• Wind and Structures
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• v.26 no.1
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• pp.45-56
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• 2018

Wind damage of urban trees arises to be a serious issue especially in the typhoon-prone areas. As a family of tree species widely-planted in Southeast China, the structural behaviors of Plane tree is investigated. In order to accommodate the complexities of tree morphology, a fractal theory based finite element modeling method is proposed. On-site measurement of Plane trees is performed for physical definition of structural parameters. It is revealed that modal frequencies of Plane trees distribute in a manner of grouped dense-frequencies; bending is the main mode of structural failure. In conjunction with the probability density evolution method, the fragility assessment of urban trees subjected to wind excitations is then proceeded. Numerical results indicate that small-size segments such as secondary branches feature a relatively higher failure risk in a low wind level, and a relatively lower failure risk in a high wind level owing to windward shrinks. Besides, the trunk of Plane tree is the segment most likely to be damaged than other segments in case of high winds. The failure position tends to occur at the connection between trunk and primary branches, where the logical protections and reinforcement measures can be implemented for mitigating the wind damage.

• Journal of Korea Multimedia Society
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• v.3 no.1
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• pp.77-90
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• 2000

Real-Time verification is a procedure that verifies the correctness of specification related to requirement in time as well as in logic. One serious problem encountered in the verification task is that the state space grows exponentially owing to the unboundedness of time, which is termed the state space explosion problem. In this paper, we propose a real-time verification technique checking the correctness of specification by showing that a system model described in timed automata is equivalent to the characteristic of system property specified in timed modal-mu calculus. For this, we propose a local model checking method based on the value of the formula in initial state with constructing product graph concerned to only the nodes needed for verification process. Since this method does not search for every state of system model, the state space is reduced drastically so that the proposed method can be applied effectively to real-time system verification.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.13 no.2
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• pp.1-10
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• 1993

Frequency response functions(FRF) are the most fundamental data for the frequency domain identifications of structural systems. In this paper, an improved method for estimating FRF's is presented. The new FRF estimator takes the weighted average of two conventional estimators, $H_1$(f) and $H_2$(f), utilizing the fact that $H_2$(f) gives more accurate estimate at resonance, while $H_1$(f) yields better results at antiresonances. Based on the estimated FRF's, the modal parameters of the structures, such as, natural frequencies, damping ratios and mode shapes, are also estimated. The effectiveness of the proposed method is investigated through numerical and experimental studies. The estimated results indicate that the proposed estimator gives more accurate results than other estimators.

• Journal of Advanced Marine Engineering and Technology
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• v.10 no.2
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• pp.146-155
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• 1986

Until recently, the calculation of torsional vibration for the marine diesel engine shafting has been performed only for vibratory stresses of resonant points and vibratory stresses for other engine speeds are determined by the estimation. With the advent of energy-saving engines which have a long stroke and a small number of cylinders, the first major critical torsional vibration of the propulsion shaft appears ordinarily near the MCR speed of engine and the flank of its vibratory stress exceeds now and then the limit stress defined by the rules of Classification Society. In order to know the above condition in the design stage of propulsion shafting, it is necessary to calculate the forced torsional vibration with the damping of propulsion shafting for all orders and to synthesize its calculated results according to their phase angles. In this study, the forced torsional vibrations with the damping of propulsion shafting are calculated for several orders by mechanical impedance method, and their results are synthesized. A computer program for above calculations are developed and some test-runs of the developed program are performed for propulsion shaftings of actual ships. The results of calculations are compared with measured values and also with those of the modal analysis method. They show fairly good agreements and the developed program is checked up on its reliability.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.24 no.8 s.179
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• pp.1949-1957
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• 2000

All the loads in the real world act dynamically on structures. Since dynamic loads are extremely difficult to handle in analysis and design, static loads are utilized with dynamic factors. The dyna mic factors are generally determined based on experiences. Therefore, the static loads can cause problems in precise analysis and design. An analytical method based on modal analysis has been proposed for the transformation of dynamic loads into equivalent static load sets. Equivalent static load sets are calculated to generate an identical displacement field in a structure with that from dynamic loads at a certain time. The process is derived and evaluated mathematically. The method is verified through numerical tests. Various characteristics are identified to match the dynamic and the static behaviors. For example, the opposite direction of a dynamic load should be considered due to the vibration response. A dynamic bad is transformed to multiple equivalent static loads according to the number of the critical times. The places of the equivalent static load can be different from those of the dynamic load. An optimization method is defined to use the equivalent static loads. The developed optimization process has the same effect as the dynamic optimization which uses the dynamic loads directly. Standard examples are solved and the results are discussed

