• 천문학회보
• /
• 제35권2호
• /
• pp.75.2-75.2
• /
• 2010

We have studied the statistical properties of turbulence in molecular clouds identified in the Boston University - Five College Radio Astronomy Observatory (BU-FCRAO) Galactic Ring Survey (GRS). Toward this end, the probability density function (PDF) and velocity distribution were measured for about 50 molecular clouds. We found there exists a good correlation between the PDF width and the velocity dispersion for these molecular clouds. In order to investigate how general properties of astrophysical turbulence depends on the plasma parameters such as magnetic field strength and sonic Mach number, we performed three-dimensional MHD simulations. We then examined if the observed characteristics of interstellar turbulence are consistent with theoretical results from MHD simulations.

• 대한기계학회논문집
• /
• 제17권9호
• /
• pp.2294-2303
• /
• 1993

This paper investigated the characteristics of the turbulent incompressible flow past the orifice ring in an axi-symmetric pipe. The flow field was the turbulent pulsatile flow for Reynolds number of $2{\times}10^{5}$ which was defined based on the maximum velocity and the pipe diameter at the inlet, with oscillating frequence $(f_{os})=1/4{\pi}$ which was considered as quasi-steady state frequence. In the present investigation, finite analytic method was used to solve the governing equations in Navier Stokes and turbulent transport formulations. Particularly at high Reynolds number and low oscillation frequency, the effects of orifice ring on the flow were numerically investigated. The separation zone behind the orifice ring during the acceleration phase was found to be decreased. However, during the deceleration phase, the separation behind the orifice ring for pulsatile flow continuously grow to a size even larger than that in steady flow. The pressure drop in steady flow was found to be constant and always positive while for pulsatile flow the pressure drop change with time. And large turbulent kinetic energy, dissipation rate were found to be located in the region where the flow passes through the orifics ring. The maximum turbulent kinetic energy, generally occurs along the shear layer where the velocity gradient is large.

• 대한기계학회논문집B
• /
• 제27권8호
• /
• pp.1174-1181
• /
• 2003

The flow around a circular cylinder was controlled by attaching O-rings to reduce drag force acting on the cylinder. Four experimental models were tested in this study; one smooth cylinder of diameter D (D=60mm) and three cylinders fitted with O-rings of diameters d=0.0167 D, 0.05D and 0.067 D with pitches of PPD=2D, 1D, 0.5D and 0.25D. The drag force, mean velocity and turbulence Intensity profiles in the near wake behind the cylinders were measured for Reynolds numbers based on the cylinder diameter in the range of Re$_{D}$=7.8$\times$10$^3$~1.2$\times$10$^{5}$ . At Re$_{D}$=1.2$\times$10$^{5}$ , the cylinder fitted with O-rings of d=0.0167D in a pitch interval of 0.25D shows the maximum drag reduction of about 5.4%, compared that with the smooth cylinder. The drag reduction effect of O-rings of d=0.067D is not so high. For O-ring circulars, as the Reynolds number increases, the peak location of turbulence intensity shifts downstream and the peak magnitude is decreased. Flow field around the cylinders was visualized using a smoke-wire technique to see the flow structure qualitatively. The size of vortices and vortex formation region formed behind the O-ring cylinders are smaller, compared with the smooth cylinder.der.

• 한국추진공학회:학술대회논문집
• /
• 한국추진공학회 2006년도 제26회 춘계학술대회논문집
• /
• pp.171-176
• /
• 2006

연료 과농 가스발생기에 대한 실험적 연구가 수행되었다. 연소 성능에 주요한 영향을 미치는 분사기 헤드에는 1.5의 리세스 수를 갖는 내부 혼합형 이중 스월 분사기 37개가 배치되었다. 본 논문에서는 연소실 길이, 직경, 그리고 교반링 등의 연소실 설계 변경에 따른 실물형 가스발생기의 연소 안정성 특성에 대하여 살펴보았다. 연소시험 결과 공진 주파수가 고주파 영역에서 생성됨에 따라 동압의 세기는 전반적으로 감소하였으나, 연소 불안정을 완전히 억제시키지는 못하는 것으로 나타났다.

• Wind and Structures
• /
• 제28권1호
• /
• pp.1-17
• /
• 2019

Internal ring beams are primary components of new ring-stiffened cooling towers. In this study, numerical simulation of the internal flow field of a cooling tower with three ring beams under wind-thermal coupling effect is performed. The studied cooling tower is a 220-m super-large hyperbolic indirect natural draft cooling tower that is under construction in China and will be the World's highest cooling tower, the influence of peripheral radiators in operating cooling tower is also considered. Based on the simulation, the three-dimensional effect and distribution pattern of the wind loads on inner surface of the cooling tower is summarized, the average wind pressure distributions on the inner surface before and after the addition of the ring beams are analyzed, and the influence pattern of ring beams on the internal pressure coefficient value is derived. The action mechanisms behind the air flows inside the tower are compared. In addition, the effects of internal ring beams on temperature field characteristics, turbulence kinetic energy distribution, and wind resistance are analyzed. Finally, the internal pressure coefficients are suggested for ring-stiffened cooling towers under wind-thermal coupling effect. The study shows that the influence of internal stiffening ring beams on the internal pressure and flow of cooling towers should not be ignored, and the wind-thermal coupling effect should also be considered in the numerical simulation of cooling tower flow fields. The primary conclusions presented in this paper offer references for determining the internal suction of such ring-stiffened cooling towers.

