• 대한기계학회논문집B
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• 제24권6호
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• pp.822-833
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• 2000

This experimental study concerns the characteristics of a transitional flow in a concentric annulus with a diameter ratio of 0.52, whose outer cylinder is stationary and inner one rotating. The pressure drops and skin-friction coefficients have been measured for the fully developed flow of water and that of glycerine-water solution (44%) at a inner cylinder rotational speed of $0{\sim}600$ rpm, respectively. The transitional flow has been examined by the measurement of pressure drops and the visualization of flow field, to reveal the relation of the Reynolds and Rossby numbers with the skin-friction coefficients and to understand the flow instability mechanism. The present results show that the skin-friction coefficients have the significant relation with the Rossby numbers, only for laminar regime. The occurrence of transition has been checked by the gradient changes of pressure drops and skin-friction coefficients with respect to the Reynolds numbers. The increasing rate of skin-friction coefficient due to the rotation is uniform for laminar flow regime, whereas it is suddenly reduced for transitional flow regime and, then, it is gradually declined for turbulent flow regime. Consequently, the critical (axial-flow) Reynolds number decreases as the rotational speed increases. Thus, the rotation of inner cylinder promotes the early occurrence of transition due to the excitation of taylor vortices.

• Nuclear Engineering and Technology
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• 제51권1호
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• pp.73-83
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• 2019

It is important to clarify the mechanisms of molten-fuel-pool sloshing behavior that might be encountered during a core disruptive accident of sodium-cooled fast reactors. In this study, motivated by acquiring some evidence for understanding the characteristics of this behavior at more realistic conditions, a number of experiments are newly performed by injecting nitrogen gas into a water pool with the accumulation of solid particles. To achieve comprehensive understanding, various parameters including particle bed height, particle size, density, shape, gas pressure along with the gas-injection duration, were employed. It is found that due to the different interaction mechanisms between solid particles and the gas bubble injected, three kinds of regimes, termed respectively as the bubble-impulsion dominant regime, the transitional regime and the bed-inertia dominant regime, could be identified. The performed analyses also suggest that under present conditions, all our experimental parameters employed can have noticeable impact on the regime transition and resultant sloshing intensity (e.g. maximum elevation of water level at pool peripheries). Knowledge and fundamental data from this work will be used for the future verifications of fast reactor severe accident codes in China.

• 해양환경안전학회지
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• 제22권1호
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• pp.145-151
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• 2016

수온과 기후 지수 자료를 이용하여 한국주변해 상층부의 수온변동과 북태평양 기후체제와의 관계를 분석하였다. 1970, 1980 그리고 1990년대 후반에 발생한 기후체제전환은 해양생태계의 구조 변화에 중요한 영향을 끼쳤다. 북서태평양 대륙주변부에 위치한 우리나라 주변해역 또한, 10년 이상의 장주기적인 변화의 영향을 받지만, 해역별 반응은 다르다. 동해, 서해, 남해 상층부(10 m)의 경우 1988년의 기후체제전환은 3개 해역의 상층부 수온변화에서 동시에 나타난다. 반면, 1998년은 동해남부해역과 서해, 1976년의 기후체제전환은 전 해역의 겨울철 수온변화에서만 그 영향이 나타난다. 1998년 기후체제전환 이후, 서해, 동중국해, 동해 남부의 수온은 점차 감소하지만, 동해 북부해역은 1988년 기후체제전환 이후 증가하는 형태이다.

• 한국수자원학회:학술대회논문집
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• 한국수자원학회 2015년도 학술발표회
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• pp.106-106
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• 2015

The information of flow regimes continues to be norm in water resource and watershed management, in that stream flow regime is a crucial factor influencing water quality, geomorphology, and the community structure of stream biota. The objectives of this study were to estimate Korean stream flows from landscape variables, classify stream flow gages using hydraulic characteristics, and then apply these methods to ungaged biological monitoring sites for effective ecological assessment. Here I used a linear modeling approach (MLR, PCA, and PCR) to describe and predict seasonal flow statistics from landscape variables. MLR models were successfully built for a range of exceedance discharges and time frames (annual, January, May, July, and October), and these models explained a high degree of the observed variation with r squares ranging from 0.555 (Q95 in January) to 0.899 (Q05 in July). In validation testing, predicted and observed exceedance discharges were all significantly correlated (p<0.01) and for most models no significant difference was found between predicted and observed values (Paired samples T-test; p>0.05). I classified Korean stream flow regimes with respect to hydraulic and hydrologic regime into four categories: flashier and higher-powered (F-HP), flashier and lower-powered (F-LP), more stable and higher-powered (S-HP), and more stable and lower-powered (S-LP). These four categories of Korean streams were related to with the characteristics of environmental variables, such as catchment size, site slope, stream order, and land use patterns. I then applied the models at 684 ungaged biological sampling sites used in the National Aquatic Ecological Monitoring Program in order to classify them with respect to basic hydrologic characteristics and similarity to the government's array of hydrologic gauging stations. Flashier-lower powered sites appeared to be relatively over-represented and more stable-higher powered sites under-represented in the bioassessment data sets.

