• Journal of Mechanical Science and Technology
• /
• 제20권8호
• /
• pp.1256-1265
• /
• 2006

The supersonic flows around tandem cavities were investigated by two-dimensional and three-dimensional numerical simulations using the Reynolds-Averaged Navier-Stokes (RANS) equation with the k- ω turbulence model. The flow around a cavity is characterized as unsteady flow because of the formation and dissipation of vortices due to the interaction between the freestream shear layer and cavity internal flow, the generation of shock and expansion waves, and the acoustic effect transmitted from wake flow to upstream. The upwind TVD scheme based on the flux vector split with van Leer's limiter was used as the numerical method. Numerical calculations were performed by the parallel processing with time discretizations carried out by the 4th-order Runge- Kutta method. The aspect ratios of cavities are 3 for the first cavity and 1 for the second cavity. The ratio of cavity interval to depth is 1. The ratio of cavity width to depth is 1 in the case of three dimensional flow. The Mach number and the Reynolds number were 1.5 and $4.5{\times}10^5$, respectively. The characteristics of the dominant frequency between two- dimensional and three-dimensional flows were compared, and the characteristics of the second cavity flow due to the first cavity flow was analyzed. Both two dimensional and three dimensional flow oscillations were in the 'shear layer mode', which is based on the feedback mechanism of Rossiter's formula. However, three dimensional flow was much less turbulent than two dimensional flow, depending on whether it could inflow and outflow laterally. The dominant frequencies of the two dimensional flow and three dimensional flows coincided with Rossiter's 2nd mode frequency. The another dominant frequency of the three dimensional flow corresponded to Rossiter's 1st mode frequency.

• 한국항해항만학회지
• /
• 제32권6호
• /
• pp.453-458
• /
• 2008

원형실린더에 크기가 다른 제어봉을 부착하여 2차원 단면 유통특성에 대해서 시간평균유동장을 계측하여 근접후류 제어에 관하여 PIV기법을 적용하여 연구를 수행하였다. 모델시험은 각각 크기가 다른 제어봉을 적용하였으며($d/D=0.1{\sim}d/D=0.5$) 레이놀즈수는 원형실린더의 직경을 기준으로 Re=15,000으로 적용하였다. 유동장의 속도분포론 획득하기 위해 2프레임 상호상관법을 이용하였다. 실험에 적용된 원형실린더(D=50mm)에 제어봉을 부착한 결과를 상호 비교함으로서 시간평균속도분포와 제어봉에 의한 유동제어 효과를 알 수 있었다.

• 한국추진공학회지
• /
• 제9권2호
• /
• pp.97-104
• /
• 2005

많은 장점에도 불구하고 유동함수를 이용한 수치해석용 격자생성 좌표변환기법의 단점은 저속영역에서의 격자간격이 고속영역에 비해 상대적으로 큼에 따라 수치적 처리에 많은 오차를 내포하고 있다는 점이다. 본 연구에서는 이러한 저속영역에서의 단점을 보완하기 위하여 격자간격을 속도크기 및 영역에 따라 적절히 조절할 수 있도록 수학적으로 변형된 압축성 유동함수를 이용한 좌표변환기법을 제안하고 가스터빈엔진에 주로 적용되는 유동모델로서 동심원상 두개 이상의 난류제트혼합유동에 대해 적용하였으며 해당 실험치, 즉 축 방향 평균속도분포, 난류운동에너지, 그리고 난류전단응력분포와 비교하여 난류운동에너지가 약간 과소평가 된 대칭축을 제외한 혼합경계층 내에서 $3.5\%$ 이내의 신뢰성을 확보하였다. 본 기법은 특히 터보팬엔진에 대한 내부흐름들의 혼합유동을 규명하거나 또는 난류전단응력에 의한 제트소음발생 및 저감방법을 도모하는데 유용하게 활용될 것으로 기대된다.

