• Journal of the Computational Structural Engineering Institute of Korea
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• v.33 no.3
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• pp.171-178
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• 2020

In this study, we develop MPI-OpenMP hybrid parallelization for multibody peridynamic simulations. Peridynamics is suitable for analyzing complicated dynamic fractures and various discontinuities. However, compared with a conventional finite element method, nonlocal interactions in peridynamics cost more time and memory. In multibody peridynamic analysis, the costs increase due to the additional interactions that occur when computing the nonlocal contact and ghost interlayer models between adjacent bodies. The costs become excessive when further refinement and smaller time steps are required in cases of high-velocity impact fracturing or similar instances. Thus, high computational efficiency and performance can be achieved by parallelization and optimization of multibody peridynamic simulations. The analytical code is developed using an Intel Fortran MPI compiler and OpenMP in NURION of the KISTI HPC center and parallelized through MPI-OpenMP hybrid parallelization. Further parallelization is conducted by hybridizing with OpenMP threads in each MPI process. We also try to minimize communication operations by model-based decomposition of MPI processes. The numerical results for the impact fracturing of multiple bodies show that the computing performance improves significantly with MPI-OpenMP hybrid parallelization.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.19 no.1
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• pp.69-74
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• 2018

With the increasing development of industrial society and the availability of high quality electrical energy, the simplification of operation and maintenance procedures is required, in order to ensure the reliability and safety of electrical systems. In this paper, the dielectric breakdown characteristics of $N_2-O_2$ mixed gas solid insulation, which is used as an alternative to SF6 in various electric power facilities, are verified. When the gas mixture has a composition ratio similar to that of the atmosphere, the dielectric breakdown characteristics are relatively stabilized. It was confirmed that the breakdown voltage of the gas in the electrode near an equal electric field increased with increasing pressure according to Paschen's rule. The breakdown voltage of the surface increased linearly with increasing pressure, and the difference was caused by the mixing ratio of $O_2$ gas. This change in the surface insulation breakdown voltage was caused by the influence of the electrically negative $O_2$ gas and the intermolecular collision distance. In this study, the influence of the intermolecular impact distance was larger (than that in the absence of the electrically negative $O_2$ gas). The breakdown voltage relation applicable to Teflon according to the surface insulation characteristics was calculated. The characteristics of the surface insulation properties of Teflon, which is used as a solid insulation material, were derived as a function of pressure. It is thought that these results can be used as the basic data for the insulation design of electric power facilities.

• Journal of Korean Society of Environmental Engineers
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• v.36 no.3
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• pp.172-177
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• 2014

Low pressure air flotation (LAF) pilot plant for sludge thickening was installed in Chung Nam N.S. municipal waste water treatment plant to verify its application possibility. Effects of operating conditions such as coagulant dosages and microbubble water ratio on thickening of the mixed sludge were examined. Microbubbles which were generated in the chamber of $1.5kgf/cm^2$ by high speed collision method with foaming agent were used to float sludge. Solid loading of $30kg/m^2/hr$, solid contents in thickened sludge of 60,300 mg/L and SS removal efficiency of 99% were obtained through long period operating LAF in conditions of mixed sludge concentration of 14,400 mg/L, coagulant dosage of 27.6 mg/L, foaming agent addition of 4.0 mg/L and microbubble water injection ratio of 9.7%.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.36 no.6
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• pp.609-617
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• 2012

The impact, spreading and recoil processes of a nanoparticle-laden droplet impacting on a horizontal solid surface are numerically investigated by solving the conservation equations for mass, momentum, energy and mass fraction. The liquid-air interface is tracked using a level-set method that is modified to include the effect of contact angle hysteresis at the wall. The species transport equation including a thermal diffusion term is additionaly solved to determine the nanoparticle distribution in the droplet. The effect of nanoparticle concentration and contact angle are also studied.

• Transactions of the Korean Society of Automotive Engineers
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• v.14 no.4
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• pp.132-138
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• 2006

Numerical analysis about atomization and wall impingement process of hollow-cone fuel spray is performed by a modified KIVA code with hybrid model. The atomization process is modeled by using hybrid breakup model that is composed of Linearized Instability Sheet Atomization(LISA) model and Aerodynamically Progressed Taylor Analogy Breakup(APTAB) model. The Gosman model, which is based on the droplet behaviors after impingement determined by experimental correlations, is used for spray-wall impingement process. The LIEF technique was used to compare the results with those of experiment. The calculations and experiments are carried out at the ambient pressures of 0.1 MPa and 0.5 MPa and the ambient temperature of 293K. It was found that the calculated results show satisfactory agreement with experimental ones.

• Journal of Korea Game Society
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• v.19 no.6
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• pp.99-106
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• 2019

This paper addresses a method to compute the closest approach distance between two ellipsoids in their inter-center direction. This is the key technique for collision detection and response between ellipsoids. We formulate a set of conditions with the inter-center distance, the contact point and the contact normal vector of the two externally-contacting ellipsoids. The equations are solved robustly and efficiently using a hybrid of Newton's method and the bisection method with root bracketing. We demonstrate the robustness and efficiency of the proposed method in various experiments.

