• 한국마린엔지니어링학회:학술대회논문집
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• 한국마린엔지니어링학회 2006년도 전기학술대회논문집
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• pp.59-60
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• 2006

Some useful flow regime data are obtained from the experiments using the flight producing hyper-gravity(2g) conditions and on ground(1g) with the identical flow conditions. The flow regime data obtained at 1g and 2g conditions are compared with new dimensionless flow regime map using Fr, Bo and We number related with gravity, surface tension and inertia force.

• 한국전산유체공학회지
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• 제20권3호
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• pp.54-61
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• 2015

In the present study, a flow solver using a kinetic BGK scheme was developed for the compressible Navier-Stokes equation. The kinetic BGK scheme was used to simulate flow field from the continuum up to the transitional regime, because the kinetic BGK scheme can take into account the statistical properties of the gas particles in a non-equilibrium state. Various numerical simulations were conducted by the present flow solver. The laminar flow around flat plate and the hypersonic flow around hollow cylinder of flare shape in the continuum regime were numerically simulated. The numerical results showed that the flow solver using the kinetic BGK scheme can obtain accurate and robust numerical solutions. Also, the present flow solver was applied to the hypersonic flow problems around circular cylinder in the transitional regime and the results were validated against available numerical results of other researchers. It was found that the kinetic BGK scheme can similarly predict a tendency of the flow variables in the transitional regime.

• 대한기계학회논문집B
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• 제24권8호
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• pp.1038-1045
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• 2000

An experimental study was carried out to delineate the flow characteristics in a closed countescurrent two-phase thermo syphon with concentric tubes. This is to be installed in the HANARO research reactor as a part of a Cold Neutron Source(CNS). In the present investigation, experiments ata room temperature with Freon-II3 as a moderator were performed. Results show that, based on the magnitude of pressure fluctuation, the flow regimes could be divided into 4 distinct ones in the ($V_f,\;Q_i$) plane, where $V_f$ represents the volume of the charged liquid and $Q_i$ the heat load: a stable flow regime, an oscillatory flow regime, a restablized flow regime and a dryout flow regime. For $V_f$>2.5l, the flow is stable at low $Q_i$. However, as $Q_i$ increases, the flow becomes oscillatory and finally restablizes As $V_f$ increases, the oscillation amplitude decreases, reaching to the restablized flow region at low $Q_i$, and the liquid level in the moderator cell remains high. In the oscillatory flow regimes, for a fixed VI; the oscillating period of time varies with $Q_i$, having a minimum value at a certain value of $Q_i$. The heat load, where the oscillating period of time is minimum, decreases as $V_f$ increases.

• 대한기계학회논문집
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• 제18권12호
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• pp.3297-3304
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• 1994

Experimental investigation have been made to study the double-diffusive nature of convection of an initially stratified salt-water solution heated from below in a cylindrical cavity. The objective is to examine the process of mixed-layer formation, the flow phenomena, the heat transfer characteristics, and temperature and concentration distribution according to the changes in the effective Rayleigh number based on the reference height which represents the relation of temperature and concentration gradient. The types of initially formed flow pattern are categorized in three regimes depending on the effective Rayleigh number ; stagnant flow regime, single mixed-layer flow regime and successively formed multiple mixed-layer flow regime. The temperature and concentration profiles are both uniform in each layer due to convective mixing in the layered flow regime, but both linear in stagnant flow regime and single mixed-layer flow regime. At the interface between adjacent layers, the temperature changes smoothly but the concentration changes rapidly. The layers expand by diffusion of concentration through the interface along with its random fluctuation.

• 대한기계학회논문집
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• 제19권7호
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• pp.1731-1740
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• 1995

Experimental investigations have been made to study the double-diffusive nature of convection of an initially stratified salt-water solution due to heating from below in a rotating cylindrical cavity. The objective is to examine the flow phenomena and the heat transfer characteristics according to the changes in temperature gradient, concentration gradient and rotating velocity of cavity. Thermal and solutal boundary conditions at side wall are adiabatic and impermeable, respectively. The top and bottom plate are maintained each at constant temperature and concentration. The cavity is put into a state of solid body rotation. Like the stationary case, the types of initially-formed flow pattern are classified into three regimes depending on the effective Rayleigh number and Taylor number; stagnant flow regime, single mixed-layer flow regime and successively formed multi-mixed layer flow regime. At the same effective Rayleigh number, the number of initially-formed mixed layer and its growth rate decrease as the effect of rotation increases. The temperature and concentration profiles are both uniform in each layer due to convective mixing in the layered-flow regime, but look both liner in stagnant flow regime and single mixed-layer flow regime. At the interface between adjacent layers, the temperature changes smoothly but the concentration changes rapidly.

