• Journal of Mechanical Science and Technology
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• 제15권3호
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• pp.392-402
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• 2001

The paper reports a multiple source modeling of low-Reynolds-number dissipation rate equation with aids of DNS data. The key features of the model are to satisfy the wall limiting conditions of the individual source terms in the exact dissipation rate equation using the wall damping functions. The wall damping functions are formulated in term of dimensionless dissipation length scale ι(sup)+(sub)D(≡ι(sub)D($\upsilon$$\xi$)(sup)1/4/$\upsilon$) and the invariants of small and large scale turbulence anisotropy tensors. $\alpha$(sub)ij(=$\mu$(sub)i$\mu$(sub)j/$\kappa$-2$\delta$(sub)ij/3) and e(sub)ij(=$\xi$(sub)ij/$\xi$-2$\delta$(sub)ij/3). The model constants are optimized with aids of DNS data in a plane channel flow. Adopting the dissipation length scale as a parameter of damping function, the applicabilities of $\kappa$-$\xi$ model are extended to the turbulent flow calculation of complex flow passages.

• 수산해양기술연구
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• 제29권3호
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• pp.200-213
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• 1993

A model experiment on a midwater rope trawl net which is used in the North Pacific to catch alaska pollack is carried out in the circulating tank to examine the basic efficiency of the net. The prototype is the net used by M/S Hanil(1, 179GT, 2, 700PS), a Korean trawler. The model net was made according to the Tauti's Similarity Law of Fishing Gear in 1/100 scale by considering the condition of the tank. To measure the basic efficiency of the standard model net, the vertical opening and width between some points marked on the net were measured, and the hydrodynamic resistance were determined. Then the constructive conditions of the net were varied as follows and the factors were measured again to compare the efficiency of those nets with that of the standard net(A-1 type) front weight multiplied 1.5 times: A-2 type. buoyancy and depressing force multiplied 1.7 times: A-3 type. front weight multiplied 1.5 times on A-3 type: A-4 type. depressors rigged at ground rope: B type. cod-end stuffed with cashmylon wad: C type. The results obtained can be summarized as follows: 1. The vertical opening at the center of head rope was steeply decreased with the flow velocity increasing and the vertical opening H(m) can be expressed in H=1.2v super(-1.2)(v : flow velocity in m/sec). The width of the net varied a little when the flow velocity was over 0.4m/sec, and the width of net mouth showed about 37% of the distance between the fore tips of net pendant. The shape of net mouth was almost a circle at 0.2m/sec and then steeply flatted elliptically with the flow velocity increasing and the area of mouth S(m super(2)) can be expressed in S=(1.65-2.3v)$\times$10 super(-2). The hydrodynamic resistance of the net increased almost linearly with the flow velocity increasing and the resistance R(kg) can be expressed in R=3.2$\times$d/l$\times$abv. where d/l denotes the mean of d(diameter of netting twine) and l(length of a leg in a mesh) from wing tip to the end of bag-net except cod-end on the side pannel, and a denotes the strectched circumference of the net at the fore end of a meshed part and b the stretched length of the whole net from wing tip to the end of cod-end. 2. In the condition-varied nets, the vertical opening of head rope showed some increase in every type net except the C type, and the increase showed the greatest in the B type by 30~54%, whereas it showed decrease in the C type by 5~10%. Variation of the area of net mouth showed almost the same tendency as the vertical opening and the increase showed the greatest in the B type by 20%, whereas it showed decrease in the C type by 12%. Hydrodynamic resistance showed some increase in every type compared with the standard net, and the rate of increase indicated 5~10% in the A-2, A-3 and A-4 type, 22% in the B type and 3% in the C type.

• Journal of Biosystems Engineering
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• 제19권1호
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• pp.50-61
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• 1994

This research was conducted to determine a preliminary design parameters for the development of twin screw extruder using Booy's screw design method, and to analyze a extrusion process using Janssen's mathematical flow model. In processing process, the experimental mass flow rates of com flour and wheat flour were almost the same as theoretical values corrected by correction factors of 0.95 and 0.6 respectively. It was appeared that the mathematical model would be suitable to predict the conveying capacity.