• Proceedings of the KSME Conference
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• 2003.11a
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• pp.508-513
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• 2003

Experiments are performed to investigate the detailed structure of underexpanded twin jet impinging on a perpendicular flat plate. The major parameters, such as nozzle operating pressure and nozzle spacing, are varied to create different jet flow fields resulted from the complicated interactions of the twin jets. From the surface pressure measurements and shadowgraphs taken by schlieren optical system, the jet structure is strongly dependent on the nozzle operation pressure and the spacing. The results obtained show that the closer nozzle spacing may induce to decrease the diameter of the Mach disk within the first shock cell in the underexpanded twin jet. With the increasing nozzle operating pressure and decreasing the nozzle spacing, a new shock wave appears at the entrainment region between the two jets, due to the enhancement of mixing effect of the both jets. The closer nozzle spacing makes the overall impinging pressure level higher, while severe pressure oscillation along the axis of symmetry. Furthermore it is recommended the wider spacing to obtain higher thrust under the present experimental conditions.

• Proceedings of the Korean Vacuum Society Conference
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• 2010.08a
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• pp.75-75
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• 2010

배변 후 toilet flushing 시 다량의 세균을 포함한 물방울들이 화장실 곳곳으로 퍼지는 현상이 있다. 이러한 현상을 방지하기 위해 변기 뚜껑에 자기 세정 효과를 갖는 초발수 표면을 위해 플라즈마를 이용한 표면 처리가 시도되고 있으며, 이 연구의 일환으로 flushing시의 변기내의 유동 분석을 초고속 카메라를 이용하여 수행하였다. Toilet flushing 시 물 튀김 현상은 육안으로는 잘 관찰하기 어렵지만 최고 1000 frame/sec의 속도를 갖는 CCD camera를 이용하여 정량적으로 물 튀김에 의한 오염 가능성을 촬영 분석하였다, 두 번째로 소변 시의 변기 표면에서의 튀김현상을 분석하기 위하여 소변의 발사각도 및 속도를 가장 실제와 유사한 조건으로 설정하고 이를 상용 전산 유체 역학 소프트웨어인 CFD- ACE+의 자유 표면 계산 기능과 두 가지 유체(액체 및 기체)의 혼합 계산 모델을 사용한 계산 결과와 비교 하였다. 그 결과 변기 표면의 표면장력을 아주 작게 설정한 경우(작은 접촉각, 친수성)에는 중력의 영향을 고려하였음에도 불구하고 소변이 변기에 충돌 후 상부로 상당부분 튀어 올라가는 결과를 얻었다. 여러 가지 각도와 발사 속도, 실제의 인체와 유사한 발사 부위의 형상 변화로 인한 유체 표면의 난류 발생과 이에 따른 변기 표면 충돌 현상 변화 등을 수치적으로 고찰하였다. 한 예로 5.6 mm 직경의 노즐에서 소변이 나오는 경우를 발사 속도 3 m/s, 각도 $10^{\circ}$로 주고 중력을 고려하여 10초 동안을 계산하면, 방뇨 시 toilet bowl 내부에서의 물의 유동과 toilet 표면을 맞고 튀기는 현상을 그림 1과 같이 볼 수 있었다.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.31 no.12
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• pp.971-978
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• 2007

A spray-wall impingement process of a hollow-cone fuel spray from the high-pressure swirl injector in the Gasoline Direct Injection (GDI) engine were experimented and calculated at various wall geometries. The Linearized Instability Sheet Atomization (LISA) & the Aerodynamically Progressed Taylor Analogy Breakup (APTAB) model and the Gosman model were applied to model the breakup and the wall impingement process of the hollow-cone fuel spray. The numerical modelings were implemented in the modified KIVA code. The calculation results of spray characteristics, such as a spray development process and a radial distance after wall impingement, compared with the experimental results by the Laser Induced Exciplex Fluorescence (LIEF) technique. The droplet size distribution and the ambient gas velocity field, which are generally difficult to obtain by the experimental methods, were also calculated and discussed. It was found that the radial distance after wall impingement and Sauter Mean Diameter (SMD) decreased with increasing a cavity angle.

• Transactions of the Korean hydrogen and new energy society
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• v.27 no.2
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• pp.220-229
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• 2016

A numerical study on lifted flame structure in impinging jet geometry with syngas composition ratio was investigated. The numerical calculations including chemical kinetic analysis were conducted using SPIN application of the CHEMKIN Package with Davis-Mechanism. The flame temperature and velocity profiles were calculated at the steady state for one-dimensional stagnation flow geometry. Syngas mixture compositions were adjusted such as $H_2:CO=10:90(10P)$, 20 : 80 (20P), 30 : 70 (30P), 40 : 60 (40P), 50 : 50 (50P). As composition ratios are changed from 10P to 50P, the axial velocity and flame temperature increase because the contents of hydrogen that have faster burning velocity increase. This phenomenon is due to increase in good reactive radicals such as H, OH radical. As a result of active reactivity, the burning velocity is more faster and this is confirmed by numerical methods. Consequently, combustion reaction zone was moved to burner nozzle.