• 한국해양공학회지
• /
• 제17권3호
• /
• pp.46-51
• /
• 2003

Acoustic emission (AE) signals, emanated during local failure of aluminum alloys, have been the subject of numerous investigations. It is well known that the characteristics of AE are strongly influenced by the previous thermal and mechanical treatment of the sample. Possible sources of AE during deformation have been suggested as the avalanche motion of dislocations, fracture of brittle particles, and debonding of these particles from the alloy matrix. The goal of the present study is to determine if AE occurring as the result of fatigue crack propagation could be evaluated by the joint time-frequency analysis method, short time Fourier transform (STFT), and Wigner-Ville distribution (WVD). The time-frequency analysis methods can be used to analyze non-stationary AE more effectively than conventional techniques. STFT is more effective than WVD in analyzing AE signals. Noise and frequency characteristics of crack openings and closures could be separated using STFT. The influence of various fatigue parameters on the frequency characteristics of AE signals was investigated.

• Journal of Ship and Ocean Technology
• /
• 제4권3호
• /
• pp.1-12
• /
• 2000

A number of numerical methods like Computational Fluid Dynamics(CFD) have been developed to predict the flow fields of a vessel but the present study is developed to infer the wake fields on propeller plane by Statistical Fluid Dynamics(SFD) approach which is emerging as a new technique over a wide range of industrial fields nowadays. Neural network is well known as one prospective representative of the SFD tool and is widely applied even in the engineering fields. Further to its stable and effective system structure, generalization of input training patterns into different classification or categorization in training can offer more systematic treatments of input part and more reliable result. Because neural network has an ability to learn the knowledge through the external information, it is not necessary to use logical programming and it can flexibly handle the incomplete information which is not easy to make a definition clear. Three dimensional stern hull forms and nominal wake values from a model test are structured as processing elements of input and output layer respectively and a neural network is trained by the back-propagation method. The inferred results show similar figures to the experimental wake distribution.

• 한국광학회지
• /
• 제24권4호
• /
• pp.189-195
• /
• 2013

본 논문에서는 두 매의 BD-2 렌즈와 비구면 회절 광학을 포함하는 F/1.2, 시야각 $15.2^{\circ}$와 $8-14{\mu}m$의 원적외선 파장대역을 가지는 광학계를 설계하였다. 이 광학계는 uncooled camera에 사용될 수 있으며, 광학계의 분석은 BSP로 진행되었다. 회절광학은 주어진 두께를 가지는 물리적인 면으로 취급하였으며, 분석결과를 출사동공법과 같은 기존 방법들에 의한 것과 회절차수 합성법에 의한 것과 비교하였다.

• 한국재료학회지
• /
• 제12권7호
• /
• pp.522-527
• /
• 2002

To investigate the SAW properties of GaN films on c-plane sapphire substrates, we carried out both the experimental measuring and theoretical calculation. The experimental characterization of SAW propagation properties was performed with a linear array of interdigital transducer (IDT) structures, while SAW velocities were calculated by matrix methods. HVPSAW mode with the propagation velocity over 10,000m/s and PSAW mode as well as GSAW could be observed in experimental determination. These results were verified by matching with the theoretical calculation.

• ETRI Journal
• /
• 제43권3호
• /
• pp.377-388
• /
• 2021

Owing to the difficulties associated with conducting millimeter-wave (mmWave) field measurements, especially in high-speed train (HST) environments, most propagation channels for mmWave HST have been studied using methods based on simulation rather than measurement. In this study, considering a linear cell layout in which base stations are installed along a railway, measurements were performed at 28 GHz with a speed up to 170 km/h in two prevalent HST scenarios: viaduct and tunnel scenarios. By observing the channel impulse responses, we could identify single- and double-bounced multipath components (MPCs) caused by railway static structures such as overhead line equipment. These MPCs affect the delay spread and Doppler characteristics significantly. Moreover, we observed distinct path loss behaviors for the two scenarios, although both are considered line-of-sight (LoS) scenarios. In the tunnel scenario, the path loss exponent (PLE) is 1.3 owing to the waveguide effect, which indicates that the path loss is almost constant with respect to distance. However, the LoS PLE in the viaduct scenario is 2.46, which is slightly higher than the free-space loss.

• Steel and Composite Structures
• /
• 제31권4호
• /
• pp.341-359
• /
• 2019

Non-monolithic concrete structural connections are commonly used both in new constructions and retrofitted structures where anchors are used for connections. Often, flaws are present in anchor system due to poor workmanship and deterioration; and methods available to check the quality of the composite system afterward are very limited. In case of presence of flaw, load transfer mechanism inside the anchor system is severely disturbed, and the load carrying capacity drops drastically. This raises the question of safety of the entire structural system. The present study proposes a wave propagation technique to assess the integrity of the anchor system. A chemical anchor (embedded in concrete) composite system comprising of three materials viz., steel (anchor), polymer (adhesive) and concrete (base) is considered for carrying out the wave propagation studies. Piezoelectric transducers (PZTs) affixed to the anchor head is used for actuation and the PZTs affixed to the surrounding concrete surface of the concrete-anchor system are used for sensing the propagated wave through the anchor interface to concrete. Experimentally validated finite element model is used to investigate three types of composite chemical anchor systems. Studies on the influence of geometry, material properties of the medium and their distribution, and the flaw types on the wave signals are carried out. Temporal energy of through time domain differentiation is found as a promising technique for identifying the flaws in the multi-layered composite system. The present study shows a unique procedure for monitoring of inaccessible but crucial locations of structures by using wave signals without baseline information.

