• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2004년도 제22회 춘계학술대회논문집
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• pp.671-686
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• 2004

The shock-induced combustion ramjet (shcramjet) is a hypersonic airbreathing propulsion concept which over-comes the drawbacks of the long, massive combustors present in the scramjet by using a standing oblique detonation wave (a coupled shock-combustion front) as a means of nearly instantaneous heat addition. A novel shcramjet combustor design that makes use of wedge-shaped flameholders to avoid detonation wave-wall interactions is proposed and analyzed with computational fluid dynamics (CFD) simulations in this study. The laminar, two-dimensional Navier-Stokes equations coupled with a non-equilibrium hydrogen-air combustion model based on chemical kinetics are used to represent the physical system. The equations are solved with the WARP (window-allocatable resolver for propulsion) CFD code (see: Parent, B. and Sislian, J. P., “The Use of Domain Decomposition in Accelerating the Convergence of Quasihyperbolic Systems”, J. of Comp. Physics, Vol. 179, No. 1,2002, pages 140-169). The solver was validated with experimental results found in the literature. A series of steady-state numerical simulations was conducted using WARP and it was deter-mined by means of thrust potential calculations that this combustor design is a viable one for shcramjet propulsion: assuming a shcramjet flight Mach number of twelve at an altitude of 36,000 m, the geometrical dimensions used for the combustor give rise to an operational range for combustor inlet Mach numbers between six and eight. Different shcramjet flight Mach numbers would require different combustor dimensions and hence a variable geometry system in or-der to be viable.

• 한국항해항만학회지
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• 제27권4호
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• pp.397-402
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• 2003

컨테이너 시큐어링 시스템은 컨테이너 운반선 상갑판에 적재되는 컨테이너의 배치 설계를 위한 시스템이다. 컨테이너의 배치는 각 선급의 가이드라인에 기초하여 설계되어지며, 주어진 배치에 따른 반력들과 변력의 한계값을 제공하고 있다. 컨테이너 설계를 위해서는 컨테이너 구속 교량(lashing bridge)과 수직 구속 장치(vertical lashing) 등을 고려해야 하며, 최대 수직 중량 중심(vertical center of gravity, VCG)과 최대 화물 중량(stack weight)을 갖는 배치안을 제시할 수 있어야 한다. 본 연구에서는 이를 위한 평형방정식을 정립하였으며, 배치안 계산을 위해 등식 제한조건(equality constraint)을 처리할 수 있는 최적화 알고리즘을 적용하여 새로운 컨테이너 시큐어링 시스템을 개발하였다. 개발된 시스템은 컨테이너 배치 설계 시간을 크게 줄여주며, 설계자가 원하는 배치 설계안을 제시해 준다.

• Structural Engineering and Mechanics
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• 제71권6호
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• pp.603-625
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• 2019

Main cable configurations under final dead load and in the unloaded state and critical construction parameters (e.g. unstrained cable length, unstrained hanger lengths, and pre-offsets for tower saddles and splay saddles) are the core considerations in the design and construction control of a suspension bridge. For the purpose of accurate calculations, it is necessary to take into account the effects of cable strands over the anchor spans, arc-shaped saddle top, and tower top pre-uplift. In this paper, a method for calculating the cable configuration under final dead load over a main span, two side spans, and two anchor spans, coordinates of tangent points, and unstrained cable length are firstly developed using conditions for mechanical equilibrium and geometric relationships. Hanger tensile forces and unstrained hanger lengths are calculated by iteratively solving the equations governing hanger tensile forces and the cable configuration, which gives careful consideration to the effect of hanger weight. Next, equations for calculating the cable configuration in the unloaded state and pre-offsets of saddles are derived from the cable configuration under final dead load and the conditions for unstrained cable length to be conserved. The equations for the main span, two side spans and two anchor spans are then solved simultaneously. In the proposed methods, coupled nonlinear equations are solved by turning them into an unconstrained optimization problem, making the procedure simplified. The feasibility and validity of the proposed methods are demonstrated through a numerical example.

• 한국재료학회지
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• 제33권5호
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• pp.189-194
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• 2023

This study uses first-principles calculations to investigate the mechanical properties and effect of strain on the electronic properties of the 2D material 1H-PbX₂ (X: S, Se). Firstly, the stability of the 1H Pb-dichalcogenide structures was evaluated using Born's criteria. The obtained results show that the 1H-PbS₂ material possesses the greatest ideal strength of 3.48 N/m, with 3.68 N/m for 1H-PbSe₂ in biaxial strain. In addition, 1H-PbS₂ and 1H-PbSe₂ are direct semiconductors at equilibrium with band gaps of 2.30 eV and 1.90 eV, respectively. The band gap was investigated and remained almost unchanged under the strain ε_xx but altered significantly at strains ε_yy and ε_bia. At the fracture strain in the biaxial direction (19 %), the band gap of 1H-PbS₂ decreases about 60 %, and that of 1H-PbSe₂ decreases about 50 %. 1H-PbS₂ and 1H-PbSe₂ can convert from direct to indirect semiconductor under the strain ε_yy. Our findings reveal that the two structures have significant potential for application in nanoelectronic devices.