• Journal of Theoretical and Applied Mechanics
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• 제1권1호
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• pp.97-109
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• 1995

In this paper, a finite viscoelastic continuum model for rubber and its finite element analysis are presented. This finite viscoelatic model based on continuum mechanics is an extended model of Johnson and Wuigley's 1-D model. In this extended model, continuum based kinematic measures are rigorously defied and by using this kinematic measures, elastic stage law and flow rule are introduced. In kinematics, three configuration are introduced. In kinematics, three configuration are introduced. They are reference, current and virtual visco configurations. In elastic state law, it is assumed that at a certain time, there exists an elastic potential which describes the recoverable elastic energy. From this elastic potential, elastic state law is derived. The proposed flow rule is based on phenomenological observation. The flow rule gives precise relaxation response. In finite element approximation, mixed Lagrangian description is used, where total and similar method of updated Lagrangian descriptions are used together. This approach reduces the numerical job and gives simple nonlinear syatem of equations. To satisfy the incompressible condition, penalty-type modified Mooney-Rivlin energy function is adopted. By this method nearly incompressible condition is obtain the virtual visco configuration. For verification, uniaxial stretch tests are simulated for various stretch rates. The simulated results show good agreement with experiments. As a practical experiments. As a preactical example, pressurized rubber plate is simulated. The result shows finite viscoelastic effects clearly.

• Geomechanics and Engineering
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• 제9권4호
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• pp.513-530
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• 2015

Commonly, the base stability of sheeted excavation pits against seepage failure by heave is evaluated by using two-dimensional groundwater flow models and Terzaghi's failure criterion. The objective of the present study is to investigate the effect of three-dimensional groundwater flow on the heave for sheeted excavation pits with various dimensions. For this purpose, the steady-state groundwater flow analyses are performed by using the finite element program ABAQUS 6.12. It has been shown that, in homogeneous soils depending on the ratio of half of excavation width to embedment depth b/D, the ratio of safety factor obtained from 3D analyses to that obtained from 2D analyses $FS_{(3D)}/FS_{(2D)}$ can reach up to 1.56 and 1.34 for square and circular shaped excavations, respectively. As failure body, both an infinitesimal soil column adjacent to the wall (Baumgart & Davidenkoff's criterion) and a three-dimensional failure body with the width suggested by Terzaghi for two-dimensional cases are used. It has been shown that the ratio of $FS_{(Terzaghi)}/FS_{(Davidenkoff)}$ varies between 0.75 and 0.94 depending on the ratio of b/D. Additionally, the effects of model size, the shape of excavation pit and anisotropic permeability on the heave are studied. Finally, the problem is investigated for excavation pits in stratified soils, and important points are emphasized.

• 한국안전학회지
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• 제31권2호
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• pp.76-82
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• 2016

A 2-D hydrodynamic model for predicting the movement of debris flow was developed. The developed model was validated against a dam break flow problem conducted in EU CADAM project, and the performance of the model was shown to be satisfactory. In order to suggest structural alternative for hazardous zone vulnerable to debris flow disaster, two types of initial mass distribution and two shapes of defensive structure were considered. It was found that 1) the collapse of debris mass initiated with square pyramid shape induced more damage compared with that of cubic shape; and 2) a defensive structure with semi-circular shape was vulnerable to debris flow disaster in terms of debris control or primary defense compared with that of rectangular-shaped structure.

• 대한전자공학회논문지SP
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• 제38권1호
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• pp.19-32
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• 2001

본 논문에서는 영상의 밝기 값을 마치 3차원 지형과 같은 개념으로 간주하여 여기에 물이 흐르는 개념을 적용하여 국부 적응 (locally adaptive) thresholding 방법을 제안하였다. 문자를 추출해내기 위해 제안한 방법에서는 지형 표면에 물을 붓는 과정을 수행하였다. 물은 지형의 낮은 곳으로 흐르게 되어 계곡 (valley)에 쌓인다. 이때 문자와 배경으로 구성된 문서영상에서 쌓인 물의 양을 기준으로 두 개의 영역을 구분을 하였다. 제안한 water flow model의 개념을 적용한 threholding 방법은 국부 적응 thresholding의 특성을 나타낸다. 합성 영상과 실제 영상을 이용하여 전산 모의 실험을 수행함으로써 제안한 방법이 문서 영상을 효과적으로 이진화할 수 있음을 보였다.