• Advances in aircraft and spacecraft science
• /
• 제11권2호
• /
• pp.153-175
• /
• 2024

In jet engines, turbine blade cooling has an extremely important role. The pin-fin array, which is situated close to the trailing edge of the blade, aids in internal cooling of the gas turbine blades and preserves the structural integrity of the blade. Previous studies often focused on pin-fin configurations, but the current research focuses on improving the geometry at the endwalls to reduce wake vortices behind the pin-fins and enhance heat transfer at the endwalls location. Using the k-ω turbulence model, a numerical study was conducted on a ribbed shape situated on the walls between pin-fin arrays, spanning a Reynolds number range of 7400 to 36000, in order to determine the heat transport characteristics. The heat transfer efficiency coefficient and Nusselt number increase dramatically with the revised wall configuration, according to the numerical data. The channel's heat transfer efficiency is increased by enlarging the heat transfer areas near the pin-fins and by the interaction of the flow with the endwalls. The addition of ribs causes the Nusselt number of the new model to climb from 78% to 96% at the previously given Reynolds numbers, and the heat transfer efficiency index to rise from 60% to 73%. The height (Hr), position (Lr), forward width (Wf), and backward width (Wb) of the ribs are among the geometric elements that were looked at in order to determine how they affected the performance of heat transmission. In comparison to the reference design, the parametric study results demonstrate that the best forward width (Wf/R=18.75%) and backward width (Wb/R=31.25%) increase the heat transfer efficiency index by 0.4% and 1.3%, respectively.

• 한국항공우주학회지
• /
• 제46권1호
• /
• pp.41-51
• /
• 2018

무장 헬리콥터에서 발사되는 무유도 로켓은 로터 블레이드에 의한 내리흐름과 전후좌우 기동으로 인한 외풍에 의해 전체 궤적 및 사거리가 변화하므로, 내리흐름 효과를 고려하여 무유도 로켓의 궤적을 예측하는 것이 중요하다. 내리흐름 효과를 고려한 무유도 로켓의 궤적 및 사거리를 예측하기 위해, 본 연구에서 여러 외풍 조건에 따른 후류 영역을 Actuator Disk Model(ADM)로 계산하고 6 자유도 (6 DOF) 운동 해석으로 무유도 로켓의 자세 및 전체 비행 궤적을 예측할 수 있는 알고리즘을 개발하였다. 개발된 알고리즘은 ADM 해석 결과를 6 자유도에 반영하여 다양한 초기 발사조건에서 무유도 로켓의 전체 궤적을 예측할 수 있고, 기존 Inflow model을 이용한 내리흐름 해석과는 다르게 동체와의 간섭효과를 고려하여 비교적 정확한 내리흐름 및 다양한 외풍 환경 조건으로 궤적을 예측 할 수 있다. 개발된 알고리즘을 이용하여, 내리흐름 효과에 의한 무유도 로켓의 자세 및 궤적 변화 메커니즘을 유효 받음각 변화와 기수 자세 안정성으로 규명하였다. 그리고 외풍으로 인해 변화하는 내리흐름 효과를 고려하여 무유도 로켓의 궤적변화와 사거리를 계산한 결과, 후방 외풍 시 최대 13% 사거리 증가를 보였다. 사거리 증가의 주요 요인으로 내리흐름 영역과 강도, 부차적 요인으로 외풍과 동체와의 간섭효과, 동압의 크기인 것을 밝혔다. 또한 사거리 변화량이 가장 큰 후방 외풍에서, 후방 외풍의 풍속이 증가함에 따라 로켓의 사거리가 증가하였다. 하지만 특정 후방 외풍 크기 이상에서 더 이상 로켓 사거리가 증가하지 않는 한계를 보였다.

