Device instabilities of graphene metal-oxide-semiconductor field effect transistors such as hysteresis and Dirac point shifts have been attributed to charge trapping in the underlying substrate, especially in SiO2. In this letter, trapping time constants around 87 μs and 1.76 ms were identified using a short pulse current-voltage method. The values of two trapping time constants with reversible trapping behavior indicate that the hysteretic behaviors of graphene field effect transistors are due to neither charge trapping in the bulk SiO2 or tunneling into other interfacial materials. Also, it is concluded that the dc measurement method significantly underestimated the performance of graphene devices.
This study evaluates the variation characteristics of wave fields (transmission ratio, wave height, timeaveraged velocity and time-averaged turbulent kinetic energy) for two-dimensional low-crested structure by ola-Flow model based on the two-phases flow numerically. In addition, the present numerical results are verified by comparing with the existing experimental results. The time-averaged velocity, one of various numerical results is formed counterclockwise circulating cell on the front of structure and is occurred strong uni-directional flow on onshore side. It is shown that these are closely related to the factors such as overtopping, etc.
요 지 : 본 연구는 olaFlow 모델에 의한 혼상류수치해석법을 적용하여 저천단구조물에 의한 불규칙파랑의 변동특성 을 수치적으로 검토한다. 목표스펙트럼에 의한 불규칙파형과 측정된 불규칙파형과의 비교 및 이로부터 산정된 두 주 파수스펙트럼의 비교로부터 olaFlow의 타당성을 검증하고, 2차원저천단구조물에 의한 불규칙파의 주파수스펙트럼의 변동, 파전달율, H r m s 의 공간분포, 평균유속과 평균난류운동에너지의 공간분포 등을 수치적으로 검토한다. 이로부터 LCS 외해측의 전면에서 시계방향의 해빈류와 반시계방향의 순환류셀이 형성되고, 내해측의 정수면 근방에서 항내로 향하는 강한 일방향흐름이 발생되는 등의 중요한 결과를 얻을 수 있었다. 핵심용어 : 저천단구조물, olaFlow 모델, 불규칙파스펙트럼, 파전달율, Hr m s , 평균유속(해빈류), 평균난류운동에너지 Abstract : This study evaluates the variation characteristics of irregular wave fields for two-dimensional Low-Crested Structure (LCS) by olaFlow model based on the two-phases flow by numerical analysis. The numerical results of olaFlow model are verified by comparing irregular wave profile of target wave spectrum and measured one, and their spectra. In addition, spacial variation of irregular wave spectrum, wave transmission ratio, rootmean square wave height, time-averaged velocity and time-averaged turbulent kinetic energy by two-dimensional LCS are discussed numerically. The time-averaged velocity, one of the most important numerical results is formed counterclockwise circulating cell and clockwise nearshore current on the front of LCS, and strong uni-directional flow directing onshore side around still water level. Keywords : low-crested structure, olaFlow model, irregular wave spectrum, wave transmission ratio, H r m s , timeaveraged velocity (nearshore current), time-averaged turbulent kinetic energy 1. 서 론 저천단구조물(LCS; Low-Crested Structure)과 잠제(Submerged breakwater)는 연안재해와 해빈침식의 방지라는 거의 동일한 목적으로 천해역에 설치되지만(Lee et al., 2019b), 국내에서 는 잠제가 대부분을 차지하는 상황이다. 하지만, 잠제의 과다 한 공사비에 대한 비경제성과 해빈침식방지에 대한 비효율성 때문에 유럽을 비롯한 해외에서는 LCS의 천단상에서 급류의 형성에 따른 구조적인 취약성과 안전상의 문제점이 있을 수 있지만 LCS에 대한 연구가 활발히 진행되어 왔다. 대표적으 로 EU에서 수행된 DELOS프로젝트(2003)를 들 수 있고, 이 외에 van der Meer and Daemen(1994), Seabrook et al. (1998), van der Meer et al.(2000) 및 Goda and Ahrens (2008) 등에 의한 파전달율과 Briganti et al.(2003), van der Meer et al.(2005) 및 Buccino et al.(2009) 등에 의한 파스 펙트럼의 변동에 관한 해외의 연구사례를 들 수 있다. 국내 에서는 Kim and Lee(2017), Lee et al.(2017a) 및 Lee et al.(2019a, b) 등의 연구가 수행된 바 있다. 이상의 연구들은 대부분 실험에 기반한 연구이며, 경사제 와 같은 사석 혹은 이형블록으로 구성된 단순한 단면형상이 적용되었다. 그리고, 실험에서는 파랑제어에 관한 파고전달율 이나 구성재료의 안정중량에 집중되었지만 LCS 주변에서 흐 름과 난류 등의 변동특성은 거의 규명되어 있지 않은 상황이 고, 투과성구조물과 케이슨이 병용되는 본 연구의 복합단면 이 적용된 사례는 거의 없다. 한편, 본 연구의 전편에 해당하는 Lee et al.(2019b)은 2차 원규칙파동장 하의 수치파동수로 내에 설치된 복합단면의 LCS를 대상으로 LCS에 의한 파고전달율과 LCS 주변에서 특
On the many coasts of South Korea, including the eastern side, it has been recently increasing the coastal disaster such as the severe coastal erosion and road damage swept away by the wave. As one of the alternatives to prevent the coastal disaster, it has been widely studied the coastal disaster reduction method by the Low-Crested Structure (LCS) in the many countries including several European countries. In this study, the ola-FLow model is used to simulate the permeable LCS and wave field of the LCS through the three-dimensional irregular waves numerical analysis on the basis of the previous research. From the numerical analysis, it is evaluated the H r m s , nearshore current and time-averaged turbulent kinetic energy. In addition, the pattern of nearshore current and spatial distribution of time-averaged turbulent kinetic energy are compared with the case of submerged breakwater under the irregular wave fields. As one of significant results, it is confirmed that the pattern of nearshore current is different with the case of submerged breakwater.
Chaff is a passive jammer widely used to disrupt radar or radio-frequency sensors. A mass of chaff fibers dispersed in the air is commonly referred to as a chaff cloud. It is nearly impossible to numerically simulate in real-time the enormous amount of chaff fibers composing the chaff cloud. In this paper, we model the behavior of numerically estimated chaff clouds as probability density functions (PDFs) and apply approximation techniques to estimate the radar cross-section (RCS) of the chaff cloud in real time. To model the aerodynamics of the chaff cloud, we represented the combination of PDFs as functions of time and wind speed. The applied approximation techniques—vector radiative transfer and generalized equivalent conductor method—showed a computation time that cannot be achieved by low-frequency methods such as the method of moments or finite-difference time-domain. Moreover, the dynamic RCS results of the approximation techniques showed a similar trend to those of other studies simulating similar situations. The proposed scheme is effective for real-time chaff cloud simulation, and the modeled dynamics and estimated dynamic RCSs can be a standard baseline for developing new analysis methods for chaff clouds. In the future, the proposed scheme will extend to more chaff fibers and more diverse environmental parameters.
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