Flow Direction When Fan Shaped Geometry is Applied in Gas-Assisted Injection Molding : 1. Flow Model Theory and its Criteria for Predicting Flow Directions

Lim, Kwang-Hee 한국화학공학회 2004 Korean Journal of Chemical Engineering Vol.21 No.1

In part 1 of this paper a qualitative analytical method to predict the preferred gas flow direction in gas-assisted injection molding (GAIM), which involves flow through panel-areas of various fan-shaped geometries, and the criteria to apply the method were presented with appropriate assumptions. Then the definition of a resistance to initial velocity was proposed as a rule of thumb, by which the gas directions of GAIM were predicted under various fan-shaped geometries. Upon performing the simulation on them with commercial software (MOLDFLOW), we compared the ratio of simulated gas penetration lengths to both directions with the predicted ratio of resistances as well as the predicted direction of the gas flow in GAIM using the suggested rule of thumb herein presented. The predictions with the suggested rule of thumb were generally quite consistent with the results of simulation (MOLDFLOW). However the discrepancy between the ratio of gas penetration lengths and the ratio of resistances was observed to increase as the ratio (?) of the values of H/R_(0) on both sides of fan-shaped cavities became bigger even though the suggested rule of thumb was assumed adequate to use until the case met the condition of (H/R_(0))^(2) 1/Θ^(2)≪1 and (H/R_(0))^(2)≪1. Neverthelsee, the suggested rule of thumb was still effective as far as the direction of gas flow was concerned.

Flow Direction When Fan Shaped Geometry is Applied in Gas-Assisted Injection Molding: 2. Development of Flow Model and its Predictions

임광희,홍수현 한국화학공학회 2004 Korean Journal of Chemical Engineering Vol.21 No.1

In part 2 of the paper simplified unsteady-mass (and momentum-) balance equations of melt polymer resinin the cavities of GAIM were proposed, as a time-dependent rule of thumb, to constitute a novel flow model in GAIMunder the configuration of two fan-shaped geometries connected with a gas nozzle. Upon performing a simulation onthem with commercial software (MOLDFLOW), we compared the time evolution of simulated gas penetration lengthswith the those of unsteady trajectory on the gas flow in GAIM by the suggested novel flow model in the fan-shapedcavities in order to check the precision of model-predicted gas penetration lengths as well as the consistency of itspredicted direction. The results by the suggested novel flow model were satisfactory to fit the trajectory simulated withcommercial software (MOLDFLOW).

Flow Direction When Fan Shaped Geometry is Applied in Gasisted-Ass Injection Molding : 2. Development of Flow Model and its Predictions

Lim, Kwang-Hee,Hong, Soo Hyeun 한국화학공학회 2004 Korean Journal of Chemical Engineering Vol.21 No.1

In part 2 of the paper simplified unsteady-mass (and momentum-) balance equations of melt polymer resin in the cavities of GAIM were proposed, as a time-dependent rule of thumb, to constitute a novel flow model in GAIM under the configuration of two fan-shaped geometries connected with a gas nozzle. Upon performing a simulation on them with commercial software (MOLDFLOW), we compared the time evolution of simulated gas penetration lengths with the those of unsteady trajectory on the gas flow in GAIM by the suggested novel flow model in the fan-shaped cavities in order to check the precision of model-predicted gas penetration lengths as well as the consistency of its predicted direction. The results by the suggested novel flow model were satisfactory to fit the trajectory simulated with commercial software(MOLDFLOW).

DSMC 및 IP 방법을 이용한 2차원 아음속 희박기체 유동 해석

최영재,권오준 한국전산유체공학회 2017 한국전산유체공학회지 Vol.22 No.4

In the present study, to investigate the characteristics of subsonic rarefied gas flows efficiently, the information preservation(IP) method was applied to the direct simulation Monte-Carlo(DSMC) method to reduce statistical scatters from the DSMC method for low speed rarefied gas flows. To find the effects of the IP method, the results of the IP method were compared to the results of the DSMC method for the same numbers of sampling. Before the comparison, it was necessary to verify the DSMC solver. The verification of the solver was conducted by simulations of the micro-channel flows by comparing to the other researchers’ DSMC results. The velocity profiles at the 2/3 section of the channel agreed well with the other researchers’ results. In addition, the normalized slip velocity distributions on the wall and the pressure distributions along the centerline also agreed well with the other researchers’ results. After the DSMC solvers were verified, two types of Couette flows were considered. One is the flows that have temperature differences between two plates, and the other is the flows that have moving plates. For the first Couette flows, the temperature of 373K was used for the upper plate, and of 173K for lower plate. For the second Couette flows, the upper plate is moving at 300m/s, and the temperature of 273K was used for both two plates. All simulations of Couette flows were conducted from near-continuum to free-molecular regimes. From both Couette flow simulations, it was found that the temperature jump and the velocity slip occurred on the plate surface. In addition, it was also shown that the temperature and the velocity differences between the plate and the gas became larger as the flow fields became more rarefied. Lastly, flow simulations around a NACA0012 airfoil were conducted to identify the effects of the IP method. The freestream Mach number of 0.8 was used, and the Knudsen number of 0.014 was considered. It was observed that more clear contours were obtained from the IP method than the DSMC method, since the statistical scatter error was reduced by the IP method. In addition, it was found that the velocity slip on the airfoil surface occurred due to the effects of the rarefied atmospheric environment.

