• 한국수자원학회논문집
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• 제56권spc1호
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• pp.1049-1058
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• 2023

보와 저낙차 댐과 같은 하천횡단구조물을 통과하는 흐름은 도수 현상을 동반하는 급변류가 지배적이다. 구조물 하류에서 도수로 인한 유속과 수면의 강한 비정상성은 수공구조물의 안정에 영향을 줄 수 있다. 특히, 높은 Froude 수 조건에서 발생하는 정상도수는 공기연행이 현저하게 발생하여 흐름 특성은 더욱 복잡해진다. 이 연구에서는 Froude 7.3 조건에서 발생하는 정상도수를 모의하기 위해서 큰와모의 기법과 하이브리드 VoF 기법을 이용한 수치모의를 수행하였다. 수치모의 결과는 구조물 하류 바닥면에서 계측된 순간최대압력과 시간평균압력 분포를 유사하게 재현하는 것으로 나타났다. 단, 구조물 직하류에서의 순간최소압력 분포는 대상으로 하는 실험 계측값과 반대의 양상을 보이지만, 유사한 다른 시험과는 같은 양상을 보임으로써 본 연구에서 수행한 수치모의는 합리적으로 압력변동을 예측하는 것으로 판단된다. 도수 중앙부에서의 연직방향 유속분포와 공기농도분포는 유사한 조건의 실험 결과들과 자기상사성을 보이면서 양호하게 일치하는 것으로 나타났다. 이러한 결과는 본 연구에서 적용한 큰와모의 기법과 하이브리드 VoF 기법이 높은 Froude 수 조건에서 강한 공기연행을 동반하는 도수현상을 양호하게 재현할 수 있음을 보여준다.

• Fibers and Polymers
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• 제7권4호
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• pp.389-397
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• 2006

The beneficial effects of graduated compression stockings (GCS) in prophylaxis and treatment of venous disorders of human lower extremity have been recognized. However, their pressure functional performances are variable and unstable in practical applications, and the exact mechanisms of action remain controversial. Direct surface pressure measurements and indirect material properties testing are not enough for fully understanding the interaction between stocking and leg. A three dimensional (3D) biomechanical mathematical model for numerically simulating the interaction between leg and GCS in dynamic wear was developed based on the actual geometry of the female leg obtained from 3D reconstruction of MR images and the real size and mechanical properties of the compression stocking prototype. The biomechanical solid leg model consists of bones and soft tissues, and an orthotropic shell model is built for the stocking hose. The dynamic putting-on process is simulated by defining the contact of finite relative sliding between the two objects. The surface pressure magnitude and distribution along the different height levels of the leg and stress profiles of stockings were simulated. As well, their dynamic alterations with time processing were quantitatively analyzed. Through validation, the simulated results showed a reasonable agreement with the experimental measurements, and the simulated pressure gradient distribution from the ankle to the thigh (100:67:30) accorded with the advised criterion by the European committee for standardization. The developed model can be used to predict and visualize the dynamic pressure and stress performances exerted by compression stocking in wear, and to optimize the material mechanical properties in stocking design, thus, helping us understand mechanisms of compression action and improving medical functions of GCS.

• 한국항공우주학회지
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• 제37권4호
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• pp.383-391
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• 2009

JP-7/산소 혼합기의 데토네이션 파 특성을 상세 반응 기구로부터 얻은 일 단계 유도 변수 모델을 (IPM) 이용하여 살펴보았다. 탄화수소 혼합기에 대한 상세 화학 반응 모델로 부터 신뢰할 만한 일 단계 반응 모델을 얻기 위한 일반적 과정을 본 연구에서 제시하였다. IPM은 상세 반응 모델 라이브러리로부터 획득한 유도 시간 데이터베이스를 재구성하여 얻었으며, 상세 반응 모델에 의한 결과와 비교하여 확인하였다. 이후 IPM을 유체역학해석 코드에 적용하였으며, 데토네이션 파 전파에 대한 수치해석에 이용하였다. 수치해석 결과는 탄화수소 연료 연소의 상세 반응 기구를 직접 적용해서는 가능하지 않은, JP-7/산소 혼합기의 데토네이션 파 전파 특성의 상세한 특징을 보여주었다.

