are compared to each other through a standard deviation. From the results of experiments, the sloshing is greatly influenced by the length of the rectangular tank than the width of that under the periodic translational motion in the length direction. The rapid amplification of sloshing by resonance is also confirmed experimentally.

• 한국소음진동공학회논문집
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• 제19권2호
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• pp.192-199
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• 2009

This paper presents the free vibration analysis of a circular plate with a concentric square hole. The boundary condition is the edge of the square hole is free and the outer circular plate is simply supported. Due to the geometric abnormality, this analysis does not permit an exact solution. The present problem deals with the numerical calculation of the natural frequencies and mode shapes of vibration of the structure by independent coordinate coupling method(ICCM). The numerical results show the efficacy of the ICCM and changes in natural frequencies and modes due to the square hole size.

• 풍력에너지저널
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• 제9권4호
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• pp.16-23
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• 2018

A damage detection method for the tripod support structure of offshore wind turbines is presented for structural health monitoring. A finite element model of a prototype tripod support structure is established and the modal properties are calculated. The degree and location of the damage are estimated based on the neural network technique using the changes of natural frequencies and mode shape due to the damage. The stress distribution occurring in the support structure is obtained by a dynamic analysis for the wind turbine system to select the output data of the neural network. The natural frequencies and mode shapes for 36 possible damage scenarios were used for the input data of the learned neural network for damage assessment. The estimated damages agreed reasonably well with the accurate ones. The presented method could be effectively applied for damage detection and structural health monitoring of various types of support structures of offshore wind turbines.

• Structural Engineering and Mechanics
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• 제82권6호
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• pp.701-711
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• 2022

The nonlocal elasticity as well as Mindlin's first-order shear deformation plate theory are proposed to investigate thermal vibrational of a nanoplate placing on a three-factor foundation. The Winkler-Pasternak elastic foundation is connected with the viscous damping to obtain the present three-parameter viscoelastic model. Differential equations of motion are derived and resolved for simply-supported nanoplates to get their natural frequencies. The influences of the nonlocal index, viscous damping index, and temperature changes are investigated. A comparison example is dictated to validate the precision of present results. Effects of other factors such as aspect ratio, mode numbers, and foundation parameters are discussed carefully for the vibration problem. Additional thermal vibration results of nanoplates resting on the viscoelastic foundation are presented for comparisons with future investigations.

• 한국기계가공학회지
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• 제18권11호
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• pp.57-62
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• 2019

This paper describes the prediction of eigenfrequencies due to changes in stiffness and mass in the connection area of the lap joint beam in terms of linear and torsional stiffness as well as connection length. The sensitivities of mass and stiffness in the finite element model were derived by using the first-order differential and algebraic equation and were thereafter applied to obtain new natural frequencies that were compared with theoretical exact solutions. Newly predicted natural frequencies due to only a change in stiffness were in relatively good agreement with those in lower modes for rigid joints, while further investigation was needed for flexible joints. On the other hand, only the change in mass resulted in a large discrepancy in the flexible joint case. It may be strongly anticipated that this study will provide a useful tool for estimating modal parameters by change in any design variable, such as the structural dimension, material property, or connection type for a large-scale structure, even though the proposed methodology is currently limited to a jointed beam.

• Smart Structures and Systems
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• 제15권2호
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• pp.335-353
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• 2015

Previous long-term measurements of the Uldolmok tidal current power plant showed that the structure's natural frequencies fluctuate with a constant cycle-i.e., twice a day with changes in tidal height and tidal current velocity. This study aims to improve structural health monitoring (SHM) techniques for offshore structures under a harsh tidal environment like the Uldolmok Strait. In this study, lab-scale experiments on a simplified offshore structure as a lab-scale test structure were conducted in a circulating water channel to thoroughly investigate the causes of fluctuation of the natural frequencies and to validate the displacement estimation method using multimetric data fusion. To this end, the numerical study was additionally carried out on the simplified offshore structure with damage scenarios, and the corresponding change in the natural frequency was analyzed to support the experimental results. In conclusion, (1) the damage that occurred at the foundation resulted in a more significant change in natural frequencies compared with the effect of added mass; moreover, the structural system became nonlinear when the damage was severe; (2) the proposed damage index was able to indicate an approximate level of damage and the nonlinearity of the lab-scale test structure; (3) displacement estimation using data fusion was valid compared with the reference displacement using the vision-based method.

• Structural Engineering and Mechanics
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• 제91권1호
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• pp.87-102
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• 2024

In recent years, the focus on vibration analysis of multilayer smart structures has attracted considerable attention in many engineering applications. In this work, vibration analysis of a three-layer microporous beam with a core amplified by a composite material reinforced with graphene platelets and two piezoelectric thin films is discussed. It is assumed that piezoelectric layers with a thickness of 0.01 core are very thin and the properties of the matrix and reinforcement vary in the thickness directions. The governing equations of motion are obtained using an energy approach and the method of numerical differential quadrature to solve them. The results of this work are compared to other research and there is good agreement between them. The influences of the volumetric weight fraction of graphene wafers, different graphene platelets distributions, porosity distribution, mass scale parameters and thin ratio of graphene platelets take into account the natural dimensionless frequencies of the micro-beam. The results of this study show that the symmetric distribution of graphene platelets based on the symmetric porosity distribution has a great influence on the natural frequencies without basic dimension of the micro-beam, while the shape ratios of graphene platelets do not have a significant influence on natural frequency changes.