• Geomechanics and Engineering
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• 제31권4호
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• pp.339-352
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• 2022

Studying slope stability is an important branch of civil engineering. In this way, engineers have employed machine learning models, due to their high efficiency in complex calculations. This paper examines the robustness of various novel optimization schemes, namely equilibrium optimizer (EO), Harris hawks optimization (HHO), water cycle algorithm (WCA), biogeography-based optimization (BBO), dragonfly algorithm (DA), grey wolf optimization (GWO), and teaching learning-based optimization (TLBO) for enhancing the performance of adaptive neuro-fuzzy inference system (ANFIS) in slope stability prediction. The hybrid models estimate the factor of safety (F_S) of a cohesive soil-footing system. The role of these algorithms lies in finding the optimal parameters of the membership function in the fuzzy system. By examining the convergence proceeding of the proposed hybrids, the best population sizes are selected, and the corresponding results are compared to the typical ANFIS. Accuracy assessments via root mean square error, mean absolute error, mean absolute percentage error, and Pearson correlation coefficient showed that all models can reliably understand and reproduce the F_S behavior. Moreover, applying the WCA, EO, GWO, and TLBO resulted in reducing both learning and prediction error of the ANFIS. Also, an efficiency comparison demonstrated the WCA-ANFIS as the most accurate hybrid, while the GWO-ANFIS was the fastest promising model. Overall, the findings of this research professed the suitability of improved intelligent models for practical slope stability evaluations.

• 공업화학
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• 제9권6호
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• pp.829-834
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• 1998

환경문제와 관련하여 오존층 파괴의 주요 원인으로 알려진 CFC를 열플라즈마를 이용하여 완전하게 분해하였다. CFC113($C_2Cl_3F_3$)을 선정하여 열플라즈마 분해에서의 적절한 공정 조건을 검토하였다. 실험에 앞서, 상압에서 300 K~5000 K범위에서 CFC113, $H_2$, $O_2$간의 열역학적 화학평형조성을 고찰함으로써 CFC113의 분해 생성의 경향을 알 수 있었다. 실험은 상압, 상온에서 CFC113과 $H_2$, $O_2$혼합가스의 주입량, 그리고 냉각관 직경의 변화에 따른 분해생성물을 조사하였고 이를 기체크로마토그래피로 분석하였다. 그 결과, 각각 99.99%이상의 분해율을 보였다. CFC113/$H_2$=1/3에서 $O_2$비가 증가할수록 CO로의 전화율은 감소하였다. CFC113/$O_2$=1/1, 1/1.5, 1/2에서 $H_2$비가 3이상 증가될수록 CO로의 전화율이 증가하였다. 그 이유는 $H_2$첨가가 증가할수록 환원분위기에서 $H_2O$가 생성되고 $CO_2$생성량이 감소하기 때문이다. DC power가 증가하여도 CO로의 전화율 변화는 차이가 없었으며, 총유량이 증가할 경우 CO전화율이 약간 감소하는 경향을 보였다. 냉각관의 직경을 8 mm에서 4 mm로 작게 할 경우 빠른 냉각속도로 인하여 CO로의 전화율이 증가하였다.

• 지질공학
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• 제23권4호
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• pp.447-455
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• 2013

본 연구에서는 다공질 지반체내의 투수계수를 계산하기 위하여 정방형의 배열형태를 갖는 유동관망(pipe network) 유동해석 모델을 개발하였다. 본 유동관망을 통한 유체의 흐름 메커니즘은 통계적 침투이론(percolation theory)에 기초하여 정의된다(Stauffer and Aharony, 1994). 여기서, 개별 유동관의 직경들이 주어진 다공질 매질의 공극률과 공극크기 분포특성을 기초로 하여 통계적으로 지정됨으로 계산된 유체흐름은 불균일한 채널 유동 형태로 나타난다. 본 유동해석에서는 유동관망 모델의 한쪽 경계면에 가압된 유체가 투입되고 다른 측면 경계면들은 흐름을 억제하는 경계조건을 두어 한 방향으로 유동관망을 통해 유체의 흐름을 유도하여 모델링된다. 이때, 흐름을 허용할지를 정의하는 확산조건(percolation condition)이 각 유동관에 부여되며, 이는 각 유동 경로의 직경과 재료면 특성을 기초로 계산된 삼투압(capillary pressure) 수준에 의해 정의된다. 유체가 유입되는 면의 수압에 대해 전체 유동관망 모델 내의 수압 분포가 평형을 이루면 유출되는 면의 수압이 일정해 지며, 유입면의 수압과 계산된 유출면의 수압 및 유동량을 Darcy 방정식에 적용하면 유동관망 모델로 모사된 다공질 매질의 투수계수를 얻어 낼 수 있다. 본 연구에서는, 민감할 것으로 예상된 유동 격자망의 규모의 투수계수 결과값에 대한 민감도를 검토하였으며, 실제 석유개발 현장에서 수집된 시추코어에 대해 측정된 투수계수값과 제안 네트워크 모델을 이용한 계산값과 비교하여 합리적인 범위 내에서 잘 부합됨을 보였다.