• Advances in nano research
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• 제6권3호
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• pp.243-255
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• 2018

Polystyrene granules were combined with nanosilver to form a nanocomposite film. One-side migration was conducted to test into three food simulants (3% acetic acid, 10% ethanol and 95% ethanol) at $40^{\circ}C$ temperature on different period of time (2, 4, 6, 8 and 10 days). It was found that, among the simulants, the highest migration amount was obtained with 3% acetic acid, while the 95% ethanol revealed the least migration level. Diffusion coefficients of nanosilver particles into simulants were estimated by inverse simulation using experimental data of concentration variation in the simulants. The finite element method used to solve the mass transfer equation and the numerical results indicates the sameresponse with the experimental data. The numerical results confirmed that the highest diffusion coefficient for acetic acid 3% (1.82E-10 to $1.76E-9m^2\;s^{-1}$) and the lowest diffusion coefficient for ethanol 95% from 2 to 10 days were obtained, respectively. Also, results of diffusion coefficient - concentration relation showed, the diffusion coefficient had in direct correlation with time and concentration. The results indicated that, in the 3% acetic acid, due to the increasing of diffusion coefficient of silver nanoparticles, they are released faster and distributed uniformly.

• 대한기계학회논문집B
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• 제27권9호
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• pp.1262-1272
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• 2003

A two-dimensional direct numerical simulation is performed to investigate the dynamic behaviors of a single vortex in counter reacting and non-reacting flow field. A predictor-corrector-type numerical scheme with a low Mach number approximation is used in this simulation. A 16-step augmented reduced mechanism is adopted to treat the chemical reaction. The budget of the vorticity transport equation is examined to reveal a mechanism leading to the formation, destruction and transport of a single vortex according to the direction of vortex generation in reacting and non-reacting flows. The results show that air-side vortex has more larger strength than that of fuel-side vortex in both non-reacting and reacting flows. In reacting flow, the vortex is more dissipated than that in non-reacting flow as the vortex approach the flame. The total circulation in reacting flow, however, is larger than that in non-reacting flow because the convection transport of vorticity becomes much large by the increased velocity near the flame region. It is also found that the stretching and the convection terms mainly generate vorticity in non-reacting and reacting flows. The baroclinic torque term generates vorticity, while the viscous and the volumetric expansion terms attenuate vorticity in reacting flow. Furthermore, the contribution of volumetric expansion term on total circulation for air-side vortex is much larger than that of fuel-side vortex. It is also estimated that the difference of total circulation near stagnation plane according to the direction of vortex generation mainly attributes to the convection term.

• 한국지진공학회논문집
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• 제19권1호
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• pp.29-36
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• 2015

Base isolation is considered as a seismic protective system in the design of next generation Nuclear Power Plants (NPPs). If seismic isolation devices are installed in nuclear power plants then the safety under a seismic load of the power plant may be improved. However, with respect to some equipment, seismic risk may increase because displacement may become greater than before the installation of a seismic isolation device. Therefore, it is estimated to be necessary to select equipment in which the seismic risk increases due to an increase in the displacement by the installation of a seismic isolation device, and to perform research on the seismic performance of each piece of equipment. In this study, modified NRC-BNL benchmark models were used for seismic analysis. The numerical models include representations of isolation devices. In order to validate the numerical piping system model and to define the failure mode, a quasi-static loading test was conducted on the piping components before the analysis procedures. The fragility analysis was performed by using the results of the inelastic seismic response analysis. Inelastic seismic response analysis was carried out by using the shell finite element model of a piping system considering internal pressure. The implicit method was used for the direct integration time history analysis. In addition, the collapse load point was used for the failure mode for the fragility analysis.

• Computers and Concrete
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• 제21권6호
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• pp.623-635
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• 2018

In order to research the influence of different recycled aggregate contents on the mechanical properties of pervious concrete, the experimental study and numerical simulation analysis of the mechanical properties of pervious concrete with five kinds of recycled aggregates contents (0%, 25%, 50%, 75% and 100%) are carried out in this paper. The experimental test were first performed on concrete specimens of different sizes in order to determine the influence of recycled aggregate on the compressive strength and splitting tensile strength, direct tension strength and bending strength. Then, the development of the internal cracks of pervious concrete under different working conditions is studied more intuitively by $PFC^{3D}$. The experimental results show that the concrete compressive strength, tensile strength and bending strength decrease with the increase of the recycled aggregate contents. This trend of reduction is not only related to the brittleness of recycled aggregate concrete, but also to the weak viscosity of recycled aggregate and cement paste. It is found that the fracture surface of pervious concrete with recycled aggregate is smoother than that of natural aggregate pervious concrete by $PFC^{3D}$, which means that the bridging effect is weakened in the stress transfer between the left and right sides of the crack. Through the analysis of the development of the internal cracks, the recycled aggregate concrete generated more cracks than the natural aggregate concrete, which means that the recycled aggregate concrete is easier to form a coalescence fracture surface and eventually break.