• 한국항공우주학회지
• /
• 제33권6호
• /
• pp.64-70
• /
• 2005

본 논문에서는 충돌형 분사기를 장착한 연료 과잉 가스발생기에서 수행한 설계점 및 탈설계점 연소시험의 전반적인 결과에 대하여 논하였다. 가스발생기는 충돌형 분사기와 추진제 공급 메니폴드로 구성된 분사기 헤드, 물냉각 채널을 가진 연소실, 혼합을 증가시키는 turbulence ring, 온도 및 압력을 측정하는 링, 그리고 노즐로 구성되었다. 여러 운영조건에서 연소시험은 성공적이었으며 가스발생기 손상은 발생하지 않았다. 체류시간 4～6msec 정도에는 출구온도변화가 거의 없었지만 압력변동에 따라 출구온도는 변하였다. 측정되는 압력, 유량 그리고 노즐목 크기로 계산한 연소효율는 출구에서 측정한 온도의 제곱근에 비례하는 관계식을 저 혼합비 가스발생기에서도 갖고 있었다. 가스발생기의 O/F 비 변화에 가장 민감하게 출구온도가 변화하였으며 이에 대한 관계식을 도출하여 향후 설계 기초 자료로 활용되게 하였다.

• 대한기계학회:학술대회논문집
• /
• 대한기계학회 2003년도 춘계학술대회
• /
• pp.2089-2094
• /
• 2003

The flow around a circular cylinder was controlled by attaching O-rings to reduce drag force acting on the cylinder. Four experimental models were tested in this study; one smooth cylinder of diameter D (D=60mm) and three cylinders fitted with O-rings of diameters d=0.0167D, 0.05D and 0.067D with pitches of PPD=1D, 0.5D and 0.25D. The drag force, mean velocity and turbulent intensity profiles in the near wake behind the cylinders were measured for Reynolds numbers based on the cylinder diameter in the range of $Re_D=7.8{\times}10^3{\sim}1.2{\times}10^5$. At $Re_D=1.2{\times}10^5$, the cylinder fitted with O-rings of d=0.0167D in a pitch interval of 0.25D shows the maximum drag reduction of about 5.4%, compared with the smooth cylinder. The drag reduction effect of O-rings of d=0.067D is not so high. For O-ring circulars, as the Reynolds number increases, the peak location of turbulence intensity shifts downstream and the peak magnitude is decreased. Flow field around the cylinders was visualized using a smoke-wire technique to see the flow structure qualitatively. The size of vortices and vortex formation region formed behind the O-ring cylinders are smaller, compared with the smooth cylinder.

• 한국추진공학회지
• /
• 제11권1호
• /
• pp.11-17
• /
• 2007

연소실 설계에 따른 실물형 가스발생기의 연소 안정성 특성에 대한 실험적 연구가 수행되었다. 연소성능에 주요한 영향을 미치는 분사기 헤드에는 1.5의 리세스 수를 갖는 내부 혼합형 이중 스월 분사기 37개가 배치되었다. 본 논문에서는 연소실 길이, 직경, 그리고 교반링 등 연소실 설계 변경에 따른 연소 안정성 특성에 대하여 살펴보았다. 연소시험 결과 연소실의 유효 길이가 줄어듦에 따라 불안정 주파수는 고주파 영역에서 발생되며, 압력의 강도는 전반적으로 줄어들어 유해하지 않은 수준으로 감소가 가능하였다. 하지만 축방향 공진주파수에 해당하는 압력 섭동을 완전히 제거시키지는 못하는 것으로 판단된다.

• Journal of Chest Surgery
• /
• 제33권1호
• /
• pp.45-50
• /
• 2000

Background: The CarboMedics prosthetic heart valve was produced in an attempt to improve the existing valve designs and was especially concerned with easily the implantation and further reduction of turbulence. Precise positioning of the valve in situ was achieved by the abilityof the valve to rotate relative to the sewing ring. Improved monitoring is possible due to increased radiopacity and the dacron sewing ring is coated with carbon to reduce pannus overgrowth. The leaflets have an opening angle of 78 degrees that apparently allows a rapid synchronous closure The aim of this study was to analyze the clinical performance of the CarboMedics valve prostheses(45 mitral 13 aortic and 7 double aortic-mitral valve replacement) were implanted in 65 patients(mean age 48.75$\pm$9.74 years) Result: The operative mortality was 3.1%(2/65) causes of death were low cardiac output syndrome. Total follow up was 1831 patient-months and mean follow up was 29.06$\pm$10.97 months/patient. No structural failure hemorrhage valve thrombosis and late death have been observed. Embolism occurred at a rate of 0.65%/Patient-year. Actuarial survival and thrombo-mbolism free rate at 36 months were 96.9% and 98.4% respectively. Consclusions: The CarboMedics valve stands for low valve related complicatons.

• 한국추진공학회지
• /
• 제14권3호
• /
• pp.9-15
• /
• 2010

과팽창 로켓노즐에서 발생하는 충격파 박리패턴의 천이 유동장을 예측하기 위해 축대칭 수치해석적연구를 수행하였다. 비정상, 압축성 N-S 방정식에 k-$\omega$ SST 난류모델을 적용하여, 유한 체적법으로 계산하였다. 종래의 실험적 연구 결과와 비교하였으며, 계산된 결과와 정성적으로 잘 일치하였다. 본 연구의 결과로부터 RSS에서 FSS로 천이할 때 가장 큰 횡력이 발생하며, 이는 비점성 제트 코어 영역에서 발생하는 Vortex ring의 발달로 기인됨을 예측하였다. 또한 엔진 시동과정과 정지과정에서 발생하는 히스테리시스 현상을 잘 모사하였다.