• 신재생에너지
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• 제18권4호
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• pp.12-21
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• 2022

The pyrolysis characteristics of high-density polyethylene (HDPE), low-density polyethylene (LDPE), and polypropylene (PP) were analyzed numerically using a 1D plug flow reactor (PFR) model. A lumped kinetic model was selected to simplify the pyrolysis products as wax, oil, and gas. The simulation was performed in the 400-600℃ range, and the plastic pyrolysis and product generation characteristics with respect to time were compared at various temperatures. It was found that plastic pyrolysis accelerates rapidly as the temperature rises. The amounts of the pyrolysis products wax and oil increase and then decrease with time, whereas the amount of gas produced increases continuously. In LDPE pyrolysis, the pyrolysis time was longer than that observed for other plastics at a specified temperature, and the amount of wax generated was the greatest. The maximum mass fraction of oil was obtained in the order of HDPE, PP, and LDPE at a specified temperature, and it decreased with temperature. Although the 1D model adopted in this study has a limitation in that it does not include material transport and heat transfer phenomena, the qualitative results presented herein could provide base data regarding various types of plastic pyrolysis to predict the product characteristics. These results can in turn be used when designing pyrolysis reactors.

• 한국수자원학회논문집
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• 제49권1호
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• pp.61-72
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• 2016

얕은 비피압 충적층이 대부분인 우리나라 해안지역 대수층에서의 흐름과 분산을 분석하기 위하여, 비선형 Boussinesq 방정식에 대한 FDM 수치해석 모형을 개발하고, 이와 관련한 분석, 검증을 수행하였다. 수치해석 과정에서 비선형 문제를 해결하기 위하여 반복법을 사용하였으며, 수치모형은 자유 지하수면과 바닥의 굴곡을 고려하는 평면 2차원 모형이므로, 결국 3차원 형태를 나타낸다. 모형의 검증을 위해서, 흐름방정식의 경우 Lee (1989)가 제시한 급상승 혹은 급하강의 경계조건을 갖는 1차원 부정류 해석해와 비교하여 거의 일치하는 결과를 나타내었으며, 분산방정식의 mass balance 산출 결과는 10% 내외의 오차범위를 나타내었다. 개발된 모형은 비피압 대수층에서의 흐름 및 분산에 대한 평가 및 설계에 이용될 수 있다. 지하댐이 설치되어 있는 쌍천 하구역에서 본 모형을 적용하여 역해석에 의하여 관련 매개변수를 도출한 결과 투수계수는 90m/day, 종분산지수는 15 m로 산출되었다. 도출된 매개변수를 대상지역에 적용하여 갈수기를 기준으로 분석한 결과, 양수정은 7호 양수정을 제외하고는 서로 영향권 내에 있는 것으로 나타났다. 대수층을 통한 해안유출량은 $3700m^3/day$로 산출되었다. 또한 지하댐이 없을 경우 양수정의 염소이온($cl^-$) 농도는 1000 mg/L 이상 증가하는 것으로 나타나 지하댐의 역할이 큰 것으로 분석되었다.

• Nuclear Engineering and Technology
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• 제54권11호
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• pp.4322-4328
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• 2022

The two-fluid model is widely used to describe two-phase flows in complex systems such as nuclear reactors. Although the two-phase flow was successfully simulated, the standard two-fluid model suffers from an ill-posed nature. There are several remedies for the ill-posedness of the one-dimensional (1D) two-fluid model; among those, artificial viscosity is the focus of this study. Some previous works added artificial diffusion terms to both mass and momentum equations to render the two-fluid model well-posed and demonstrated that this method provided a numerically converging model. However, they did not consider mass conservation, which is crucial for analyzing a closed reactor system. In fact, the total mass is not conserved in the previous models. This study improves the artificial viscosity model such that the 1D incompressible two-fluid model is well-posed, and the total mass is conserved. The water faucet and Kelvin-Helmholtz instability flows were simulated to test the effect of the proposed artificial viscosity model. The results indicate that the proposed artificial viscosity model effectively remedies the ill-posedness of the two-fluid model while maintaining a negligible total mass error.