• 한국세라믹학회지
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• 제42권12호
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• pp.854-859
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• 2005

Performance of micro scale intermediate temperature solid oxide fuel cell system has been successfully evaluated by computer simulation based on macro modeling. Two systems were studied in this work. The one is designed that the ceria-based electrolyte placed between composite electrodes and the other is designed that electrodes alternately placed on the electrolyte. The injected gas was composed of hydrogen and air. The polarization curve was obtained through a series of calculations for ohmic loss, activation loss and concentration loss. The calculation of each loss was based on the solving of mathematical model of multi physical-phenomena such as ion conduction, fluid dynamics and diffusion and convection by Finite Element Method (FEM). The performance characteristics of SOFC were quantitatively investigated for various structural parameters such as distance between electrodes and thickness of electrolyte.

• Ocean Systems Engineering
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• 제1권1호
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• pp.17-28
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• 2011

The purpose of this paper is to demonstrate the efficacy of modeling a surface effect ship's air-cushion flexible seal utilizing a two-dimensional beam under steady state conditions. This effort is the initial phase of developing a more complex three-dimensional model of the air-seal-water fluid-structure interaction. The beam model incorporates the seal flexural rigidity and mass with large deformations while assuming linear elastic material response. The hydrodynamic pressure is derived utilizing the OpenFOAM computational fluid dynamic (CFD) solver for a given set of steady-state flow condition. The pressure distribution derived by the CFD solver is compared with the pressure required to deform the seal beam model. The air pressure, flow conditions and seal geometry are obtained from experimental analysis. The experimental data was derived from large-scale experimental tests utilizing a test apparatus of a canonical surface effect ship's flexible seal in a towing tank over a variety of test conditions.

• Structural Engineering and Mechanics
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• 제85권3호
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• pp.369-379
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• 2023

Reliability-Based Design Optimization (RBDO) is an appropriate framework for obtaining optimal designs by taking uncertainties into account. Large-scale problems with implicit limit state functions and problems with discrete design variables are two significant challenges to traditional RBDO methods. To overcome these challenges, this paper proposes a hybrid method to perform RBDO of structures that links Firefly Algorithm (FA) as an optimization tool to advanced (finite element) reliability methods. Furthermore, the Genetic Algorithm (GA) and the FA are compared based on the design cost (objective function) they achieve. In the proposed method, Weighted Simulation Method (WSM) is utilized to assess reliability constraints in the RBDO problems with explicit limit state functions. WSM is selected to reduce computational costs. To performing RBDO of structures with finite element modeling and implicit limit state functions, a First-Order Reliability Method (FORM) based on the Direct Differentiation Method (DDM) is utilized. Four numerical examples are considered to assess the effectiveness of the proposed method. The findings illustrate that the proposed RBDO method is applicable and efficient for RBDO problems with discrete and continuous design variables and finite element modeling.

• Structural Monitoring and Maintenance
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• 제9권2호
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• pp.179-200
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• 2022

This paper proposes a data-driven methodology for online early damage identification under changing environmental conditions. The proposed method relies on two data analysis methods: feature-based method and hybrid principal component analysis (PCA) and kernel PCA to separate damage from environmental influences. First, spectral sub-band features, namely, spectral sub-band centroids (SSCs) and log spectral sub-band energies (LSSEs), are proposed as damage-sensitive features to extract damage information from measured structural responses. Second, hybrid modeling by integrating PCA and kernel PCA is performed on the spectral sub-band feature matrix for data normalization to extract both linear and nonlinear features for nonlinear procedure monitoring. After feature normalization, suppressing environmental effects, the control charts (Hotelling T² and SPE statistics) is implemented to novelty detection and distinguish damage in structures. The hybrid PCA-KPCA technique is compared to KPCA by applying support vector machine (SVM) to evaluate the effectiveness of its performance in detecting damage. The proposed method is verified through numerical and full-scale studies (a Bridge Health Monitoring (BHM) Benchmark Problem and a cable-stayed bridge in China). The results demonstrate that the proposed method can detect the structural damage accurately and reduce false alarms by suppressing the effects and interference of environmental variations.

• 한국정보과학회논문지:시스템및이론
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• 제33권1_2호
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• pp.15-34
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• 2006

본 논문에서는, 소규모 시뮬레이션을 통하여 수집된 표본에 통계적인 추정방법을 적용하여 모델을 구하는 경험적 모델링 기법을 제안한다. 이 기법을 이용하여 대칭형 구조를 갖는 다중프로세서 시스템에서의 캐시접근실패율을 위한 두 종류의 모델을 구하였다. 목표시스템의 사양이 고정되었을 때 입력데이타의 크기변화에 따르는 모델과, 입력데이타의 크기가 고정되었을 때 목표시스템의 프로세서 수의 변화에 따르는 모델이다. 모델의 정확성을 제고하기 위하여 한 프로그램에 존재하는 공유데이타들에 대하여 종류별 캐시접근실패에 대한 개별적인 모델들을 구한 후 이들을 종합함으로써 최종적인 모델을 구하였다. 또한 최소 제곱 추정법과 로버스트 추정법을 병용하여 이탈점으로 인한 왜곡을 최소화함으로써 모델의 정확도를 향상시켰다. 경험적 모델링은 표본에 대한 분석이 필요 없으면서도 모델의 정확도가 매우 높다. 또한 소규모의 시뮬레이션만 수행하면 되고, 실험을 통하여 일련의 표본을 수집할 수만 있으면 모든 분야의 연구에 적용할 수 있다. 경험적 모델을 이용한 24가지 경우의 예측시도 중 17번의 경우에는 $1\%$ 미만의 예측오차율을 보였으며, 나머지 경우에도 매우 높은 정확도를 보였다. 특히 프로그램의 실행양식이 불규칙하거나, 표본의 수가 충분하기에는 부족한 경우에도 좋은 결과를 보여준다.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제10권8호
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• pp.3602-3620
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• 2016

