• 한국공작기계학회논문집
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• 제17권2호
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• pp.45-50
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• 2008

Molecular flows inside a guide block in the OLED(organic luminescent emitting device) deposition process have been simulated using DSMC(direct simulation Monte Carlo) method. Because the organic materials are evaporated under vacuum, molecules flow at a high Knudsen number of the free molecular regime, where the continuum mechanics is not valid. A guide block is designed as a part of the linear cell source to transport the evaporated materials to a deposition chamber, When solving the flows, the inlet boundary condition is proved to affect significantly the whole flow pattern. Thus, it is proposed that the pressure should be specified at the inlet. From the analysis of the density distributions at the nozzle exit of the guide block, it is shown that the longer nozzle can emit molecules more straightly. Finally, a nondimensionalized mass flow profile is obtained by numerical experiments, where various nozzle widths and inlet pressures are tested.

• 대한기계학회논문집B
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• 제26권10호
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• pp.1406-1418
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• 2002

The direct simulation Monte Carlo(DSMC) method is employed to calculate the etch rate on Al wafer. The etchant is assumed to be Cl$_2$. The etching process of an Al wafer in a helicon plasma etcher is examined by simulating molecular collisions of reactant and product. The flow field inside a plasma etch reactor is also simulated by the DSMC method fur a chlorine feed gas flow. The surface reaction on the Al wafer is simply modelled by one-step reaction: 3C1$_2$+2Allongrightarrow1 2AIC1$_3$. The gas flow inside the reactor is compared for six different nozzle locations. It is found that the flow field inside the reactor is affected by the nozzle locations. The Cl$_2$ number density on the wafer decreases as the nozzle location moves toward the side of the reactor. Also, the present numerical results show that the nozzle location 1, which is at the top of the reactor chamber, produces a higher etch rate.

• 대한기계학회논문집B
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• 제35권7호
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• pp.657-664
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• 2011

이 연구는 기체로 채워진 1 차원 평행 공간에서 경계 온도를 추정하는 역해석 기법을 제안한다. 평판사이의 거리는 마이크론 이하의 크기부터 1 밀리미터 까지를 고려한다. 한쪽 경계에서는 온도와 열유속이 동시에 활용 가능하지만 다른 경계에서는 아무런 측정이 불가한 상황을 가정한다. 한쪽 경계의 온도는 알려진 열유속과 온도를 이용하여 거꾸로 결정하여야 한다. 이 연구는 이 온도를 몬테카를로 모사를 통하여 산정하는 절차를 제안하였는데 직접 문제는 DSMC 를 사용하고 역문제는 모사 어닐링을 이용한다.

• 대한기계학회논문집B
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• 제21권4호
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• pp.509-517
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• 1997

Results of flat plate compressible boundary layer calculation, based on discrete formulation of DSMC method, are presented in low Mach number and low Knudsen number range. The free stream is a uniform flow of pure nitrogen at various Mach numbers in low pressures (i.e. rarefied gas). Complete thermal accommodation and diffuse molecular reflections are used as the wall boundary condition, replacing unreal no-slip condition used in continuum calculations. In the discrete formulation of DSMC method, there is no need to use ad hoc assumptions on transport properties like viscosity and thermal conductivity, instead viscosity is calculated from values of other field variables (velocity and shear stress). Also the results are compared with existing self-similar continuum solutions. In all Mach number cases computed, velocity slip is most pronounced in regions near the leading edge where continuum formulation renders the solution singular. As the boundary layer develops further downstream, velocity slips asymptote to values that are between 10 to 20% of the magnitude of free stream velocity. When the free stream number density is reduced, so the gas more rarefied, the velocity slip increases as expected.

• 항공우주시스템공학회지
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• 제12권5호
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• pp.8-15
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• 2018

발사체로부터 분리된 정지궤도위성은 천이궤도로 진입한 후에 액체원지점엔진을 사용하여 충분한 속도증분을 얻음으로써 정지궤도로 진입하게 된다. 이때 우주공간으로 배출되는 액체원지점엔진의 배기가스 중 일부는 고진공 환경에서 팽창하는 동안 위성체 방향으로 역류하는 후방유동으로 발달하게 된다. 이러한 후방유동은 위성체에 충돌하면서 자세제어 교란, 표면 오염, 열전달 등의 영향을 끼치게 되므로 정지궤도위성 임무성능의 저하를 유발할 수 있다. 따라서 본 연구에서는 정지궤도위성에 사용되는 400 N급 액체원지점엔진에서 배출되는 배기가스의 거동을 해석하였다. 이를 위해 볼츠만방정식에 기반을 둔 직접모사법(DSMC)을 사용하였다. 해석결과로 액체원지점엔진에서 배출된 배기가스의 온도 및 수밀도와 같은 열유동 특성을 확인할 수 있었다.