• Nuclear Engineering and Technology
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• 제54권2호
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• pp.546-553
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• 2022

Narrow rectangular channels are employed in nuclear research reactors that use plate-type nuclear fuels, high heat-flux compact heat exchangers, and high-performance micro-electronics cooling systems. Two-phase flow in narrow rectangular channels is important, and it needs to be better understood because it is considerably different than that in round tubes. In this study, mechanistic models were developed for the flow regime transition criteria for various flow regimes in co-current air-water two-phase flow for vertical downward flow inside a narrow rectangular channel. The newly developed criteria were compared to a flow regime map of downward air-water two-phase flow inside a narrow rectangular channel with a 2.35-mm gap width under ambient temperature and pressure conditions. Overall, the proposed model showed good agreement with the experimental data.

• Wind and Structures
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• 제11권3호
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• pp.221-240
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• 2008

The spanwise flow structure around a rigid smooth circular cylinder model in cross-flow has been investigated based on the experimental data obtained from a series of wind tunnel tests. Surface pressures were collected at five spanwise locations along the cylinder over a Reynolds number range of $1.14{\times}15^5$ to $5.85{\times}10^5$, which covered sub-critical, single-bubble and two-bubble regimes in the critical range. Separation angles were deduced from curve fitted to the surface pressure data. In addition, spanwise correlations and power spectra analyses were employed to study the spatial structure of flow. Results at different spanwise locations show that the transition into single-bubble and two-bubble regimes could occur at marginally different Reynolds numbers which expresses the presence of overlap regions in between the single-bubble regime and its former and later regimes. This indicates the existence of three-dimensional flow around the circular cylinder in cross-flow, which is also supported by the observed cell-like surface pressure patterns. Relatively strong spanwise correlation of the flow characteristics is observed before each transition within the critical regime, or formation of first and second separation-bubbles. It is also noted that these organized flow structures might lead to greater overall aerodynamic forces on a circular cylinder in cross-flow within the critical Reynolds number regime.

• 한국습지학회지
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• 제21권spc호
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• pp.1-8
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• 2019

하천에서 물이용을 위한 이수사업은 피할 수 없는 하천유황의 변화를 가져왔고, 이로 인하여 하천 생태계의 건전성을 훼손하고 있다. 우리나라의 경우 1990년대 들어 변화된 하천환경을 자연형 하천으로 복원하고자 하는 노력이 본격화 되었다. 하천 복원사업의 주된 목적은 수로를 복원하고, 오염된 하천의 개선, 생태계를 위한 서식처 설치 및 인간의 친수활동을 위한 공간 확보였으며, 상대적으로 생태계보전에 가장 중요한 요소인 하천에 흐르는 유황의 복원에 대한 관심은 적었다. 하천유황은 생태계의 모든 측면에서 큰 영향을 주기 때문에 유황과 생태계의 관계 규명과 하천에 유지하여야 할 목표유황의 설정 및 하천 유황복원 방법을 정립할 필요가 있다. 본 연구에서는 국내에서 아직 정립되지 않은 유황과 생태계의 관계를 환경유량의 개념을 통하여 소개하고 자연유황의 역할과 Richter et al.(1997)이 제안한 유량변동분석법(Range of Variability Approach, RVA)을 이용한 유황 평가 및 목표유량 설정 방법을 한강 유역의 사례를 통하여 고찰하였다.

• 대한설비공학회지:설비저널
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• 제17권4호
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• pp.342-349
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• 1988

The characteristics of flow pattern transitions in a horizontal cocurrent gas-liquid flow have been investigated by means of a statistical analysis of instantaneous pressure drop curves at an orifice. The dimensionless intensity of pressure drop fluctuation shows a sudden change during the course of flow transitions, indicating that it may be a good measure to identify the flow regime transitions. The probability density function of the curves feature a unique pattern depending upon the flow regimes and the statistical properties of the PDF also have particular ranges for each flow regime. In conclusion, the statistical analysis of instantaneous pressure drops may be a powerful tool for predicting the flow regime transitions.

• International Journal of Air-Conditioning and Refrigeration
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• 제15권1호
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• pp.1-9
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• 2007

This study concerns the characteristics of helical flow in a concentric and eccentric annulus with a diameter ratio of 0.52 and 0.9, whose outer cylinders are stationary and inner ones are rotating. Pressure losses and skin friction coefficients have been measured for fully developed flows of water and 0.2% aqueous of sodium carboxymethyl cellulose (CMC), respectively, when the inner cylinder rotates at the speed of 0-500 rpm. The effect of rotation on the skin friction is significantly dependent on the flow regime. In all flow regimes, the skin friction coefficient is increased by the inner cylinder rotation. The change of skin friction coefficient corresponding to the variation of rotating speed is large for the laminar flow regime, whereas it becomes smaller as Re increases for the transitional flow regime and, then, it gradually approach to zero for the turbulent flow regime.