• 한국해양공학회:학술대회논문집
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• 한국해양공학회 2006년 창립20주년기념 정기학술대회 및 국제워크샵
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• pp.290-293
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• 2006

From the hydraulic experiment, it was concluded that upwelling could be enhanced when the relative structure height (the ratio of structure height to water depth) was 0.3 and stratification parameter was 3.0. In addition, the optimum size of rubbers was determined that the effect of the mean horizontal length of block was affected incident velocity than size of block. In the numerical experiment, the relation between the shape of rubber and stratification parameter was verified, ana the hydraulic characteristics of 3-D flow field around the artificial structures were investigated. Phenomena of flow field around the artificial upwelling structures corresponded with the results of hydraulic experiment. The position with maximum velocity in artificial upwelling structure was the center of top of its front side and the slip stream occurred at the inside and behind-bottom of artificial upwelling structures. The velocity of slip stream and early amplitude of velocity were higher in the inside than the behind-bottom.

• Nuclear Engineering and Technology
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• 제52권11호
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• pp.2491-2498
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• 2020

The paper concentrates on an experimental study of the pressure drop in double-layered packed beds formed by glass spheres, having the configuration of horizontal and vertical stratification. Both single-phase and two-phase flow tests are performed. The pressure drop during the test is recorded and the measured data are compared with those of homogeneous beds consisting of mono-size particles. The results show that for the horizontally stratified bed with fine particles atop coarse particles, the pressure drop in top layer is found higher than those of homogenous bed consisting of the same smaller size particles, while the measured pressure drop of bottom part is similar with those of similar homogenous bed. But for the homologous bed with upside-down structure, the stratification has little or no effect on the pressure drop of the horizontally stratified bed, and the pressure drop of each layer is almost same as that of homogeneous bed packed with corresponding spheres. Additionally, in vertically stratified bed, the pressure drops on the left and right side is almost equal and between those in homogeneous beds. It is speculated that vertically stratified structure may lead to lateral flow which redistributes the flow rate in different parts of packed bed.

• 해양환경안전학회지
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• 제14권1호
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• pp.21-27
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• 2008

삼각형상 구조물은 바다에서 풍요로운 어장을 만들기 위해 용승류를 발생시키기 위한 인공용승구조물의 기본적인 형태로 이용되고 있다. 인공용승류는 많은 양의 영양염류를 포함하고 있는 저층의 바닷물을 해저로부터 표층으로 끌어 올리는 작용을 한다. 이 연구의 목적은 여러 가지 성층계수에 따른 삼각형상 수중구조물 주위의 유동특성을 규명하는 것이다. 회류수조 내에서 삼각형상 구조물 모델을 대상으로 유동특성을 조사하기 위하여 유동가시화 방법을 이용한 실험적 연구를 수행하였고, 입자영상유속계(PIV)를 이용하여 수중구조물 주위의 유동장을 계측하였다. 실험결과 구조물 후상부 영역에서의 용승효과는 수심이 구조물 높이의 2배이고, 성층계수가 약 3.0일 때 가장 좋은 결과가 나타났다. 이러한 정량적인 데이터는 인공용승구조물의 기능적 효율을 결정하는 데에 유용하게 이용될 수 있을 것이다.

• 한국자동차공학회논문집
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• 제16권5호
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• pp.157-163
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• 2008

The HCCI combustion mode poses its own set of narrow engine operating by knocking. In order to solve this, inhomogeneity method of mixture and temperature is suggested. The purpose of this research is to get fundamental knowledge about the effect of thermal stratification on HCCI combustion of PRF -Air mixture. The temperature stratification is made by buoyancy effect in combustion chamber of RCM. The analysis items are pressure, temperature of in-cylinder gas and combustion duration. In addition, the structure of flames using the two dimensional chemiluminescence's images by a framing camera are analyzed. Under stratification, the LTR starting time and the HTR starting time are advanced than that of homogeneous. Further, the LTR period of homogeneous conditions became shorter than that of the stratified conditions. With the case of homogeneous condition, the luminosity duration becomes shorter than the case of stratified condition. Additionally, under stratified condition, the brightest luminosity intensity is delayed longer than at homogeneous condition.

