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위상을 포함한 다방향 불규칙파 조건에서의 이안류 수치모의
최준우(Junwoo Choi) 한국해안해양공학회 2015 한국해안해양공학회 논문집 Vol.27 No.4
Choi et al.(2015)은 최근 Boussinesq 방정식 모형인 FUNWAVE를 이용하여, 다방향 불규칙파 조건으로 SandyDuck 현장실험 조건의 연안흐름을 수치모의하여, 연안류 유속분포 뿐만 아니라 전단파동 등의 계산결과가 관측결과와 잘 일치함을 보였다. 이 연구모형을 기반으로 SandyDuck 지형과 더불어 다방향 위상의 상호작용에 의해 발생되는 연안방향의 파에너지 비균등성이 이안류 발달에 미치는 영향을 고찰하기 위해 수치모의를 수행하였다. 이 결과 지형에 의해 발생하는 이안류가 지배적인 규칙파 조건과 달리 다방향 불규칙파 위상의 상호작용에 따른 돌발적 이안류의 발생이 추가되어 매우 복잡한 쇄파대의 연안 순환흐름을 재현함을 알 수 있었다. Recently, Choi et al.(2015) showed that a numerical simulation of the SandyDuck experiment under a directional random wave environment agreed well with the experimental data including the wave height distribution of the random waves, the well-developed longshore current and its energetic fluctuation. Based on the Boussinesq modeling, this study investigates the effect of the alongshore variations, which are induced by not only the field topography but also the phase interaction of multidirectional random waves in the surf zone wave field, on the rip currents. As a result, transient rip currents as well as topographical rip currents cause the complicated surfzone circulation and mixing process due to their interactions in a multi-directional random wave condition while the topographical rip currents are dominant in a monochromatic wave condition.
Thermally stable, planar hybrid perovskite solar cells with high efficiency
Choi, Kyoungwon,Lee, Junwoo,Kim, Hong Il,Park, Cheol Woong,Kim, Guan-Woo,Choi, Hyuntae,Park, Sungjin,Park, Sang Ah,Park, Taiho The Royal Society of Chemistry 2018 ENERGY AND ENVIRONMENTAL SCIENCE Vol.11 No.11
<P>We report a highly effective interface engineering strategy for thermally stable perovskite solar cells (PSCs) by employing a zwitterion-modified SnO2 electron transport layer (ETL) and a dopant-free hole transport layer (HTL). A zwitterionic compound, 3-(1-pyridinio)-1-propanesulfonate, is used to modify the SnO2 ETL. The zwitterion, which forms interfacial dipoles, plays a few important roles: (1) it causes shifts in the work function of SnO2 resulting in more efficient charge extraction and an increase in the built-in potential. (2) It pulls electrons from perovskite layers to the ETL/perovskite interface, enhancing the electron transport ability. (3) Interfacial dipoles prevent back transfer of electrons from the ETL to the perovskite and suppress charge recombination. (4) Positively charged atoms in the zwitterion passivate Pb-I antisite defects, improving the stability of devices. With these desirable properties, the PSC with doped Spiro-OMeTAD obtained a power conversion efficiency of 21.43%. In addition, the PSC with the dopant-free HTL exhibited a record high efficiency of 20.5% among dopant-free polymeric HTLs using green solvents. The resulting PSCs without encapsulation showed excellent thermal stability. Accordingly, this work suggests that the use of a modified ETL and a dopant-free HTL is a promising strategy to overcome the thermal instability of planar-PSCs (P-PSCs).</P>
Choi, Junwoo,Lee, Dongkyu,Hwang, Hyun-Su,Kim, Byungkyu SAGE Publications 2015 Proceedings of the Institution of Mechanical Engin Vol.229 No.4
<P>In this paper, we address the design, fabrication and test of a qualification model of a wire thermal cutting based non-explosive actuation (NEA) device for a small satellite. The important design parameters such as maximum allowable preload and release time are investigated through theoretical analysis. After it is fabricated based on theoretical results, we implemented the mechanical performance tests to assure operability and reliability for the compact size NEA device. From the result of the mechanical performance tests such as maximum preload, shock level and release time tests, we found that the proposed NEA device can be operated under a preload of 1000 kg<SUB>f</SUB> within 550 ms stably and it generates a low shock of 58.57 G. The margin calculation shows that the proposed NEA device can hold a solar panel of 20 kg with a margin of safety (<I>MS</I>) of +0.32. In addition, we carried out the environment tests such as the vibration test and thermal vacuum test to qualify the proposed NEA device. From the vibration test, we found the resonant shapes after sine and random vibration were the same which means the proposed NEA device was successfully designed to have a stable structure. From the result of the thermal vacuum test, in addition, the proposed NEA device was well operated in a space environment condition.</P>
Intra Prediction based on the Rate-distortion Cost in HEVC
Junwoo Choi,Yung-Lyul Lee 대한전자공학회 2017 IEIE Transactions on Smart Processing & Computing Vol.6 No.5
High Efficiency Video Coding (HEVC) provides high coding efficiency, compared with H.264/AVC. In particular, intra coding efficiency is improved due to reference-sample filtering and unfiltering methods before intra prediction, where the reference-filtering methods are applied according to the block size and intra prediction mode. In this paper, a rate-distortion (RD) costbased intra prediction that computes the minimum RD cost between the filtered reference samples and the unfiltered reference samples is proposed. Intra coding efficiency is improved by using the probability update between the reference sample filtering in the current coding unit and the neighbor coding units, because the predefined reference-sample filtering in HEVC is not optimal in terms of intra coding efficiency. Experiment results show that the proposed method reduces the Bjøntegaard delta (BD) rates of the luma and chroma (Cb and Cr) by approximately 0.5% and 0.4%, respectively.
( Junwoo Lee ),( Byoung-soo Choi ),( Donghyun Seong ),( Jewon Lee ),( Sang-hwan Kim ),( Jimin Lee ),( Jang-kyoo Shin ),( Pyung Choi ) 한국센서학회 2018 센서학회지 Vol.27 No.6
A complementary metal oxide semiconductor (CMOS) binary image sensor is proposed for low-power and low-noise operation. The proposed binary image sensor has the advantages of reduced power consumption and fixed pattern noise (FPN). A gate/body-tied (GBT) p-channel metal-oxide-semiconductor field-effect transistor (PMOSFET)-type photodetector is used as the proposed CMOS binary image sensor. The GBT PMOSFET-type photodetector has a floating gate that amplifies the photocurrent generated by incident light. Therefore, the sensitivity of the GBT PMOSFET-type photodetector is higher than that of other photodetectors. The proposed CMOS binary image sensor consists of a pixel array with 394 (H) × 250 (V) pixels, scanners, bias circuits, and column parallel readout circuits for binary image processing. The proposed CMOS binary image sensor was analyzed by simulation. Using the dynamic comparator, a power consumption reduction of approximately 99.7% was achieved, and this performance was verified by the simulation by comparing the results with those of a two-stage comparator. Also, it was confirmed using simulation that the FPN of the proposed CMOS binary image sensor was successfully reduced by use of the double sampling process.