• Wind and Structures
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• 제16권2호
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• pp.157-178
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• 2013

Output-only modal parameter identification is based on the assumption that external forces on a linear structure are white noise. However, harmonic excitations are also often present in real structural vibrations. In particular, it has been realized that the use of forced acceleration responses without knowledge of external forces can pose a problem in the modal parameter identification, because an external force is imparted to its impulse acceleration response function. This paper provides a three-stage identification procedure as a solution to the problem of harmonic and white noise excitations in the acceleration responses of a linear dynamic system. This procedure combines the uses of the mode indicator function, the complex mode indication function, the enhanced frequency response function, an iterative rational fraction polynomial method and mode shape inspection for the correlation-related functions of the force-embedded acceleration responses. The procedure is verified via numerical simulation of a five-floor shear building and a two-dimensional frame and also applied to ambient vibration data of a large-span roof structure. Results show that the modal parameters of these dynamic systems can be satisfactorily identified under the requirement of wide separation between vibration modes and harmonic excitations.

• 천문학회보
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• 제43권2호
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• pp.42.1-42.1
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• 2018

Collisionless shocks with low sonic Mach numbers, M<4, are expected to accelerate cosmic ray (CR) protons via diffusive shock acceleration (DSA) in the intracluster medium (ICM). However, observational evidence for CR protons in the ICM has yet to be established. Performing particle-in-cell simulations, we study the injection of protons into DSA and the early development of a nonthermal particle population in weak shocks in high ${\beta}$ plasmas. Reflection of incident protons, self-excitation of plasma waves via CR-driven instabilities, and multiple cycles of shock drift acceleration are essential to the early acceleration of CR protons in supercritical quasi-parallel shocks. We find that only in ICM shocks with $M{\geq}2.3$, a sufficient fraction of incoming protons are reflected by the overshoot in the shock electric potential and magnetic mirror at locally perpendicular magnetic fields, leading to efficient excitation of magnetic waves via CR streaming instabilities and the injection into the DSA process. Since a significant fraction of ICM shocks have M < 2.3 CR proton acceleration in the ICM might be less efficient than previously expected. This may explain why the diffuse gamma-ray emission from galaxy clusters due to proton-proton collisions has not been detected so far.

• Nuclear Engineering and Technology
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• 제55권4호
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• pp.1428-1438
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• 2023

A novel Fission Diffusion Synthetic Acceleration (FDSA) method is developed and implemented as a part of a hybrid neutronics method for source convergence acceleration and variance reduction in Monte Carlo (MC) criticality calculations. The acceleration of the MC calculation stems from constructing a synthetic operator and solving a low-order problem using information obtained from previous MC calculations. By applying the P₁ approximation, two correction terms, one for the scalar flux and the other for the current, can be solved in the low-order problem and applied to the transport solution. A variety of one-dimensional (1-D) and two-dimensional (2-D) numerical tests are constructed to demonstrate the performance of FDSA in comparison with the standalone MC method and the coupled MC and Coarse Mesh Finite Difference (MC-CMFD) method on both intended purposes. The comparison results show that the acceleration by a factor of 3-10 can be expected for source convergence and the reduction in MC variance is comparable to CMFD in both slab and full core geometries, although the effectiveness of such hybrid methods is limited to systems with small dominance ratios.

• Smart Structures and Systems
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• 제31권5호
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• pp.501-516
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• 2023

Dynamic irregularity and acceleration of bridges subjected to high-speed trains provide crucial information for comprehensive evaluation of the health state of under-track structures. This paper proposes a novel approach for real-time estimation of vertical track dynamic irregularity and bridge acceleration using deep generative adversarial network (GAN) and vibration data from in-service train. The vehicle-body and bogie acceleration responses are correlated with the two target variables by modeling train-bridge interaction (TBI) through least squares generative adversarial network (LSGAN). To realize supervised learning required in the present task, the conventional LSGAN is modified by implementing new loss function and linear activation function. The proposed approach can offer pointwise and accurate estimates of track dynamic irregularity and bridge acceleration, allowing frequent inspection of high-speed railway (HSR) bridges in an economical way. Thanks to its applicability in scenarios of high noise level and critical resonance condition, the proposed approach has a promising prospect in engineering applications.

• 농업과학연구
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• 제50권3호
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• pp.469-484
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• 2023

In this study, three types of structures-stepped gabion retaining walls, vertical gabion retaining walls, and parapets-were installed on the dam floor crest to prevent the overflow of deteriorative homogeneous reservoirs. The acceleration response, displacement behavior, and pore water pressure ratio behavior were compared and evaluated using shaking-table model tests. The experimental conditions were set to 0.154 g in consideration of the domestic standard and the seismic acceleration range according to the magnitude of the earthquake, and the input waveform was applied with Pohang, Gongen, and artificial earthquake waves. The acceleration response according to the design ground acceleration increased as the height of the embankment increased, and the observed value were larger in the range of 1.1 to 2.1 times the input acceleration for all structures. The horizontal and vertical displacements exhibited maximum values on the upstream slope, and the embankment was evaluated as stable and included within the allowable range for all waveforms. The settlement ratio considering the similarity law exhibited the least change in the case of the parapet structure. The amplification ratio was 1.1 to 1.5 times in all structures, with the largest observed in the dam crest. The maximum excess pore water pressure ratio was in the range of 0.010 - 0.021, and the liquefaction evaluation standard was within 1.0, which was considered very stable.

