• 전자공학회논문지SC
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• 제48권4호
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• pp.80-85
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• 2011

시간 측정 시 신호 발생 시점의 기준 펄스를 발생시키는데 사용되는 time pickoff 회로는 기준 펄스의 발생 시점이 입력 신호의 크기에 영향을 받는 time walk로 인해 측정 오차를 겪는다. 본 논문에서는 이와 같은 time walk를 감소시키기 위해 자동 이득 조절(AGC: Automatic Gain Control)을 이용한 time pickoff 방식을 제안한다. 자동 이득 조절부는 가변 증폭부와 바이어스부, 입력 신호의 크기를 측정하는 크기 획득부로 구성되며, 기준 펄스를 발생하는 비교기 앞에 위치한다. 그리고, 비교기 입력 신호의 크기를 거의 일정하게 조절함으로써, time walk 발생 원인을 최소화하고 기준 펄스 발생 시점의 변화를 감소시킨다. 제안한 time pickoff 회로의 동작은 SPICE 시뮬레이션과 실험을 통하여 확인하였다. 20dB의 dynamic range와 4 ns의 상승 시간을 가지는 입력 신호에 대해 측정된 time walk는 2 ns로, 일반적으로 널리 사용되는 leading edge discriminator에 비해 약 65% 개선된 성능을 보였다.

• 한국연소학회지
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• 제8권3호
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• pp.15-23
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• 2003

Partial quenching structure of turbulent diffusion flames in a turbulent mixing layer is investigated by the method of flame hole dynamics in order to develop a prediction model for turbulent flame lift off. The essence of flame hole dynamics is derivation of the random walk mapping, from the flame-edge theory, which governs expansion or contraction of flame holes initially created by local quenching events. The numerical simulation for flame hole dynamics is carried out in two stages. First, a direct numerical simulation is performed for constant-density fuel-air channel mixing layer to obtain the turbulent flow and mixing fields, from which a time series of two dimensional scalar dissipation rate array is extracted at a fixed virtual flame surface horizontally extending from the end of split plate to the downstream. Then, the Lagrangian simulation of the flame hole random walk mapping projected to the scalar dissipation rate array yields temporally evolving turbulent extinction process and its statistics on partial quenching characteristics. The statistical results exhibit that the chance of partial quenching is strongly influenced by the crossover scalar dissipation rate while almost unaffected by the iteration number of the mapping that can be regarded as a flame-edge speed.

• Journal of Mechanical Science and Technology
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• 제19권12호
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• pp.2312-2320
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• 2005

The aerodynamic effects of leading-edge accretion can raise important safety concerns since the formulation of ice causes severe degradation in aerodynamic performance as compared with the clean airfoil. The objective of this study is to develop a numerical simulation strategy for predicting the particle trajectory around an MS-0317 airfoil in the test section of the NASA Glenn Icing Research Tunnel and to investigate the impingement characteristics of droplets on the airfoil surface. In particular, predictions of the mean velocity and turbulence diffusion using turbulent flow solver and Continuous Random Walk method were desired throughout this flow domain in order to investigate droplet dispersion. The collection efficiency distributions over the airfoil surface in simulations with different numbers of droplets, various integration time-steps and particle sizes were compared with experimental data. The large droplet impingement data indicated the trends in impingement characteristics with respect to particle size ; the maximum collection efficiency located at the upper surface near the leading edge, and the maximum value and total collection efficiency were increased as the particle size was increased. The extent of the area impinged on by particles also increased with the increment of the particle size, which is similar as compared with experimental data.

• 대한전자공학회논문지SD
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• 제45권8호
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• pp.61-66
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• 2008

최근 모바일 기기에 3차원 그래픽 디지털 콘텐츠들이 증가함에 따라 휴대용 기기에 적합한 3차원 그래픽 가속기의 연구와 설계는 점점 중요한 이슈가 되고 있다. 본 논문에서는 저전력 3차원 그래픽 파이프라인에 적합한 효율적인 클리핑 구조를 제안한다. 많은 연산 사이클과 연산기를 필요로 하는 클리핑 연산을 두 단계로 나누어서 기하변환 엔진에서는 컬링 정렬(cull and sort) 유닛으로 구현하고, 실질적인 클리핑은 스캔 변환(scan conversion)에서 구현한다. 즉, 스캔 변환 처리기를 구성하고 있는 변처리 (edge walk) 유닛에서 Y축 클리핑을 함께 수행하고 스팬처리 (span processing) 유닛에서 X축과 Z축 클리핑을 함께 수행한다. 제안하는 기하 변환 엔진의 컬링 정렬 유닛은 기존 클리핑 유닛에 비해 면적과 동작 사이클이 크게 줄었고 스캔 변환 처리기의 면적은 거의 증가하지 않아 전반적으로 동작 속도 및 동작 효율을 높였다. 제안하는 클리핑 구조를 적용한 3차원 그래픽 가속기는 Verilog-HDL을 이용하여 설계하고 FPGA를 이용하여 검증하였다.