희박기체 유동 해석에 대한 CFD와 Kinetic 모델의 비교 연구

김상훈,박웅휘,전은지 한국항공우주학회 2023 韓國航空宇宙學會誌 Vol.51 No.12

일반적으로 Navier-Stokes-Fourier 방정식에 근거한 Computational Fluid Dynamics (CFD) 방법은 연속체 영역에 적용되고, Boltzmann 방정식에 근거한 Direct Simulation Monte Carlo (DSMC) 방법은 희박기체 영역에 적용된다. 하지만 재진입 비행체와 같이 공기역학의 많은 문제는 국소적으로 밀도가 크게 변하는 멀티-스케일 희박 유동으로 알려져 있다. 멀티-스케일 희박 유동에 대해서는 CFD 방법만으로는 정확도 측면에서 한계가 있고, DSMC 방법만으로는 효율성 측면에서 제한이 있다. 최근 CFD와 DSMC 사이의 격차를 메우기 위해 대안적인 입자 시뮬레이션 기법들이 제시되고 있다. 가장 활발히 연구되고 있는 대상은 Bhatnagar-Gross-Krook (BGK) 방법과 Fokker-Planck (FP) 방법이 있다. 하지만 수치 방법들의 성능을 일관성 있게 평가하는 비교 연구는 아직 부족한 상황이다. 본 연구에서는 희박기체 유동 해석에 대해 CFD, DSMC, BGK, FP 방법을 적용하고 성능을 분석하였다. In practice, Computational Fluid Dynamics (CFD) based on the Navier-Stokes-Fourier equation is applied in the continuum regime, while Direct Simulation Monte Carlo (DSMC) based on the Boltzmann equation is applied in the rarefied regime. However, many aerodynamic engineering problems, such as re-entry vehicles, involve multi-scale rarefied gas flows where the density significantly varies locally. For multi-scale rarefied gas flows, CFD has limitations in terms of accuracy, while DSMC has limitations in terms of efficiency. Recently, alternative particle Monte Carlo methods have been proposed to bridge the gap between CFD and DSMC. These include the Bhatnagar-Gross-Krook (BGK) method and the Fokker-Planck (FP) method. However, there is a scarcity of comprehensive comparative studies to assess their performance. The present paper provides consistent comparison results among CFD, DSMC, BGK, and FP for rarefied gas flows.

직접모사법을 이용한 지구 저궤도 파라볼릭 안테나 탑재 위성의 항력 예측

신소민(Somin Shin),나경수(Kyung-Su Na),이주영(Juyoung Lee),조기대(Ki-Dae Cho) 한국항공우주학회 2014 韓國航空宇宙學會誌 Vol.42 No.7

저궤도에서 운용되는 위성은 대기 저항에 의한 연료소모가 크며, 연료소모는 임무수명 및 발사무게에 영향을 미치게 되어 위성 형상에 따른 항력의 예측이 중요하다. 본 논문에서는 직접모사법을 이용하여 파라볼릭 안테나를 탑재한 저궤도 위성의 임무고도의 변화와 받음각에 따른 항력 및 항력 계수의 변화를 살펴보았다. 저궤도의 희박 기체의 거동을 모사하는 직접모사법의 적용성을 검증하기 위해 스타샤인(Starshine) 위성의 비행데이터를 이용하여 고도, 대기와 표면의 상호작용에 따른 항력 계수를 비교하였다. 결론적으로 계산결과로부터 저궤도 위성의 정밀한 궤도수명 계산에 적합한 항력 계수를 도출하였다. Consumption of the fuel on the satellite operating in low earth orbit, is increased due to the air resistance and the amount of increase makes the satellite lifetime decrease or the satellite mass risen. Therefore the prediction of drag force of the satellite is important. In the paper, drag force and drag coefficient analysis of the parabolic antenna satellite in low earth orbit using direct simulation monte carlo method (DSMC) is conducted according to the mission altitude and angle of attack. To verify the DSMC simulated rarefied air movement, Starshine satellite drag coefficient according to the altitude and gas-surface interaction are compared with the flight data. Finally, from the analysis results, it leads to appropriate satellite drag coefficient for orbit lifetime calculation.