• 환경영향평가
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• 제30권5호
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• pp.271-296
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• 2021

댐 저수지 수온성층은 수직혼합을 억제하여 저층의 빈산소층 형성과 퇴적물 영양염류 용출을 일으키는 원인이므로 미래 기후변화에 따른 저수지 성층구조의 변화는 수질 및 수생태 관리 측면에서 매우 중요하다. 본 연구의 목적은 대청댐 저수지를 대상으로 고빈도 자료기반의 통계적 저수지 유입 수온 예측 모델을 개발하고, RCP(Representative Concentration Pathways) 기후변화 시나리오를 고려한 미래 유입 수온변화와 대청호 성층구조의 변화를 예측하는 데 있다. 대청호 유입 수온 예측을 위해 개발한 Random Forest 회귀 예측모델(NSE 0.97, RMSE 1.86℃, MAPE 9.45%)은 실측 수온의 통계량과 변동성을 적절히 재현하였다. 지역 기후 모델(HadGEM3-RA)로 예측된 RCP 시나리오별 미래 기상자료를 Random Forest 모델에 입력하여 유입 수온을 예측하고 3차원 저수지 수리 모델을 이용하여 기후변화에 따른 대청호의 미래(2018~2037, 2038~2057, 2058~2077, 2078~2097) 수온성층 구조 변화를 예측하였다. 예측 결과, 미래 기후 시나리오별로 대기 온도와 저수지 유입 수온의 증가속도는 각각 0.14~0.48℃/10year와 0.21~0.43℃/10year의 범위로써 지속적으로 증가하였다. 계절별 분석 결과, RCP 2.6 시나리오의 봄과 겨울철을 제외한 모든 시나리오에서 유입 수온은 증가 경향이 통계적으로 유의하였으며, 탄소저감 노력이 약한 기후 시나리오로 갈수록 수온의 증가속도가 빨랐다. 저수지 표층 수온의 증가속도는 0.04~0.38℃/10year 범위였으며, 모든 시나리오에서 성층화 기간이 점진적으로 증가되었다. 특히 RCP 8.5 시나리오 적용 시 성층일수는 약 24일 증가하는 것으로 전망되었다. 연구 결과는 기후변화가 호소의 성층강도를 강화하고 성층형성 기간을 장기화한다는 선행연구 결과와 일치하며, 수온성층의 장기화는 저층 빈산소층 확대, 퇴적물-수체간 영양염류 용출량 증가, 수체 내 조류 우점종의 변화 등 수생태계 변화를 유발할 수 있음을 시사한다.

• 한국전산유체공학회지
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• 제17권4호
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• pp.41-48
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• 2012

A numerical analysis of thermal stratification in the upper plenum of the MONJU fast breeder reactor was performed. Calculations were performed for a 1/6 simplified model of the MONJU reactor using the commercial code, CFX-13. To better resolve the geometrically complex upper core structure of the MONJU reactor, the porous media approach was adopted for the simulation. First, a steady state solution was obtained and the transient solutions were then obtained for the turbine trip test conducted in December 1995. The time dependent inlet conditions for the mass flow rate and temperature were provided by JAEA. Good agreement with the experimental data was observed for steady state solution. The numerical solution of the transient analysis shows the formation of thermal stratification within the upper plenum of the reactor vessel during the turbine trip test. The temporal variations of temperature were predicted accurately by the present method in the initial rapid coastdown period (~300 seconds). However, transient numerical solutions show a faster thermal mixing than that observed in the experiment after the initial coastdown period. A nearly homogenization of the temperature field in the upper plenum is predicted after about 900 seconds, which is a much shorter-term thermal stratification than the experimental data indicates. This discrepancy is due to the shortcoming of the turbulence models available in the CFX-13 code for a natural convection flow with thermal stratification.