• 한국연소학회:학술대회논문집
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• 한국연소학회 제26회 KOSCO SYMPOSIUM 논문집
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• pp.223-227
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• 2003

The method of flame hole dynamics is demonstrated as a mean to simulate turbulent flame extinction. The core of the flame hole dynamics involves derivation of a random walk mapping for the flame holes, created by local quenching, between the burning and quenched states provided that the dynamic characteristics of flame edges is known. Then, the random walk mapping is projected to a background turbulent field. The numerical simulations are carried out with the further simplifications of flame string and unconditioned scalar dissipation rate. The simulation results show how the chance of partial quenching is influenced by the crossover scalar dissipation rate. Finally, a list of improvements, necessary to achieve more realistic turbulent flame quenching simulation, are discussed.

• Journal of Mechanical Science and Technology
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• 제17권11호
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• pp.1775-1783
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• 2003

The method of flame hole dynamics is demonstrated as a mean to simulate turbulent flame extinction. The core of the flame hole dynamics involves derivation of a random walk mapping for the flame holes, created by local quenching, between burning and quenched states provided that the dynamic characteristics of flame edges is known. Then, the random walk mapping is projected to a background turbulent field. The numerical simulations are carried out with further simplifications of flame string and unconditioned scalar dissipation rate. The simulation results show how the chance of partial quenching is influenced by the crossover scalar dissipation rate. Finally, a list of improvements, necessary to achieve more realistic turbulent flame quenching simulation, are discussed.

• 한국연소학회:학술대회논문집
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• 한국연소학회 2004년도 제28회 KOSCO SYMPOSIUM 논문집
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• pp.102-111
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• 2004

Partial quenching structure of turbulent diffusion flames in a turbulent mixing layer is investigated by the method of flame hole dynamics to develope a prediction model for the turbulent lift off. The present study is specifically aimed to remedy the problem of the stiff transition of the conditioned partial burning probability across the crossover condition by adopting level-set method which describes propagating or retreating flame front with specified propagation speed. In light of the level-set simulations with two model problems for the propagation speed, the stabilizing conditions for a turbulent lifted flame are suggested. The flame hole dynamics combined with level-set method yields a temporally evolving turbulent extinction process and its partial quenching characteristics is compared with the results of the previous model employing the flame-hole random walk mapping. The probability to encounter reacting' state, conditioned with scalar dissipation rate, demonstrated that the conditional probability has a rather gradual transition across the crossover scalar dissipation rate in contrast to the stiff transition of resulted from the flame-hole random walk mapping and could be attributed to the finite response of the flame edge propagation.

• 한국정보통신학회:학술대회논문집
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• 한국정보통신학회 2021년도 춘계학술대회
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• pp.22-24
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• 2021

본 논문은 노드와 엣지로 이루어진 그래프 구성을 통해 신경망을 활용하여 엣지 가중치 예측 방안을 제안하고자 한다. 사회 구성요소 중 하나인 브랜드들의 중요도 높은 전략 중 출점전략에 대해 초점을 맞추어보았다. 본 논문에서는 1) 브랜드 지점들을 노드로 구성하고, 지점 간 관계를 엣지로 구성한다. 그리고 지점 간 실제 도보 및 주행 가능 거릿값을 엣지 가중치로 표현한 그래프를 구성한다. 그리고 2) 엣지를 수치화하여 신경망을 학습해 엣지의 가중치인 지점 간 거릿값을 예측하는 방안을 제안한다. 제안한 방식을 활용함으로 예측되는 특정 브랜드의 출점 범위를 성공적인 브랜드의 출점전략 분석할 시에 활용할 수 있을 것으로 예상한다.

• 대한수학회보
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• 제37권4호
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• pp.649-657
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• 2000

The 2-step competition graph of D has the same vertex set as D and an edge between vertices x and y if and only if there exist (x, z)-walk of length 2 and (y, z)-walk of length 2 for some vertex z in D. The 2-step competition number of a graph G is the smallest number k such that G together with k isolated vertices is the 2-step competition graph of an acyclic digraph. Cho, et al. showed that the 2-step competition number of a path of length at least two is two. In this paper, we characterize all the minimal acyclic digraphs whose 2-step competition graphs are paths of length n with two isolated vertices and construct all such digraphs.

• 한국연소학회지
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• 제9권2호
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• pp.18-29
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• 2004

Partial quenching structure of diffusion flames in a turbulent mixing layer has been investigated by the method of flame hole dynamics in oder to develope a prediction model for the phenomenon of turbulent flame lift off. The present study is specifically aimed to remedy the shortcoming of the stiff transition of the conditioned partial burning probability across the crossover condition by employing the level-set method which enables us to include the effect of finite flame edge propagation speed. In light of the level-set simulation results with two models for the edge propagation speed, the stabilizing conditions for turbulent lifted flame are suggested. The flame hole dynamics combined with the level-set method yields a temporally evolving turbulent extinction process and its partial quenching characteristics is compared with the results of the previous model employing the flame-hole random walk mapping based on three critical scalar dissipation rates. The probability to encounter reacting state, conditioned with scalar dissipation rate, demonstrated that the conditional probability has a rather gradual transition across the crossover scalar dissipation rate. Such a smooth transition is attributed to the finite response of the flame edge propagation.