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김남형(Nam-Hyeong Kim),윤현철(Hyeon-Cheol Yun),허영택(Young-Teck Hur) 한국항해항만학회 2010 한국항해항만학회지 Vol.34 No.5
수치해석을 수행할 경우 좀 더 정확한 수치모의를 위하여 해석영역의 특성에 따라 적절하게 요소를 배치하는 것이 필요하다. 본 연구에서 사용한 개별요소법(DEM)은 입자의 마찰력 및 항력을 제외한 반발력과 인장력만을 적용하였다. 초기 쿼드트리(Quad-tree)형식으로 충진된 입자들을 DEM을 이용하여 재배치할 경우 입자의 형상이 원형이기 때문에 입자사이에 존재하는 빈 공간을 최소화 할 수 있다. 결국 입자중심점의 배치가 정삼각형에 가깝게 되는 특징을 보여준다. 이 재배치된 입자를 대상으로 Delaunay 삼각기법을 이용하여 삼각망을 구성하고, Laplace 보간을 수행하여 격자 품질을 향상시켰다. Laplace 보간 전?후의 형상비(Aspect Ratio: AR)를 비교한 결과 DEM을 이용하여 작성한 격자의 품질도 우수하지만, Laplace보간을 수행한 이후 보다 높은 품질의 격자가 생성되는 것을 확인하였다. 본 연구에서 개발한 기법은 기존의 삼각격자망 생성기법에 비해 다소 계산시간이 오래 걸리는 단점이 있지만, 복잡한 형상과 정확한 지형의 재현을 필요로 하는 파랑 해석용 유한요소 격자망 작성 및 다양한 수치모의 분야에서 그 적용 가능성이 매우 높다고 사료된다. When the numerical analysis is carried out, it is necessary to set proper elements as a feature of analysis domains for more accurate simulations. In this study, Distinct Element Method(DEM) is applied, only considering repulsive force and tensile force except for frictional force and resisting force of particle. When the filled particles with initial Quad-tree type is relocated by DEM, a blank space existing among the particles can be minimized because the shape of particle is circular. Finally, it is the effective feature that the centroidal disposion of the particles is similar to an equilateral triangle. Triangular mesh are formed by using the Delaunay triangular technique on these relocated particles, the quality of triangular mesh is more improved by carrying out Laplace interpolations. The compared result of Aspect Ratio before and after the Laplace interpolation is shown that although the quality of triangular mesh made by DEM is good, the later triangular mesh are higher quality than the formers. In this study, although the developed technique takes a longer calculational time than the previous technique to generate triangular mesh, it is considered that the applicable possibility is very high in the generation of finite element mesh about wave analysis and various numerical simulation to need a complex or reappearance of exact topography.
Optimum angles of non-standard diffusers and reducers for engineering application
최순호,김세동,최재혁,박지태,정효민 대한기계학회 2019 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.33 No.10
The fittings such as diffusers and reducers are essential flow elements in a piping system, which play a role to connect the different sized pipes or to branch a part of main stream to other piping. However, their central role is to convert the dynamic energy (velocity) of a flow into the static energy (pressure), or vice versa. When designing a piping system, the pressure drop calculation is very important due to the estimation of power consumption for maintaining a required flowrate. For this reason, many coefficient values of the standard sized fitting products are usually provided by manufacturers or by engineering handbooks. In this study, some analytical calculations were performed to find the optimum angles of non-standard reducers and diffusers based on a dimensionless aspect ratio (length/diameter). The results showed that the optimum angles decreased exponentially as the aspect ratio increased and vice versa. Especially, the optimum angle of a diffuser converged to about 5 o when the aspect ratio is over 10. Furthermore, these results were verified through the CFD calculations and the previous reported data.
Guo Xian,Muralimohan Cheepu,Jieun Yu,Sang Myung Cho,Jong‑Taek Yeom,Yoon Suk Choi,Namhyun Kang 대한금속·재료학회 2023 METALS AND MATERIALS International Vol.29 No.2
The cooling time of cryogenic vaporised Ar shielding/cooling of Ti-6Al-4 V deposits was ~ 10 times faster than that ofnormal Ar gas shielding/cooling. The columnar β grains in rapid cooling condition were observed to be in the longitudinaldirection of the deposits same as that in natural cooling condition. Under the rapid cooling, a narrow heat-affected zone band(~ 70 μm) and a wide vanadium-segregation band was observed, along with a reduced aspect ratio of the α plate, which ledto improved isotropic-tensile properties along the deposition and building directions. Rapid cooling condition also retaineda significant amount of O and N, which enhanced its hardness and tensile strength. Thus, cryogenic vaporised Ar shielding/cooling can produce Ti-6Al-4 V wire-arc additive manufacturing deposits with better tensile properties compared to normalAr gas shielding.
NH₃-SCR 촉매를 이용한 디젤기관에서의 NOx 저감에 관한 연구
박경균(Kyounggyun Park),이원석(Wonseok Lee),이성욱(Seongwock Lee),강호인(Hoin Kang),나완용(Wanyong Rha),손용기(Yonggy Son),임인혁(Inhyuk Im),황재원(Jaewon Hwang) 한국자동차공학회 2007 한국자동차공학회 춘 추계 학술대회 논문집 Vol.- No.-
Diesel engines provide merits in the aspect of higher thermal efficiency and less CO₂ emission, but more problems on PM and NOx emissions. Selective Catalyst Reduction(SCR) technology is well-known to be effective for the reduction of NOx emission. SCR system can be classified into three kinds of NSR, NH₃-SCR and HC-SCR depending on reducing agents in jet spray. Because of poisoning problem of sulfur and unstable gas condition, much stable Urea-SCR system has been preferred to use instead. However, this research used to NH₃ gas instead Urea solution to model gas test. It is general to use NH₃-SCR catalyst for the NOx reduction on a diesel engine. This research focuses on the effects of gas temperature, SV(space velocity) and AR(aspect ratio) of NOx reduction rate. In this experiment was used the model gas which is similar to a diesel exhaust gas. NOx conversion rate is higher than 90% at the temperature of near 400℃ for all cases. As SV is lower, NOx conversion rate is decreased. On the contrary, as AR is higher, NOx conversion rate is increased.