• Nuclear Engineering and Technology
• /
• 제50권7호
• /
• pp.1043-1050
• /
• 2018

For an efficient Monte Carlo (MC) burnup analysis, an accurate high-order depletion scheme to consider the nonlinear flux variation in a coarse burnup-step interval is crucial accompanied with an accurate depletion equation solver. In a Seoul National University MC code, McCARD, the high-order depletion schemes of the quadratic depletion method (QDM) and the linear extrapolation/quadratic interpolation (LEQI) method and a depletion equation solver by the Chebyshev rational approximation method (CRAM) have been newly implemented in addition to the existing constant extrapolation/backward extrapolation (CEBE) method using the matrix exponential method (MEM) solver with substeps. In this paper, the quadratic extrapolation/quadratic interpolation (QEQI) method is proposed as a new high-order depletion scheme. In order to examine the effectiveness of the newly-implemented depletion modules in McCARD, four problems in the VERA depletion benchmarks are solved by CEBE/MEM, CEBE/CRAM, LEQI/MEM, QEQI/MEM, and QDM for gadolinium isotopes. From the comparisons, it is shown that the QEQI/MEM predicts ${k_{inf}}^{\prime}s$ most accurately among the test cases. In addition, statistical uncertainty propagation analyses for a VERA pin cell problem are conducted by the sensitivity and uncertainty and the stochastic sampling methods.

• 한국항행학회논문지
• /
• 제28권2호
• /
• pp.172-177
• /
• 2024

본 논문은 레이다로 다중경로 상황에서 고각 추정을 할 때 고각 추정 오차가 커지는 문제를 극복하기 위한 방법을 제시하였다. 다중경로 상황 이란, 동일한 표적에서 반사된 레이다의 수신신호가 여러경로에서 오는 것을 의미한다. 다중경로 상황이 아닐 때는 모노펄스 방식이 정확하고, 그 반대 상황이면 최소 제곱 오차 추정 방식이 정확하다. 다중경로 상황이면서 고각이 매우 낮을 경우, 최소 제곱 오차 추정이 발산하는 특이 경우가 발생한다. 이 특이경우를 전파 인자 기반으로 판별하여, 모노펄스와 최소 제곱 오차 추정 방식을 선택적으로 운용했다. 그 결과, 고각 추정의 정확도를 높이는 데 성공했다. 본 논문에서 제안한 방법을 검증하기 위하여 매트랩 시뮬레이션을 수행했다.

• 원예과학기술지
• /
• 제28권4호
• /
• pp.535-539
• /
• 2010

국내 육성 품종인 '매향' 딸기의 육묘 기간 중 광합성유효광량자속의 환경 조절을 통하여 러너 및 러너플랜트의 발생과 생산 효율을 증대시키고자 하였다. 딸기 육묘에 이용된 증식체의 엽수와 관부 직경은 각각 $3.1{\pm}0.4$와 $7.0{\pm}1.1mm$였다. 광합성유효광량자속을 각각 140, 210 및 $280{\mu}mol{\cdot}m^{-2}{\cdot}s^{-1}$로 설정한 9기의 인공광 이용형 육묘 모듈에서 35일 동안 육묘하였다. 러너플랜트 생산 효율은 증식체당 일일 0.27주로 처리구 중 유의하게 가장 높았으며 관행의 딸기 육묘 방식의 러너플랜트 생산 효율과 비교하여도 크게 증진되었음을 확인하였다. 폐쇄형 육묘 시스템을 활용하여 육묘 기간 중의 PPF를 $280{\mu}mol{\cdot}m^{-2}{\cdot}s^{-1}$로 조절하면 국내 육성 품종인 '매향'의 러너 발생 및 러너플랜트 생산을 증진시킬 수 있음이 확인되었다. 따라서 최근 국내에서 육성된 신품종 딸기의 급속 보급을 위한 증식체계 구축에 본 연구 기술이 활용되면 육성된 품종의 조급 보급이 가능할 것으로 판단된다.

• 터널과지하공간
• /
• 제21권2호
• /
• pp.128-137
• /
• 2011

수직 시추공에 대한 일반적인 수압파쇄시험으로부터 구해지는 균열폐쇄압력은 암반의 최소수평주응력을 직접 나타내기 때문에 현지암반의 응력분포양상을 해석하는데 있어서 매우 중요한 요소이다. 그러나 수압파쇄균열의 거동과 현지암반의 응력분포양상의 관계로 인하여 대부분의 경우 이 균열폐쇄압력은 수압파쇄 압력이력곡선 상에서 애매모호한 값으로 나타난다. 본 연구에서는 수압파쇄시험으로부터 균열폐쇄압력을 산정하기 위하여 여러 연구자들에 의해 제안된 기법들의 특성을 비교해 보고자 수치해석을 실시하였다. 즉, 유체의 가압에 의한 암반 내 균열의 발생이라는 수압파쇄의 특성을 모사하기 위하여 H-M couple 해석을 적용하였으며, 또한 수치해석 모델의 형상학적 특성에 따른 균열의 전파양상을 검토하기 위해 4가지 서로 다른 형태의 요소망을 구축하여 해석을 실시하였다. 각각의 요소망에 대한 수치해석 결과, 그래픽 방법이 통계적 방법에 비해 상대적으로 낮은 수준의 균열폐쇄압력을 보였으며, 따라서 시험공 주변에서의 응력 이상대의 존재 및 복잡한 메커니즘을 수반하는 수압파쇄균열의 발생양상을 감안할 때 수압파쇄시험에 의한 균열폐쇄압력의 산정시 특별한 주의가 요구된다.