Dynamic topology is one of the main influence factors on network performability. However, it was always ignored by the traditional network performability assessment methods when analyzing large-scale mobile ad hoc networks (MANETs) because of the state explosion problem. In this paper, we address this problem from the perspective of complex network. A two-layer hierarchical modeling approach is proposed for MANETs performability assessment, which can take both the dynamic topology and multi-state nodes into consideration. The lower level is described by Markov reward chains (MRC) to capture the multiple states of the nodes. The upper level is modeled as a small-world network to capture the characteristic path length based on different mobility and propagation models. The hierarchical model can promote the MRC of nodes into a state matrix of the whole network, which can avoid the state explosion in large-scale networks assessment from the perspective of complex network. Through the contrast experiments with OPNET simulation based on specific cases, the method proposed in this paper shows satisfactory performance on accuracy and efficiency.

• Advances in nano research
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• 제9권4호
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• pp.221-235
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• 2020

The following composition establishes a nonlocal strain gradient plate model that is essentially related to mass sensors laying on Winkler-Pasternak medium for the vibrational analysis from graphene sheets. To achieve a seemingly accurate study of graphene sheets, the posited theorem actually accommodates two parameters of scale in relation to the gradient of the strain as well as non-local results. Model graphene sheets are known to have double variant shear deformation plate theory without factors from shear correction. By using the principle of Hamilton, to acquire the governing equations of a non-local strain gradient graphene layer on an elastic substrate, Galerkin's method is therefore used to explicate the equations that govern various partition conditions. The influence of diverse factors like the magnetic field as well as the elastic foundation on graphene sheet's vibration characteristics, the number of nanoparticles, nonlocal parameter, nanoparticle mass as well as the length scale parameter had been evaluated.

• International Journal of Aeronautical and Space Sciences
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• 제14권4호
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• pp.356-368
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• 2013

The Korea Aerospace Research Institute plans to launch a lunar module by 2025, and so is carrying out a preliminary study. Landing stability on the lunar surface is a key design factor of a lunar module. In this paper, a 1/6 scale model of a lunar module is investigated, for its landing stability on non-level surfaces. The lunar module has four tripod legs, with aluminum honeycomb shock absorbers in each leg strut. ADAMS$^{TM}$, the most widely used multi-body dynamics and motion analysis software, is used to simulate the module's lunar landing. Three types of dampers in the struts (rigid, viscous, and aluminum honeycomb dampers), and two types of lunar surfaces (rigid and elastic) are considered. The Sforce function is adopted, to model the aluminum honeycomb dampers. Details on the modeling and analysis of the landing stability of the 1/6 scale lunar module and the simulation results are provided in this paper.

• Smart Structures and Systems
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• 제1권4호
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• pp.385-404
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• 2005

The paper presents a model to calculate reinforcement strain using measured crack width in members under applied tension, flexure, and/or shear stress. Crack mapping using a new type of distributed coaxial cable sensors for health monitoring of large-scale civil engineering infrastructure was recently proposed and developed by the authors. This paper shows the results and performance of such sensors mounted on near surface of two flexural beams and a large scale reinforced concrete box girder that was subjected to cyclic combined shear and torsion. The main objectives of this health monitoring study was to correlate the sensor's response to strain in the member, and show that magnitude of the signal's reflection coefficient is related to increases in applied load, repeated cycles, cracking, and reinforcement yielding. The effect of multiple adjacent cracks, and signal loss was also investigated. The results shown in this paper are an important step in using the sensors for crack mapping and determining reinforcement strain for in-situ structures.

• Structural Engineering and Mechanics
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• 제50권1호
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• pp.19-36
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• 2014

This paper focuses on a model order reduction (MOR) for large-scale rotordynamic systems by using finite element discretization. Typical rotor-bearing systems consist of a rotor, built-on parts, and a support system. These systems require careful consideration in their dynamic analysis modeling because they include unsymmetrical stiffness, localized nonproportional damping, and frequency-dependent gyroscopic effects. Because of this complex geometry, the finite element model under consideration may have a very large number of degrees of freedom. Thus, the repeated dynamic analyses used to investigate the critical speeds, stability, and unbalanced response are computationally very expensive to complete within a practical design cycle. In this study, we demonstrate that a Krylov subspace-based MOR via moment matching significantly speeds up the rotordynamic analyses needed to check the whirling frequencies and critical speeds of large rotor systems. This approach is very efficient, because it is possible to repeat the dynamic simulation with the help of a reduced system by changing the operating rotational speed, which can be preserved as a parameter in the process of model reduction. Two examples of rotordynamic systems show that the suggested MOR provides a significant reduction in computational cost for a Campbell diagram analysis, while maintaining accuracy comparable to that of the original systems.