• 한국물환경학회지
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• 제26권2호
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• pp.208-214
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• 2010

This study aimed at the introduction of desktop method for assessment of environmental flows developed by International Water Management Institute (IWMI) recently and its application to Geum river basin. This scheme simulated the influence on aquatic ecosystem caused by watershed development and in turn the decrease of water quantity keeping the river's own flow regime. It was found to be as very effective method although it had simple structure. Flow duration curves for different environmental classes at Sutong and Gongjoo sites were estimated according to the natural conditional scenario of Geum river basin and the results were relatively compared well with the previous studies. The behaviors of monthly average runoff time series of both sites showed the level of A class. The results of this study would provide the fundamental data to establish the future plans of monitoring or management for aquatic ecosystem of Geum river basin.

• 대한설비공학회지:설비저널
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• 제14권2호
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• pp.87-97
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• 1985

Experimental measurements of the heat flux to a upward impinging water jet on high heated test surface were obtained in the transition and film boiling regimes. Test variables were nozzle outlet velocity, subcooled water temperature and height of supplementary water. Boiling curve of this investigation is similar to a pool boiling curve, but it has one or two cap-shaped peaks in the transition regime. In the film boiling regime, the heat transfer rates are increased along with the increment of nozzle outlet velocity and subcooled temperature. There is optimum height of supplementary water for the augmentation of heat transfer Generalized correlations of boiling heat transfer are presented for maximum heat flux, minimum heat flux and $q_c$ at each supplementary height.

• 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.

• Ocean and Polar Research
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• 제27권3호
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• pp.277-288
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• 2005

Spatial distribution and vertical structures of water masses around the Antarctic continental margin are described using synthesized hydrographic data. Antarctic Surface Water (AASW) over the shelf regime is distinguished from underlying other water masses by the cut-off salinity, varying from approximately 34.35 to 34.45 around Antarctica. Shelf water, characterized by salinity greater than the cut-off salinity and potential temperature less than $-17^{\circ}C$, is observed on the Ross Sea, off George V Land, off Wilkes Land, the Amery Basin, and the Weddell Sea, but in some shelves AASW occupies the entire shelf. Lower Circumpolar Deep Water is present everywhere around the Antarctic oceanic regime and in some places it mixes with Shelf Water, producing Antarctic Slope Front Water (ASFW). ASFW, characterized by potential temperature less than about $0^{\circ}C$ and greater than $-17^{\circ}C$, and salinity greater than the cut-off salinity, is found everywhere around Antarctica except in the Bellingshausen-Amundsen sector. The presence of different water masses over the Antarctic shelves and shelf edges produces mainly three types of water mass stratifications: no significant meridional property gradient in the Bellingshausen and Amundsen Seas, single property gradient where ASFW presents, and a V-shaped front where Shelf Water exists.

• 한국수자원학회논문집
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• 제54권11호
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• pp.985-998
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• 2021

일반적으로 상류의 댐은 하류 유황을 자연 상태에서 발전방류 조건으로 심각하게 변화시킨다. 본 연구에서는 물리서식처모의를 통하여 국내 조절하천에서 자연유황 패턴이 하류의 어류 서식처에 미치는 영향을 조사하였다. 연구 대상하도는 금강의 용담댐 하류 13.4 km 구간으로 설정하였다. 현장조사 결과, 대상하도에서 피라미, 쉬리, 그리고 끄리가 우점하고 있는 것으로 나타났으며, 이들이 전체의 70%를 차지하고 있는 것으로 파악되었다. 이중 끄리는 금강에 서식하는 토착어종이다. 이들 3종을 물리서식처모의를 위한 대상어종으로 선정하였다. 수리해석과 서식처모의를 위하여 2차원 천수방정식에 기반한 Nays2D 모형과 HIS 모형을 각각 사용하였다. 자연 유황에 따른 영향을 검토하기 위하여, 본 연구에서는 댐 유입 유량과 발전유량을 이용하였다. 물리서식처모의 결과, 자연유황 조건에서 대상하도의 복합서식처지수가 크게 증가하는 것으로 나타났다. 그리고 BBA 방법을 이용하여 대상하도에 자연유황에 대한 댐 운영 시나리오를 제시하였다. 시나리오 1을 위하여 유입유량의 양과 지속기간을 고려하는 수문학적 방법을 이용하였고, 시나리오 2는 유입유량을 월별로 평균하여 구축하였다. 시나리오 1과 시나리오 2를 통해서 구현된 자연유황 조건이 발전방류에 비해 가중가용면적을 크게 증가시키는 것으로 나타났다. 결과적으로 댐 운영을 자연유황 조건으로 변환시켜 방류하였을 때 하류의 어류 서식처를 개선하는데 도움이 되는 것으로 확인되었다.