• 대한자원환경지질학회:학술대회논문집
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• 대한자원환경지질학회 2002년도 제18차 공동학술강연회 자연저감고 지질학 (대한 자원 환경지질학회)
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• pp.81-100
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• 2002

While most of regulatory communities in abroad recognize ' 'natural attenuation " to include degradation, dispersion, dilution, sorption (including precipitation and transformation), and volatilization as governing Processes, regulators prefer "degradation" because this mechanism destroys the contaminant of concern. Unfortunately, true degradation only applies to organic contaminants and short- lived radionuclides, and leaves most metals and long-lived radionuclides. The natural attenuation Processes may reduce the potential risk Posed by site contaminants in three ways: (i)contaminants could be converted to a less toxic form througy destructive processes such as biodegradation or abiotic transformations; (ii) potential exposure levels may be reduced by lowering concentrations (dilution and dispersion); and (iii) contaminant mobility and bioavailability may be reduced by sorption to geomedia. In this review, authors will focus will focul on "sorption" among the natural attenuation processes of hazardous inorganic contaminants including radionuclides. Note though that sorption and transformation processes of inorganic contaminants in the natural setting could be influenced by biotic activities but our discussion would limit only to geochemical reactions involved in the natural attenuation. All of the geochemical reactions have been studied in-depth by numerous researchers for many years to understand "retardation" process of contaminants in the geomedia. The most common approach for estimating retardation is the determination of distrubution coefficiendts ($K_{d}$) of contaminants using parametric or mechanistic models. As typocally used in fate and contaminant transport calculations such as predictive models of the natural attenuation, the $K_{d}$ is defined as the ratio of the contaminant concentration in the surrounding aqueous solution when the system is at equilibrium. Unfortunately, generic or default $K_{d}$ values can result in significant error when used to predict contaminant migration rate and to select a site remediation alternative. Thus, to input the best $K_{d}$ value in the contaminant transport model, it is essential that important geochemical processes affecting the transport should be identified and understood. Precipitation/dissolution and adsorption/desorption are considered the most important geochemical processes affecting the interaction of inorganic and radionuclide contaminants with geomedia at the near and far field, respectively. Most of contaminants to be discussed in this presentation are relatively immobile, i.e., have very high $K_{d}$ values under natural geochemical environments. Unfortunately, the obvious containment in a source area may not be good enough to qualify as monitored natural attenuation site unless owner demonstrate the efficacy if institutional controls that were put in place to protect potential receptors. In this view, natural attenuation as a remedial alternative for some of sites contaminated by hazardous-inorganic components is regulatory and public acceptance issues rather than scientific issue.

• 방사성폐기물학회지
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• 제9권3호
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• pp.161-167
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• 2011

사용후핵연료 운반용기 표면온도가 $85^{\circ}C$를 초과할 경우, 대인용 보호막(Personnel Barrier) 또는 운반용 덮개(Transport Hood)를 설치하여 운반 중 운반용기 표면에 사람이 직접 접근할 수 없도록 하여야 한다. 운반용 덮개가 설치된 경우, 열적 안전성 평가의 한 가지 경우인 정상조건 열해석 시, 외부환경 경계조건(환경온도 및 외부복사온도)으로 적용하기 위해서 운반용 덮개 내부 열 환경 조건(내부 공기온도 및 운반용 덮개 표면온도)을 계산해야 한다. 따라서 본 연구에서는 운반용 덮개 내부 공기온도 및 표면온도를 계산하기 위한 해석적 방법 및 열전달 특성에 대한 분석을 수행하였고 CFD 해석 결과와 비교를 통해 타당성을 검증하였다.

• 한국표면공학회지
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• 제42권1호
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• pp.8-12
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• 2009

Electronic structures and chemical bonding of Li-intercalated $LiTiS_2$ and $LiTiO_2$ were investigated by using discrete variational $X{\alpha}$ method as a first-principles molecular-orbital method. ${\alpha}-NaFeO_2$ structure is the equilibrium structure for $LiCoO_2$, which is widely used as a commercial cathode material for lithium secondary battery. The study especially focused on the charge state of Li ions and the magnitude of covalency around Li ions. The average voltage of lithium intercalation was calculated using pseudopotential method and the average intercalation voltage of $LiTiO_2$ was higher than that of $LiTiS_2$. It can be explained by the differences in Mulliken charge of lithium and the bond overlap population between the intercalated Li ions and anions in $LiTiO_2$ as well as $LiTiS_2$. The Mulliken charge, which means the ionicity of Li atom, was approximately 0.12 in $LiTiS_2$ and the bond overlap population (BOP) indicating the covalency between Ti and S was about 0.339. One the other hands, the Mulliken charge of lithium was about 0.79, which means that Li is fully ionized. The BOP, the covalency between Ti and O, was 0.181 in $LiTiO_2$. Because of high ionicity of Li and the weak covalency between Ti and the nearest anion, $LiTiO_2$ has a higher intercalation voltage than that of $LiTiS_2$.