• 한국해안해양공학회:학술대회논문집
• /
• 한국해안해양공학회 1997년도 정기학술강연회 발표논문 초록집 Annual Meeting of Korean Society of Coastal and Ocean Engineers
• /
• pp.110-115
• /
• 1997

Recently, since one of the most important societal problems facing us today is the growing incidence of the contamination of coastal sea from variety of sources, several distinct numerical improvements have been made and applying the transport models to flow-dominated transport area. Application of Eulerian numerical models to the solution of sharp-front problems often results in oscillations, phase errors, peak depression, and/or numerical dispersion, unless very fine temporal and spatial steps are adopted. (omitted)

• 한국해안해양공학회지
• /
• 제10권1호
• /
• pp.37-44
• /
• 1998

본 연구에서 제안하는 혼합 방법(hybrid method)은 흐름이 우세한 영역에서의 전송 문제를 정확하고 효과적으로 해결하기 위하여 개발된 것으로 오일러-라그란쥐적 방법과는 달리 전방추적에 의하여 이송 과정이 수행되므로 보간 기법이 불필요하고 무작위 행보에 의한 라그란쥐적 방법과 달리 유한 차분법에 의하여 확산 과정이 수행되므로 많은 입자가 요구되지도 않는다. 한 점에 순간적으로 부하되는 오염원과 연속적으로 부하되는 오염원에 대한 이론적인 해와 비교하여 확산 계수와 무관하게 상당히 만족할 만한 결과를 얻었다. 현 방법은 또한 2차원 상에서 주변 5격자로부터 보간하는 오일러-라그란쥐적 방법과 무작위 행보로 입자 추적하는 순수 라그란쥐적 방법과 비교하여 정확성은 물론 계산 시간에 있어서도 상당히 월등한 방법임이 입증되었다.

• 한국수자원학회:학술대회논문집
• /
• 한국수자원학회 2023년도 학술발표회
• /
• pp.519-519
• /
• 2023

Estuarine dams are a recent and global phenomenon. While estuarine dams can provide the benefit of improved freshwater resources, they can also alter estuarine processes. Due to the wide range of estuarine types and estuarine dam configurations, the effect of estuarine dams on estuaries is not well understood in general. To develop a systematic understanding of the effect of estuarine dam location and freshwater discharge interval on a range of estuarine types (strongly stratified, partially mixed, periodically stratified, and well-mixed), this study used a coupled hydrodynamic-sediment dynamic numerical model (COAWST) and compared flow, sediment transport, and morphological conditions in the pre- and post-dam estuaries. For each estuarine type, scenarios with dam locations at 20, 55 and 90 km from the mouth and discharge intervals of a discharge every 0.5, 3, and 7 days were investigated. The results were analyzed in terms of change in tide, river discharge, estuarine classification, and sediment flux mechanism. The estuarine dam location primarily affected the tide-dominated estuaries, and the resonance length was an important length scale affecting the tidal currents and Stokes return flow. When the location was less than the resonance length, the tidal currents and Stokes return flow were most reduced due to the loss of tidal prism, the dead-end channel, and the shift from mixed to standing tides. The discharge interval primarily affected the river-dominated estuaries, and the tidal cycle period was an important time scale. When the interval was greater than the tidal cycle period, notable seaward discharge pulses and freshwater fronts occurred. Dams located near the mouth with large discharge interval differed the most from their pre-dam condition based on the estuarine classification. Greater discharge intervals, associated with large discharge magnitudes, resulted in scour and seaward sediment flux in the river-dominated estuaries, and the dam located near the resonance length resulted in the greatest landward tidal pumping sediment flux and deposition in the tide-dominated estuaries.

• 한국지하수토양환경학회지:지하수토양환경
• /
• 제20권2호
• /
• pp.10-21
• /
• 2015

The present study introduces a novel numerical approach for solving dispersion dominated problems with Cauchy boundary condition in an Eulerian-Lagrangian scheme. The study reveals the incapability of traditional Neuman approach to address the dispersion dominated problems with Cauchy boundary condition, even though it can produce reliable solution in the advection dominated regime. Also, the proposed numerical approach is applied to a real field problem of radioactive contaminant migration from radioactive waste repository which is a major current waste management issue. The performance of the proposed numerical approach is evaluated by comparing the results with numerical solutions of traditional FDM (Finite Difference Method), Neuman approach, and the analytical solution. The results show that the proposed numerical approach yields better and reliable solution for dispersion dominated regime, specifically for Peclet Numbers of less than 0.1. The proposed numerical approach is validated by applying to a real field problem of radioactive contaminant migration from radioactive waste repository of varying Peclet Number from 0.003 to 34.5. The numerical results of Neuman approach overestimates the concentration value with an order of 100 than the proposed approach during the assessment of radioactive contaminant transport from nuclear waste repository. The overestimation of concentration value could be due to the assumption that dispersion is negligible. Also our application problem confirms the existence of real field situation with advection dominated condition and dispersion dominated condition simultaneously as well as the significance or advantage of the proposed approach in the real field problem.