• Nuclear Engineering and Technology
• /
• 제55권10호
• /
• pp.3874-3897
• /
• 2023

A high-fidelity computational fluid dynamics (CFD) analysis was performed using the Large Eddy Simulation (LES) model for the lower plenum of the High-Temperature Test Facility (HTTF), a ¼ scale test facility of the modular high temperature gas-cooled reactor (MHTGR) managed by Oregon State University. In most next-generation nuclear reactors, thermal stress due to thermal striping is one of the risks to be curiously considered. This is also true for HTGRs, especially since the exhaust helium gas temperature is high. In order to evaluate these risks and performance, organizations in the United States led by the OECD NEA are conducting a thermal hydraulic code benchmark for HTGR, and the test facility used for this benchmark is HTTF. HTTF can perform experiments in both normal and accident situations and provide high-quality experimental data. However, it is difficult to provide sufficient data for benchmarking through experiments, and there is a problem with the reliability of CFD analysis results based on Reynolds-averaged Navier-Stokes to analyze thermal hydraulic behavior without verification. To solve this problem, high-fidelity 3-D CFD analysis was performed using the LES model for HTTF. It was also verified that the LES model can properly simulate this jet mixing phenomenon via a unit cell test that provides experimental information. As a result of CFD analysis, the lower the dependency of the sub-grid scale model, the closer to the actual analysis result. In the case of unit cell test CFD analysis and HTTF CFD analysis, the volume-averaged sub-grid scale model dependency was calculated to be 13.0% and 9.16%, respectively. As a result of HTTF analysis, quantitative data of the fluid inside the HTTF lower plenum was provided in this paper. As a result of qualitative analysis, the temperature was highest at the center of the lower plenum, while the temperature fluctuation was highest near the edge of the lower plenum wall. The power spectral density of temperature was analyzed via fast Fourier transform (FFT) for specific points on the center and side of the lower plenum. FFT results did not reveal specific frequency-dominant temperature fluctuations in the center part. It was confirmed that the temperature power spectral density (PSD) at the top increased from the center to the wake. The vortex was visualized using the well-known scalar Q-criterion, and as a result, the closer to the outlet duct, the greater the influence of the mainstream, so that the inflow jet vortex was dissipated and mixed at the top of the lower plenum. Additionally, FFT analysis was performed on the support structure near the corner of the lower plenum with large temperature fluctuations, and as a result, it was confirmed that the temperature fluctuation of the flow did not have a significant effect near the corner wall. In addition, the vortices generated from the lower plenum to the outlet duct were identified in this paper. It is considered that the quantitative and qualitative results presented in this paper will serve as reference data for the benchmark.

• 만화애니메이션 연구
• /
• 통권38호
• /
• pp.71-91
• /
• 2015

본 연구는 애니메이션의 전문적인 특성을 고려한 라이프 드로잉 교수법을 연구하는 과정으로 두뇌의 창작 기제를 고려한 전략을 적용한 모형과 교수방법 설계를 목적으로 한다. 최근 들어 창의성을 기반으로 하는 각 전문분야의 교육방법에 대한 대안적인 논의로 뇌 기반 학습원리를 적용한 융합적 교수법에 대한 연구결과들이 발표되고 있다. 즉, 뇌의 창의기제를 기반으로 한 융합적 교육은 미술과 드로잉 교육뿐만 아니라, 예술전반에서 적용되고 있는 것이다. 라이프 드로잉은 인체에 대한 구조적 지식을 넘어서 인지적 감각, 창의성, 그리고 동작을 통한 대상과의 소통방식을 이해한 생동감 표현법 등을 숙련할 수 있는 종합적인 교수법을 요하는 분야이다. 이에 본 연구에서는 연구의 앞선 단계에서 분석된 창의, 학습기제와 내용요소를 바탕으로 하여 라이프 드로잉 숙련을 위한 전략과 방법 그것을 정리한 교육모형 구조도를 설계하여 본다. 그 결과 이전 연구의 결과물인 뇌의 창의, 학습 기제를 기반으로 한 라이프드로잉의 능력요소와 두뇌기반 촉진요소가 유기적으로 결합되기 위해서는 5단계 인지전략단계인 뇌 활성화 준비단계, 대뇌피질 기능 활성화, 고등사고촉진단계, 고등사고단계, 통합단계를 거쳤을 때 가능하다는 결론에 도달하였다. 또한 이를 실행하기위한 전략적 방법으로는 브레인짐(brain gym), 우뇌활성화드로잉, HSP(고차인지)트레이닝으로 설계되었다. 이를 토대로 하여 설계된 교수학습모형 구조도는 이후의 연구에서 해당 회기 동안의 교수학습지도안 설계로 이어진다.