• Advances in aircraft and spacecraft science
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• 제6권3호
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• pp.239-256
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• 2019

This study develops a dual purpose: i) evaluating the effects of two different Mars atmosphere models (NASA Glenn and GRAM-2001) on aerodynamics of a capsule (Pathfinder) entering the Mars atmosphere, ii) verifying the feasibility of evaluating the ambient density and pressure by means of the methods by McLaughlin and Cassanto, respectively and therefore to re-build the values provided by the models. The method by McLaughlin relies on the evaluation of the capsule drag coefficient, the method by Cassanto relies on the measurement of pressure at a point on the capsule surface in aerodynamic shadow. The study has been carried out computationally by means of: i) a code integrating the equations of dynamics of the capsule for the computation of the entry trajectory, ii) a DSMC code for the solution of the flow field around the capsule in the altitude interval 50-100 km. The models show consistent differences at altitudes higher than about 40 km. It seems that the GRAM-2001 model is more reliable than the NASA Glenn model. In fact, the NASA Glenn model produces, at high altitude, temperatures that seem to be too low compared with those from the GRAM-2001 model and correspondingly very different aerodynamic conditions in terms of Mach, Reynolds and Knudsen numbers. This produces pretty different capsule drag coefficients by the two models as well as pressure on its surface, making not feasible neither the method by McLaughlin nor that by Cassanto, until a single, reliable model of the Mars atmosphere is not established. The present study verified that the implementation of the Cassanto method in Mars atmosphere should rely (such as it is currently) on pressure obtained experimentally in ground facilities.

• Advances in aircraft and spacecraft science
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• 제2권4호
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• pp.427-443
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• 2015

The present paper is the follow-on of a former work in which the influence of the gas-surface interaction models was evaluated on the aerodynamic coefficients of an aero-space-plane and on a section of its wing. The models by Maxwell and by Cercignani-Lampis-Lord were compared by means of Direct Simulation Monte Carlo (DSMC) codes. In that paper the diffusive, fully accommodated, semi-specular and specular accommodation coefficients were considered. The results pointed out that the influence of the interaction models, considering the above mentioned accommodation coefficients, is pretty strong while the Cercignani-Lampis-Lord and the Maxwell models are practically equivalent. In the present paper, the comparison of the same models is carried out considering the dependence of the accommodation coefficients on the angle of incidence (or partial accommodation coefficients). More specifically, the normal and the tangential momentum partial accommodation coefficients, obtained experimentally by Knetchel and Pitts, have been implemented. Computer tests on a NACA-0012 airfoil have been carried out by the DSMC code DS2V-64 bits. The airfoil, of 2 m chord, has been tested both in clean and flapped configurations. The simulated conditions were those at an altitude of 100 km where the airfoil is in transitional regime. The results confirmed that the two interaction models are practically equivalent and verified that the use of the Knetchel and Pitts coefficients involves results very close to those computed considering a diffusive, fully accommodated interaction both in clean and flapped configurations.

• Advances in aircraft and spacecraft science
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• 제8권1호
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• pp.53-67
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• 2021

The increasing interest in the exploration of Mars stimulated the authors to study aerodynamic problems linked to space vehicles. The aim of this paper is to evaluate the aerodynamic effects of a flapped wing in collaborating with parachutes and retro-rockets to reduce velocity and with thrusters to control the spacecraft attitude. 3-D computations on a preliminary configuration of a blunt-cylinder, provided with flapped fins, quantified the beneficial influence of the fins. The present paper is focused on Aerodynamics of a wing section (NACA-0010) provided with a trailing edge flap. The influence of the flap deflection was evaluated by the increments of aerodynamic force and leading edge pitching moment coefficients with respect to the coefficients in clean configuration. The study was carried out by means of two Direct Simulation Monte Carlo (DSMC) codes (DS2V/3V solving 2-D/3-D flow fields, respectively). A DSMC code is indispensable to simulate complex flow fields on a wing generated by Shock Wave-Shock Wave Interaction (SWSWI) due to the flap deflection. The flap angle has to be a compromise between the aerodynamic effectiveness and the increases of aerodynamic load and heat flux on the wing section lower surface.

• 한국항공우주학회지
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• 제30권2호
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• pp.1-11
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• 2002

본 연구에서는 특별히 PC cluster와 같은 병렬 환경에서 효율적인 계산을 수행할 수 있는 격자중심에 기초한 직접모사 기법을 개발하였다. 병렬환경 하에서의 효과적인 계산 수행을 위해서는 전체 계산 영역을 격자수와 각 격자에 할당되는 모사 입자 수를 고려한 부 영역들로 나누어주었다. 또한, 격자 사용의 효율성 증대를 위해서는 매우 성긴 격자에서부터 출발하여 점차적인 격자 적응을 수행하였다. 본 방법은 2차원의 초음속 평판 문제와 축대칭의 Rothe, 노즐 문제에 적용하였다. 그 결과로부터 본 방법을 사용하면 기존의 입자 중심 기법에 비해 매우 효율적으로 희박기체 유동을 해석할 수 있음을 알 수 있었다.

• 한국항공우주학회지
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• 제30권5호
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• pp.32-40
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• 2002

희박상태나 극소장치에 관련된 기체운동을 해석하는 문제가 최근 중요한 연구주제로 부각되고 있다. 잘 알려진 DSMC와 더불어 모우멘트 기법, Chapman-Enskog 기법으로 분류되는 고차 비평형 유동 해석모델들이 이 문제에 적용되어 왔다. 본 연구에서는 Eu의 일반유체역학을 근간으로 이원자 기체에 관한 고차 해석모델을 개발하고자 한다. 회전 비평형 효과는 기체의 용적 점성계수에 관한 초과 수직응력을 고려하여 감안하였다. 개발된 계산모델을 일차원 충격파 내부구조와 단순 형상 외부의 희박 극초음속 유동장 해석에 적용하였다. 충격파 내부구조 및 전단유동 해석을 통해 회전 비평형에 의한 용적 점성계수 효과가 중요함을 확인하였다. 충격파 내부구조에 관한 이론적 예측이 실험과 잘 일치함도 확인하였다.