• Korea-Australia Rheology Journal
• /
• 제18권2호
• /
• pp.51-65
• /
• 2006

Of concern in the present theoretical investigation is the study of blood flow and convection-dominated diffusion processes in a model bifurcated artery under stenotic conditions. The geometry of the bifurcated arterial segment having constrictions in both the parent and its daughter arterial lumen frequently appearing in the diseased arteries causing malfunction of the cardiovascular system, is constructed mathematically with the introduction of suitable curvatures at the lateral junction and the flow divider. The streaming blood contained in the bifurcated artery is treated to be Newtonian. The flow dynamical analysis applies the two-dimensional unsteady incompressible nonlinear Wavier-Stokes equations for Newtonian fluid while the mass transport phenomenon is governed by the convection diffusion equation. The motion of the arterial wall and its effect on local fluid mechanics is, however, not ruled out from the present model. The main objective of this study is to demonstrate the effects of constricted flow characteristics and the wall motion on the wall shear stress, the concentration profile and on the mass transfer. The ultimate numerical solutions of the coupled flow and diffusion processes following a radial coordinate transformation are based on an appropriate finite difference technique which attain appreciable stability in both the flow phenomena and the convection-dominated diffusion processes.

• 한국수자원학회:학술대회논문집
• /
• 한국수자원학회 2023년도 학술발표회
• /
• pp.125-125
• /
• 2023

Estuarine dams are dams constructed in estuaries for reasons such as securing freshwater resources, controlling water levels, and hydroelectric power generation. These estuarine dams alter the flow of freshwater to the coastal ocean and the tidal properties of the estuaries which has implications for the estuaries' circulation and sediment transport. A previous study has analyzed the effect of estuarine dams on 1D (along-channel) circulation and sediment transport. However, the effect of estuarine dams on the transverse variability of along-channel and across-channel circulation and sediment transport has not been studied and is not known. In this study, a coupled hydrodynamic-sediment dynamic numerical model (COAWST) was used to analyze the transverse variability of along-channel and across-channel flow and sediment transport in estuaries with estuarine dams. The estuarine dam was found to change the 3D structure of circulation and sediment transport, and the result was found to depend on the estuarine type (i.e., strongly stratified (SS) or well-mixed (WM) estuary). The SS estuary had inflow in the channel and outflow over the shoals, consistent with estuarine circulation. Longer discharge interval reduced the estuarine circulation. The WM estuary had inflow over the shoals and outflow in the channel, consistent with tide-induced circulation. As the estuarine dam was located nearer to the estuary mouth, the tide-induced circulation was reduced and replaced with estuarine circulation. The lateral circualtion was the greatest in the tide-dominated estuaries. It was reduced and changed direction due to differential advection change as the dam was located nearer the mouth. Overall, the WM estuary transverse flow structure changed the most. Lateral sediment flux was important in all estuaries, particularly for transporting sediments to the tidal flats.

• 한국지하수토양환경학회:학술대회논문집
• /
• 한국지하수토양환경학회 2000년도 추계학술대회
• /
• pp.69-73
• /
• 2000

We systematically analyze the influence of fracture junction, solute transfer characteristics on transport patterns in discrete, two-dimensional fracture network models. Regular lattices and random fracture networks with power-law length distributions are considered in conjunction with particle tracking methods. Solute transfer probabilities at fracture junctions are determined from analytical considerations and from simple complete mixing and streamline routing models. For regular fracture networks, mixing conditions at fracture junctions are always dominated by either complete mixing or streamline routing end member cases. Moreover bulk transport properties such as the spreading and the dilution of solute are highly sensitive to the mixing rule. However in power-law length networks there is no significant difference in bulk transport properties, as calculated by assuming either of the two extreme mixing rules. This apparent discrepancy between the effects of mixing properties at fracture junctions in regular and random fracture networks is explained by the statistics of the coordination number and of the flow conditions at fracture intersections. We suggest that the influence of mixing rules on bulk solute transport could be important in systematic orthogonal fracture networks but insignificant in random networks.