OpenFOAM을 이용한 희박기체 유동에서의 직접모사 몬테카를로 방법

송수동(Soodong Song),이은택(Euntaek Lee) 한국추진공학회 2018 한국추진공학회 학술대회논문집 Vol.2018 No.5

본 논문은 OpenFOAM을 이용한 직접모사 몬테카를로(DSMC) 방법의 타당성을 검증하였다. OpenFOAM은 다양한 수치해석 모듈을 가지고 있는 공개 소프트웨어다. OpenFOAM안에 dsmcFoam은 DSMC 방법을 이용하여 희박기체 유동을 해석할 수 있다. dsmcFoam은 비구조화 격자계와 임의의 형상에서 입자 추적법을 사용한다. 본 연구에서는 dsmcFoam의 검증을 위하여 3차원 극초음속 코너 유동, 평평한 원통 주면을 흐르는 3차원 극초음속 유동 그리고 평판을 지나는 3차원 극초음속 유동을 해석할 것이다. 그 결과를 참고문헌과 비교하였고 서로 일치함을 알 수 있었다. This paper represents the validation of direct simulation Monte Carlo (DSMC) method using OpenFOAM. OpenFOAM is an open source software having various numerical modules. In the framework of OpenFOAM, the solver, dsmcFoam can simulate rarified gas flows using DSMC. It employs particle tracking for unstructured grids and arbitrary geometries. In this study, 3D hypersonic corner flow, 3D hypersonic flow over a flat-nosed cylinder and 3D hypersonic flow over a flat plate are considered for the validation. The results are compared with the references and have good agreements.

Direct Laser Melting 공정시 차폐가스가 성형 특성에 미치는 영향

한상욱(S. W. Han),신세계로(S. G. R. Shin),신은경(E. K. Shin),이철환(C. H. Lee),문영훈(Y. H. Moon) 한국소성가공학회 2013 한국소성가공학회 학술대회 논문집 Vol.2013 No.5

Direct Laser Melting process is a kind of prototyping process whereby a 3-D part is built layer wise by laser scanning the powder. This process is highly influenced by shielding gas and laser parameters such as laser power, scan rate, layering thickness and rescanning. Especially, shielding gas is important in evaluating the microstructure and mechanical properties. In this study, fabrication was carried out by experimental facilities consisting of a 200W fiber laser. Cylinder was fabricated by shielding gas atmosphere such as Air, Ar and N₂ with Fe-Ni-Cr powder to investigate forming characteristics. Although surface properties was improve, surface roughness was similar without significant change. In order to evaluate the formability, flow stress curves were analyzed. The microstructure is different by scan rate and rescanning.

DSMC-IP 방법을 이용한 삼차원 희박기체 유동 해석

최영재(Y.J. Choi),권오준(O.J. Kwon) 한국전산유체공학회 2019 한국전산유체공학회지 Vol.24 No.1

In the present study, the information preservation(IP) method was employed to the three dimensional direct simulation Monte-Carlo(DSMC) solver to simulate the low-speed rarefied gas flows efficiently. It was known that the IP method can reduce the statistical scatter problems of the DSMC method. Firstly, the verification of the DSMC solver was conducted to compare with the results of the DSMC-IP solver. After that, two cases of flow problems were simulated by the developed DSMC-IP solver. One is the thermal diffusion flows, and the other is the low-speed flows around the sphere. To find the effects of the IP method, the both cases were calculated by the pure DSMC method using same number of sampling of particles. Through the results of the thermal diffusion flows, it was found that the temperature scatters were reduced in the results of the DSMC-IP solver by comparing with the results of the pure DSMC solver. It was also shown that the velocity scatters decreased in the results of the IP method from the sphere problems. In addition, it was observed that the IP method gets the reasonable results using a relatively less number of sampling of particles than the pure DSMC method. As a result, it was obtained that the developed DSMC-IP solver is a more efficient method for the low-speed rarefied gas flows than the pure DSMC solver.

희박연료 직접분사(Lean Direct Injection) 가스터빈 연소기의 이상유동 분석

이교빈(Kyobin Lee),김종찬(Jong-Chan Kim),성홍계(Hong-Gye Sung) 한국항공우주학회 2019 韓國航空宇宙學會誌 Vol.47 No.3

희박연료 직접분사(Lean Direct injection(LDI)) 가스터빈 연소기에 대한 이상유동 특성을 해석하였다. LDI 연소기에 적용된 환형분사기(hollow-cone spray injector)의 분열을 모사하기 위해 분열모델(Linearized Instability Sheet Atomization(LISA), Aerodynamically Progressed Taylor Analysis Breakup(APTAB)을 적용하였다. 침투깊이와 평균입도(Sauter Mean Diameter(SMD))를 통해 분열모델을 검증하였으며, LDI 연소기에 적용하여 이상유동특성을 분석하였다. 스월인젝터로 인해 Precessing Vortex Core(PVC)가 발생하였으며, 액적들이 PVC를 따라 미립화되는 것을 확인하였다. SMD결과를 통해 PVC가 회전하는 영역의 외곽으로 즉, 빠른 속도 영역에 액적들이 분포하며, 스톡스수 (Stokes number)는 1보다 작다. The analysis on two-phase flow in a Lean Direct Injection(LDI) combustor has been investigated. Linearized Instability Sheet Atomization(LISA) and Aerodynamically Progressed Taylor Analogy Breakup(APTAB) breakup models are applied to simulate the droplet breakup process in hollow-cone spray. Breakup model is validated by comparing penetration length and Sauter Mean Diameter(SMD) of the experiment and simulation. In the LDI combustor, Precessing Vortex Core(PVC) is developed by swirling flow and most droplets are atomized along the PVC. It has been confirmed that all droplets have Stokes number less than 1.0.