• 대기
• /
• 제23권2호
• /
• pp.205-220
• /
• 2013

Upper ocean response to typhoon Ewiniar (0603) and its impact on the following typhoon Bilis (0604) are investigated using observational data and numerical experiments. Data used in this study are obtained from the Ieodo Ocean Research Station (IORS), ARGO, and satellite. Numerical simulations are conducted using 3-dimensional Princeton Ocean Model. Results show that when Ewiniar passes over the western North Pacific, unique oceanic responses are found at two places, One is in East China Sea near Taiwan and another is in the vicinity of IORS. The latter are characterized by a strong sea surface cooling (SSC), $6^{\circ}C$ and $11^{\circ}C$ in simulation and observation, under the condition of typhoon with a fast translation speed (8m $s^{-1}$) and lowering intensity (970 hPa). The record-breaking strong SSC is caused by the Yellow Sea Bottom Cold Water, which produces a strong vertical temperature gradient within a shallow depth of Yellow Sea. The former are also characterized by a strong SSC, $7.5^{\circ}C$ in simulation, with a additional cooling of $4.5^{\circ}C$ after a storm's passage mainly due to enhanced and maintained upwelling process by the resonance coupling of storm translation speed and the gravest mode internal wave phase speed. The numerical simulation reveals that the Ewiniar produced a unfavorable upper-ocean thermal condition, which eventually inhibited the intensification of the following typhoon Bilis. Statistics show that 9% of the typhoons in western North Pacific are influenced by cold wakes produced by a proceeding typhoon. These overall results demonstrate that upper ocean response to a typhoon even after the passage is also important factor to be considered for an accurate intensity prediction of a following typhoon with similar track.

• Journal of Preventive Medicine and Public Health
• /
• 제53권1호
• /
• pp.37-44
• /
• 2020

Objectives: This study investigated the effects of comorbid sleep disorders (SD) on the incidence of cardiovascular complications among newly-diagnosed hypertension (HTN) patients. Methods: As study population, 124 057 newly-diagnosed essential HTN patients aged 30 or older, without cardiovascular complications at diagnosis with HTN, were selected from the National Health Insurance Service-National Sample Cohort. The incidence of cardiovascular complications was calculated, Cox proportional-hazards regression model was used to analyze the risk of complications, and the population attributable fraction (PAF) for cardiovascular complications of having comorbid SD at HTN diagnosis was calculated. Results: Over 10 years, 32 275 patients (26.0%) developed cardiovascular complications. In HTN patients with comorbid SD at diagnosis of HTN, the incidence of cardiovascular complications (78.3/1000 person-years; 95% confidence interval [CI], 75.8 to 80.9) was higher than in those without comorbid SD (58.6/1000 person-years; 95% CI, 57.9 to 59.3) and the risk of cardiovascular complications was 1.21 times higher (95% CI, 1.17 to 1.25), adjusting for age, gender, income, area of residence, and comorbid diabetes mellitus. The PAF of having comorbid SD at diagnosis of HTN for the incidence of cardiovascular complications was 2.07% (95% CI, 1.69 to 2.44). Conclusions: Newly-diagnosed essential HTN patients aged 30 or older who had comorbid SD at the time of their HTN diagnosis had a higher incidence of cardiovascular complications than those without comorbid SD. Age, gender, income, area of residence, and comorbid diabetes mellitus had a significant effect on the incidence of cardiovascular complications. Approximately 2% of cardiovascular complications were found to occur due to the presence of SD.

• 농촌계획
• /
• 제22권3호
• /
• pp.71-80
• /
• 2016

Korea is becoming a model country for the developing countries' agricultural and rural development. Drawing implications for Vietnam from Korea's experiences can help make development strategies and policies for other developing countries including North Korea as well as for Vietnam itself. Vietnam is facing an inefficiency in agricultural production and the gap between urban and rural growth has been widening. Farm sizes per household are small and farmlands are scattered. Diversification in rural industry is very restricted. To attack these problems, investment is urgently needed for rural infrastructure building as well as agricultural structure adjustment. In the process of rural development, there have been also encountered such problems as financial procurement, community's spontaneous participation, manpower development for adjusting to industrial structural change. Korea's experiences may be helpful for establishing rural development strategies and policies in Vietnam. Benchmark scopes can go beyond Saemaul Undong in 1970s. Korea's pre- and post-Saemaul Undong era as well as the Saemaul Undong era can be referred. In the wake of globalization, Vietnam has not only experienced compressed rapid economic growth but also encountered policy tasks to eradicate poverty, to realize self-reliance and income increase, and to lessen urban-rural development gap, at the same time. Korea's experiences show that priority needs to be put on the establishment of national and rural development strategies based on Vietnam-specific conditions, utilization of village's resources including community tradition and social capital, fund raising for rural development, farmland development and mobilization, production and living infrastructure building, technology transfer for farmers and vocational training for new job seekers.