• Ocean Science Journal
• /
• 제43권2호
• /
• pp.81-90
• /
• 2008

A study of estuarine flows during a neap tide was performed using 13-hour roving acoustic Doppler current profiles (ADCP) and conductivity-temperature-depth (CTD) profiles in the Altamaha River estuary, Georgia, U.S.A. The least-squared harmonic analysis method was used to fit the tidal ($M_2$) component and separate the flow into two components: the tidal and residual ($M_2$-removed) flows. We applied this method to depth-averaged data. Results show that the $M_2$ component demonstrates over 95% of the variability of observation data. As the flow was dominated by the $M_2$ tidal component in a narrow channel, the tidal ellipse distribution was essentially a back-and-forth motion. The amplitude of $M_2$ velocity component increased slightly from the river mouth (0.45 m/sec) to land (0.6 m/sec) and the phase showed fairly constant values in the center of the channel and rapidly decreasing values near the northern and southern shoaling areas. The residual flow and transport calculated from depth-averaged flow shows temporal variability over the tidal time scale. Strong landward flows appeared during slack waters which may be attributed to increased baroclinic forcing when turbulent mixing decreases.

• 생태와환경
• /
• 제34권1호통권93호
• /
• pp.45-53
• /
• 2001

남조류 물꽃현상이 나타나는 일본의 Suwa호에서 방류수를 통해 하류하천 (Tenryu강과 Nishitenryu 수로)으로 유출된 남조류세포와 남조류 독소 (microcystin-LR,-RR, YR)의 유하과정에서의 변동을 1998년 5월부터 10 월까지 조사하였다. 하천 내 모든 지점에서 식물플랑크톤 종조성은 상류의 호수와 일치하였다. 6월과 7월에 우점한 남조류는 M. ichthyoblabe였고, 8월부터 증가한 M. viridis는 10월까지 우점종이었다. Microcystin은 남조류의 현존량이 증가한 7월부터 검출되기 시작하여 남조류 세포밀도의 계절변동에 따라 농도가 변동하였으며, 3종류 microcystin의 조성변화는 남조류 종조성과 관련이 있었다. Microcystis. ichthyoblabe가 우점한 7월에는 MC-RR과 -LR만이 검출된 반면, M. viridis가 우점한 8월 부터 10월까지는 3종류의 microcystin이 모두 검출 되었다. Microcystin은 호수로부터 32 km 떨어진 하류지점에서도 3.2${\sim}$0.3 ${\mu}$g/l의 농도로 검출되었다. Tenryu강 지점 2와 지점 5사이의 29 km 구간 (유하시간 11시간)에서 세포밀도와 microcystin 농도의 감소율은 각각 73%, 72%이었고, 희석에 의한 세포밀도와 microcystin 농도의 감소율이 각각 61%와 57%로서 감소요인의 대부분을 차지하였다. 인공수로에서는 자연하천보다 남조류 세포와 독소의 제거율이 더 낮았다. 이러한 결과들은 남조류가 번성한 부영양호의 하류하천에서는 먼 거리까지 남조류의 독소가 전달되어 공중보건에 위해성을 줄 수 있음을 보여 주고 있다.

• 대한기계학회논문집
• /
• 제19권12호
• /
• pp.3317-3328
• /
• 1995

The objective of the present study is to predict the characteristics of heat and mass transfer around an evaporative condenser. Numerical calculations have been performed using multi-zone method to investigate heat transfer rate and evaporation rate with the variation of inlet condition(velocity, relative humidity and temperature) of the moist air, the flow rate of the cooling water and the shape of the condenser tube. From the results it is found that the profile of heat flux is the same as that of evaporation rate since heat transfer along the gas-liquid interface is dominated by the transport of latent heat in association with the vaporization(evaporation) of the liquid film. The evaporation rate and heat transfer rate is increased as mass flow rate increases or relative humidity and temperature decrease respectively. But the flow rate of the cooling water hardly affect the evaporation rate and heat flux along the gas-liquid interface. The elliptic tube which the ratio of semi-minor axis to semi-major axis is 0.8 is more effective than the circular tube because the pressure drop is decreased. But the evaporation rate and heat flux shown independency on the tube shape.