• Geomechanics and Engineering
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• 제36권2호
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• pp.157-166
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• 2024

Generally, the rubble mound installed on the slope embankment of the open-type wharf is designed based on the impact of wave force, with no consideration for the impact of seismic force. Therefore, in this study, dynamic centrifuge model test results were analyzed to examine the acceleration amplification of embankment reinforced with rubble mound under seismic conditions. The experimental results show that when rubble mounds were installed on the ground surface of the embankment, acceleration response of embankment decreased by approximately 22%, and imbalance in ground settlement decreased significantly from eight to two times. Furthermore, based on the experimental results, one-dimensional site response (1DSR) analyses were conducted. The analysis results indicated that reinforcing the embankment with rubble mound can decrease the peak ground acceleration (PGA) and short period response (below 0.6 seconds) of the ground surface by approximately 28%. However, no significant impact on the long period response (above 0.6 seconds) was observed. Additionally, in ground with lower relative density, a significant decrease in response and wide range of reduced periods were observed. Considering that the reduced short period range corresponds to the critical periods in the design response spectrum, reinforcing the loose ground with rubble mound can effectively decrease the acceleration response of the ground surface.

• 품질경영학회지
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• 제52권1호
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• pp.135-148
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• 2024

Purpose: This study is to investigate the literature on accelerated tests based on the acceleration model and to provide a compilation of results on the parameters applied in the acceleration model and the test conditions. Methods: This research is conducts a literature review on accelerated tests using the acceleration model, with a focus on test targets, test conditions, and parameter values. The study is organizing the results of this literature review to facilitate their application in the design of reliability tests. Results: A literature review investigated a variety of test targets, test conditions, and parameter values. Conclusion: The results of the literature research conducted revealed various acceleration model parameter. Such literature research on accelerated tests can establish the foundation for reliability test design and contribute to future product development and quality improvement

• 한국전산구조공학회논문집
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• 제24권1호
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• pp.107-117
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• 2011

본 논문에서는 PSC 거더 교량의 가속도-임피던스 응답 특성을 이용하는 손상 모니터링에 있어 온도유발 불확실성의 영향을 분석하였다. 먼저, 전역적 및 국부 진동 특성을 이용한 손상 모니터링 기법을 설계하였다. 전역적 및 국부 특성으로 각각 가속도 및 전기-역학적 임피던스 특성을 선정하였다. 다음으로 모형 PSC 거더 교량을 이용하여 온도유발 가속도-임피던스 응답 특성을 실험적으로 분석하였다. 실험 결과로부터 온도-가속도 및 온도-임피던스 응답 특징들에 대한 보정식을 산출하였다. 마지막으로 긴장력 감소 및 휨 강성 저하 경우들이 실험된 모형 PSC 거더를 대상으로 산출된 보정식을 이용한 가속도-임피던스기반 손상 모니터링 기법의 유용성이 평가되었다.

• 동력기계공학회지
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• 제16권2호
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• pp.43-48
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• 2012

Generally accelerated life test is well known as one method to reduce reliability qualification test period. This test is conducted under the higher stress condition than normal condition. So it can save the test time by calculating the acceleration factor from the relationship between the worst stress condition and normal condition. This paper investigates the worst stress condition for the accelerated life test to increase the acceleration factor. Especially, we focused on the method to obtain effective acceleration factors under the worst stress condition. Moreover, we considered how to decide the worst stress condition by looking for the operating limit of this system. The acceleration factor can be estimated from the ratio of the kinematic viscosity in the normal condition and the worst stress condition, the lowest temperature, by using Arrhenius relationship. Through some experiments for a refrigerator's compressor, we were able to confirm how to increase acceleration factors and how to reduce the reliability qualification test period with minimum samples.

• 대한기계학회논문집A
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• 제38권10호
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• pp.1157-1162
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• 2014

시간 이력 지진해석시 두 가지 가진 방법론[유효하중법(또는 관성법), 거대질량법]이 적용되고 있는데 균열 없는 구조물에 대해서만 두 가지 가진 방법론의 타당성을 확인한 바 있으나, 균열로 인해 강성이 변화하는 균열 배관에 대해서는 가진 방법론의 타당성에 대한 연구가 수행된 바 없다. 본 연구에서는 시간이력 Implicit 동적 탄성 지진해석을 통해 탄성 파괴역학 측면에서 관통 균열 배관에 대한 두 가지 가진 방법론의 타당성을 평가하였다. 평가 결과, 거대질량의 크기와 최대 시간 증분이 적절히 선정된다면 균열 배관에 대해서도 두 가지 가진 방법론이 모두 동일한 결과를 도출함을 확인하였다.