• Nuclear Engineering and Technology
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• 제56권4호
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• pp.1310-1319
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• 2024

The reactor cavity cooling system (RCCS) is a passive reactor safety system commonly present in the designs of High-Temperature Gas-cooled Reactors (HTGR) that removes heat from the reactor pressure vessel by means of natural convection and radiation. It is one of the factors responsible for ensuring that the reactor does not melt down under any plausible accident scenario. For the simulation of accident scenarios, which are transient phenomena unfolding over a span of up to several days, intermediate fidelity methods and system codes must be employed to limit the models' execution time. These models can quantify radiation heat transfer well, but heat transfer caused by natural convection must be quantified with the use of correlations for the heat transfer coefficient. It is difficult to obtain reliable correlations for HTGR RCCS heat transfer coefficients experimentally due to such a system's size. They could, however, be obtained from high-fidelity steady-state simulations of RCCSs. The Rayleigh number in RCCSs is too high for using a Direct Numerical Simulation (DNS) technique; thus, a Reynolds-Averaged Navier-Stokes (RANS) approach must be employed. There are many RANS models, each performing best under different geometry and fluid flow conditions. To find the most suitable one for simulating an RCCS, the RANS models need to be validated. This work benchmarks various RANS models against three experiments performed on the HTTR RCCS Mockup by the Japanese Atomic Energy Agency (JAEA) in 1993. This facility is a 1/6 scale model of a vessel cooling system (VCS) for the High Temperature Engineering Test Reactor (HTTR), which is operated by JAEA. Multiple RANS models were evaluated on a simplified 2d-axisymmetric geometry. They were found to reproduce the experimental temperature profiles with errors of up to 22% for the lowest temperature benchmark and 15% for the higher temperature benchmarks. The results highlight that the pragmatic turbulence models need to be validated for high Rayleigh natural convection-driven flows and improved accordingly, more publicly available experimental data of RCCS resembling experiments is needed and indicate that a 2d-axisymmetric geometry approximation is likely insufficient to capture all the relevant phenomena in RCCS simulations.

• Journal of Advanced Marine Engineering and Technology
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• 제33권8호
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• pp.1261-1267
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• 2009

직접토크제어는 일정한 히스테리시스 범위 내에서 전동기의 자속과 토크를 제어하는 방법으로서 최적 스위칭 테이블을 사용한 고정자 전압 공간 벡터에 의해 전동기의 자속과 토크를 제어하게 된다. 그리고 본 논문에서 사용한 센서리스 제어법은 실제 전동기와 수식 모델의 전류가 수렴하도록 고정자 전압을 인가하면 실제 전동기 회전자 속도가 속도 지령치인 수식 모델의 회전자 속도에 접근하는 제어방식이다. 이 방식들을 접목하면 PI 제어기가 필요하지 않는 간단하면서도 강인한 제어를 구현할 수 있는데 본 논문에서는 컴퓨터 모의실험을 통해 이의 유효성을 입증한다.

• 한국정밀공학회지
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• 제29권5호
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• pp.563-571
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• 2012

In 3D integration package using TSV technology, bonding is the core technology for stacking and interconnecting the chips or wafers. During bonding process, however, warpage and high stress are introduced, and will lead to the misalignment problem between two chips being bonded and failure of the chips. In this paper, a finite element approach is used to predict the warpages and stresses during the bonding process. In particular, in-plane deformation which directly affects the bonding misalignment is closely analyzed. Three types of bonding technology, which are Sn-Ag solder bonding, Cu-Cu direct bonding and SiO2 direct bonding, are compared. Numerical analysis indicates that warpage and stress are accumulated and become larger for each bonding step. In-plane deformation is much larger than out-of-plane deformation during bonding process. Cu-Cu bonding shows the largest warpage, while SiO2 direct bonding shows the smallest warpage. For stress, Sn-Ag solder bonding shows the largest stress, while Cu-Cu bonding shows the smallest. The stress is mainly concentrated at the interface between the via hole and silicon chip or via hole and bonding area. Misalignment induced during Cu-Cu and Sn-Ag solder bonding is equal to or larger than the size of via diameter, therefore should be reduced by lowering bonding temperature and proper selection of package materials.