• The Bulletin of The Korean Astronomical Society
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• v.35 no.2
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• pp.75.2-75.2
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• 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.

• Transactions of the Korean Society of Mechanical Engineers
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• v.17 no.9
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• pp.2294-2303
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• 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.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.8
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• pp.1174-1181
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• 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.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2006.05a
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• pp.171-176
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• 2006

An experimental study on a fuel-rich gas generator was carried out. Thirty seven double-swirl injectors with recess number of 1.5 were distributed and installed in the injector head, which significantly influences the combustion performance. In the paper, the characteristics of combustion stability are inspected by the parametric varations such as changing length and diameter of a combustion chamber and installing a turbulence ring. The experimental results show that as a resonant frequency took place in a high region, the amplitude of the dynamic pressure generally diminished, however, the combustion instability could not be suppressed perfectly.

• Wind and Structures
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• v.28 no.1
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• pp.1-17
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• 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.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.33 no.6
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• pp.64-70
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• 2005

The overall results of hot firing tests of fuel-rich gas generator with impinging injector at design and off-design points are described. The gas generator consists of an injector head with impinging injector, a water cooled combustor wall, a turbulence ring to enhance mixing, an instrument ring measuring temperature and pressure and a nozzle. The combustion tests were successfully performed without damage of gas generator. Test results show that the outlet temperature is not dependent on residence time of hot gas within 4～6msec but dependent on chamber pressure. The relation between outlet temperature and combustion efficiency resulting from measured pressure, mass flow rate and area of nozzle throat is shown. The overall O/F ratio is the critical parameter to determine the outlet temperature and the linear correlation between two parameters is established.

• Proceedings of the KSME Conference
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• 2003.04a
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• pp.2089-2094
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• 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.

• Journal of the Korean Society of Propulsion Engineers
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• v.11 no.1
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• pp.11-17
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• 2007

Effects of combustion chamber design on combustion stability characteristics of a full-scale gas generator were studied experimentally. Thirty seven double-swirl injectors with recess number of 1.5 were distributed in the injector head, which significantly influences combustion performance. The characteristics of combustion stability were inspected by the parametric variations such as changing length and diameter of the combustion chamber and installing a turbulence ring. The experimental result shows that as the effective length of the combustion chamber decreased, an instability frequency took place in a high-frequency region, and the amplitude of the dynamic pressure generally diminished and could be reduced to the unharmful level. However, the dynamic pressure fluctuation in the region of longitudinal resonant frequency could not be suppressed perfectly.

• Journal of Chest Surgery
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• v.33 no.1
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• pp.45-50
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• 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.

• Journal of the Korean Society of Propulsion Engineers
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• v.14 no.3
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• pp.9-15
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• 2010

Numerical investigation was carried out on axisymmetric over-expanded rocket nozzle to predict flow fields of transitional shock separation patterns. The unsteady, compressible N-S equations with k-$\omega$ SST for turbulence model closure were solved using a fully implicit finite volume scheme. Computed results were in good agreement with previous experimental works. It was found that strong side-loads were generated during the transition of RSS to FSS due to the development of a vortex ring in the inviscid jet core region. Hysteresis phenomenon exhibited by the shock-separation patterns was also found during the start-up and shut-down processes.