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2001.05a
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• pp.942-947
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• 2001

This paper provides a stability analysis of the transverse vibration of a spinning disk under the torque fluctuation from a permanent magnetic motor. An analytical model has been formulated for a flexible annular disk with its spinning velocity varying harmonically with the same frequency as the cogging torque. A perturbation method based on multiple time scales is applied to perform the stability analysis. Based on expressions for the amplitude and frequency of the parametric excitation, stability boundaries are determined in terms of a nominal spindle velocity, the least common multiple of poles and slots, the magnitude of torque fluctuation and the modal characteristics of. the disk. The stability diagrams predicted by perturbation have been verified numerically using the Floquet theory, which is in good agreement. In conclusion, the fluctuation in spinning velocity is found to affect the stability of the transverse vibration of a rotating disks. The results of this work can be applied to high precision spindle systems such as computer storage systems.

• Steel and Composite Structures
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• v.25 no.4
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• pp.485-496
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• 2017

In this paper, damage detection has been introduced as an optimization problem and a two-step method has been proposed that can detect the location and severity of damage in truss structures precisely and reduce the volume of computations considerably. In the first step, using the residual force vector concept, the suspected damaged members are detected which will result in a reduction in the number of variables and hence a decrease in the search space dimensions. In the second step, the precise location and severity of damage in the members are identified using the genetic algorithm and the results of the first step. Considering the reduced search space, the algorithm can find the optimal points (i.e. the solution for the damage detection problem) with less computation cost. In this step, the Efficient Correlation Based Index (ECBI), that considers the structure's first few frequencies in both damaged and healthy states, is used as the objective function and some examples have been provided to check the efficiency of the proposed method; results have shown that the method is innovatively capable of detecting damage in truss structures.

• Nuclear Engineering and Technology
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• v.47 no.2
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• pp.219-225
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• 2015

In this paper, a three-dimensional model for the dynamic analysis of a flywheel based on the finite element method is presented. The static structure analysis for the model provides stress and strain distribution cloud charts. The modal analysis provides the basis of dynamic analysis due to its ability to obtain the natural frequencies and the vibration-made vectors of the first 10 orders. The results show the main faults are attrition and cracks, while also indicating the locations and patterns of faults. The harmonic response simulation was performed to gain the vibration response of the flywheel under operation. In this paper, we present a Hilbert-Huang transform (HHT) algorithm for flywheel vibration analysis. The simulation indicated that the proposed flywheel vibration signal analysis method performs well, which means that the method can lay the foundation for the detection and diagnosis in a reactor main coolant pump.

• Journal of the Korean Society for Precision Engineering
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• v.28 no.8
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• pp.959-965
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• 2011

This paper presents the dynamic modeling and vibration suppression methods for axially deploying beams subjected to gravity. A modal modeling method is employed to develop the lateral vibration model for axially deploying beams. Simulation is made to validate the proposed model as well as to investigate the dynamics of axially deploying beams. This paper rigorously investigates the gravity effect as a source of vibration for axially deploying beams. In order to suppress lateral vibration for deploying beams, the moving speed command is modified by using the input shaping method, Experiments are also performed to prove the proposed vibration suppression method. The simulations and experiments show that the proposed modeling and input shaping methods are effective for the dynamic analysis and vibration suppression of axially deploying beams subjected to gravity.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 1995.10a
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• pp.62-67
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• 1995

A method for determining impulsive responses and acoustic radiation for submerged shells of finite length has been presented. The method is a modal-based method, and uses a surface variational principle to obtain data in the frequency domain. The fast Fourier transform technique is used to convert the data to the time domain. The surface pressure responses of a cylindrical shell with endcaps wer compared with those of an infinite shell. It was shown that the surface pressures coincide exactly before any significant reflections from the endcaps occur. Traces of different types of waves were identified from the dispersion relations of the infinite shell. The contributions of flexural and longitudinal waves and these due to the direct radiation from the driving force to the fluid pressure were demonstrated using near-field plots. The exchange of energy between the shell and fluid was examined for shells with and without bulkheads. It was shown that a significant amount of the energy which enters the fluid returns to the shell and most of the energy is dissipated in the shell. It was also shown that the shell with bulkheads radiate significantly more energy into the far-field than the empty shell.