• 한국환경과학회지
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• 제7권2호
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• pp.223-232
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• 1998

득량만의 조석주기에 따른 성층특성을 연구하기 위하여 우선 30여개 관측점에서 관측된 수온, 염분, 밀도, 해류, 열수지, 바람, 밀도류 자료를 조사하였다. 이 관측자료를 토대로 성층현상을 규명할 수 있는 해석적 모델을 개발하였으며 이 해석적모델을 이용하여 얻어진 연구결과에 따르면, 득량만의 성층을 이루는 요인은 조석, 태양열, 밀도,류인 것으로 나타났으며, 성층을 파과할 수 있는 조석에너지의 혼합에너지의 에너지 기여율 $\varepsilon$ 은 약 0.020 - 0.021정도 되는 것으로 나타났다.

• 대한기계학회논문집
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• 제18권9호
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• pp.2454-2462
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• 1994

The effects of thermal stratification on the flow of a stratified fluid past a heated circular cylinder were examined in a wind tunnel. Turbulent intensities, rms values of temperature and turbulent convective heat flux distributions in the heated cylinder wake with and without thermal stratification were measured by using a hot-wire and cold-wire combination probe. A phase averaging method was also used to estimated coherent motion in the near wake. It is found that the vertical turbulent motion in the stably stratified flow case dissipates faster than that of the neutral case, i.e., vertical growth of vortical structure is suppressed under the strongly stratified condition. The coherent motion of temperature makes a large contribution like velocity coherent motion. However, the coherent motions of temperature fluctuation become very different with the change of experimental conditions, though the velocity coherent motions are quite similar in all experimental conditions.

• Journal of Ship and Ocean Technology
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• 제9권4호
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• pp.1-10
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• 2005

The effect of buoyancy orientation on turbulent channel flow has been investigated using DNS (direct numerical simulation). Grashof number is kept at 9.6 $\times 10^{5}$ while changing the orientation of the buoyancy vector to be parallel or perpendicular to the channel walls. Four study cases can be distinguished during this research namely; streamwise, wall-normal unstable stratification, wall-normal stable stratification and spanwise oriented buoyancy. The driving mean pressure gradient used in all cases is adjusted to keep mass flow rate constant while friction Reynolds number is around 150. At this Grashof number, the skin friction shows decrement in the unstable and stable stratification and increment in the other two cases. Analyses of the changes of flow structure for the four cases are presented highlighting on the mean quantities and second order statistics.

• Ocean and Polar Research
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• 제30권3호
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• pp.261-275
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• 2008

The profile of a fixed site at station M ($34.77^{\circ}N,\;129.13^{\circ}E$) in the Korea Strait was studied from March 2006 to February 2007. The aim was to understand the relationship between the annual thermal stratification pattern and seasonal variation in phytoplankton community structure. Physicochemical factors including temperature, salinity and nutrient concentrations, which strongly influence the proliferation and diversity of phytoplankton, were measured. The study period was divided into three due to the characteristic of thermohaline structures; mixed I (March-May 2006), stratified (June-November 2006) and mixed II(December 2006-Feburuary 2007). Diatoms dominated during the mixed I (89%) and II (48%) periods, while nanoplankton group occupied over 83% of total population during the stratified period. The dominant species during the mixed I and II was Chaetoceros socialis (47% and 29%, respectively), while during the stratified period Gyrodinium sp.(4%) was the most dominant. Averaged total chl a concentrations during the mixed I and II periods were 0.61 mg $m^{-3}$ and 0.72 mg $m^{-3}$, respectively, which were at least two-fold higher than that during the stratified period (0.30 mg $m^{-3}$). The vertical mixing and convection process of the water column induced nutrient supply from the bottom layer to the euphotic zone. It also led to the dominance of diatoms during the mixed periods, whereas small phytoplankton prevailed over large phytoplankton as stratification blocked the upward movement of nutrients to subsurface during the stratified period. During the mixed I and II periods, microplanktonic chl a dominated concentrations (50% and 48%, respectively), while picoplanktonic chl a occupied over 37% of total chl a during the stratified period.