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하상고 차이가 있는 하천합류부에서 오염물질 혼합거동에 대한 수치모델링
Get PDF학위논문(박사) -- 서울대학교대학원 : 공과대학 건설환경공학부, 2022. 8. 서일원.하천합류부는 서로 다른 수리학적 특성을 가지는 두 개 이상의 하천이 하나로 합쳐지는 구간으로 급격한 흐름변화 및 수리학적 변화가 발생하며 하천수질 및 생태계에서도 중요한 역할을 한다. 합류수로에서의 유동특성을 다룬 연구는 많은 연구자들에 의해 꾸준히 이어져 왔으나, 대부분의 연구가 전단층, 재순환구역 등 합류부의 독특한 특성들에 영향을 주는 주요인자들을 파악하는데 초점을 두었다. 하지만 실제 자연하천에서는 준설작업 등으로 인한 하상고의 불일치, 홍수 등으로 인한 본류와 지류의 급격한 유량 변화 등 합류부의 흐름특성이 변화하는 경우는 빈번히 발생한다. 또한 이로인한 이차류의 발생 및 이차류강도의 변화는 합류부에서의 흐름 및 오염물 혼합의 3차원 거동을 더욱 강화시킨다. 따라서 기존의 연구들과 같이 일부 지점에서 계측된 한정적인 데이터들로는 합류부의 전체적인 특성을 정확하게 파악하기 매우 어렵다.
이런 이유로 본 연구에서는 합류부에서의 흐름특성 및 오염물의 혼합거동을 종합적으로 이해하기 위해 지류가 90°로 유입되는 합류수로에서 단차비와 유량비를 변화시키며 3차원 수치모의를 수행하였고 실내합류수로실험을 통해 획득한 유속자료를 통해 모의결과를 검증하였다. 흐름장 모의 결과, 유량비와 단차비가 증가할수록 재순환구역의 최대 폭은 증가하는 것을 확인할 수 있었다. 반면 최대 길이는 단차비가 증가하면 길어지지만 유량비가 증가하면 짧아지는 경향을 보였다.
유량비와 단차비가 증가할수록 시계방향으로 회전하는 와류가 점차 발달하는 것을 확인하였으며 유량비와 단차비에 의해 강화된 이차류가 합류부의 흐름 및 오염물의 혼합에 큰 영향을 미치는 것을 확인하였다. 이러한 경향을 정량적으로 평가하기 위해 이차류강도 (SCI)를 계산해 본 결과, 단차비와 유량비가 증가할수록 합류 후 수로의 이차류강도가 증가하는 것을 알 수 있었으며 특히, 단차비가 증가함에 따라 이차류의 영향이 하류 방향으로 긴 구간에 걸쳐 지속되는 것을 확인하였다. 농도장 모의 결과, 유량비와 단차비가 증가함에 따라 횡방향 혼합이 빠르게 완료되는 것을 확인하였다. 특히, 단차비가 큰 경우 지류의 수체가 본류수로의 반대편 제방과 부딛치며 반사되는데 이 경우 주수로의 우측하단에서 좌측상단 방향으로 오염물 혼합이 진행되는 것을 확인하였다.
3차원 수치모의을 통해 파악한 흐름특성을 기준으로 모의구간을 흐름변화구간, 흐름분리구간, 흐름회복구간으로 구분하였으며 이들 구역에서 속도기반 및 농도기반 분산계수산정법 통해 횡분산계수를 계산하였다. 본 연구에서 횡방향 분산계수를 산정한 이유는 합류부에서 오염물의 혼합거동을 2차원 이송분산모형을 이용하여 빠르고 정확하게 모의하기 위함이며 실제로 자연하천의 합류부에서 지류를 통해 유입되는 오염물의 경우 합류직후 짧은 구간에서 3차원거동을 보이지만 연직방향 혼합이 완료된 이후에는 긴 구간에서 걸쳐 횡방향, 종방향으로 2차원 거동을 보이기 때문에 계산의 효율성이나 정확도를 고려하였을 때 2차원 수치모형을 사용하는 것이 바람직하다고 할 수 있다.
합류부에서 2차원 이송분산모형을 적용하는 경우 가장 중요한 매개변수는 분산 계수인데 분산계수 산정시 합류부의 복잡한 특성을 반영하지 못한다면 오염물질의 횡분산을 과대산정하거나 과소 산정하는 결과가 나타난다. 본 연구에서는 흐름특성이 다른 합류부 특성구간 별 적절한 분산계수를 계산하는 2D 분산모형을 제안하였다. 본 연구에서 제안하는 2D분산모형의 검증을 위해 낙동강-금호강 합류부에서 수행한 자연추적자실험의 지형자료, 유속 및 전기전도도 자료를 사용하였으며 수치모의를 위해 2차원수치모형 RAMS+를 이용하였다. 수치모의 결과 전체모의구간에 동일한 분산계수를 적용하는 기존의 모의방법에 비해 본 연구에서 제안한 새로운 분산모형을 사용하는 경우 합류직후 뿐만 아니라 합류 후 긴 모의구간에서도 높은 정확도를 보이는 계산 결과를 도출하였다.A river confluence is an essential topographical node that occurs at the junction of two or more separate flowing channels, resulting in complex hydraulic and morphological changes and controlling the downstream flow paths and mixing of contaminants. In addition, given that the confluence is common in natural large and small rivers and an inevitable feature of river systems, understanding river flow dynamics and contaminant transport at confluence channels is the topic of river engineers garnered significant research interest.
Several studies have been conducted on the flow characteristics of the confluence channel since about 80 years ago, most of the studies had focused on the relationship of unique characteristics, such as shear layers, flow recirculation zone (RZ), and flow deflection zone with the confluences plane symmetry, angle, discharge rate, and momentum flux ratio.
However, flow and pollutant mixing characteristics at the confluence are three-dimensional behaviors, and in particular, as the discharge or discordance ratios changes, the effect of reinforced secondary flows dramatically affects the characteristics of flow and contaminants mixing. So, it is difficult to grasp the features of the confluence channel accurately with data obtained piecewise through traditional methods. Therefore, in this study, three-dimensional (3D) numerical simulation was performed under various conditions to understand the 3D behavior at the confluence channel comprehensively.
Through 3D numerical simulation, it was identified that the direction and magnitude of the lateral flow fluctuated abruptly with the streamwise direction and changes in discharge and discordance ratios affect the dimension of the RZ. It was confirmed that the maximum width of the RZ increased as the discharge and discordance ratios increased. The maximum length increased as the discordance ratio increased but shortened as the discharge ratio increased. It also confirmed that clockwise vortices strengthened as the discharge and discordance ratios increased. To evaluate this trend quantitatively, secondary current intensity (SCI) was calculated, revealing that the secondary flow dimensions in the post-confluence channel increased as the discharge and discordance ratios increased. In addition, as the step ratio increased, the secondary flow sustained its effect over a long section in the downstream direction.
The results of the pollutant simulation confirmed the occurrence of rapid lateral mixing as the discharge and discordance ratios increased. Especially, when the discordance ratio was large, the waterbody of the tributary lashed the opposite bank of the main channel and was reflected. In these cases, pollutants began to mix from the right-bottom of the main channel, and the mixing proceeded in the opposite direction of the inflow from the tributary channel.
Through the findings of 3D numerical simulation, the confluence channel was divided into specific zones based on flow characteristics. In these zones, the dispersion coefficient was calculated through velocity- and concentration-based methods and confirmed that the transverse dispersion coefficient was distributed differently in each section.
The 3D numerical simulation was conducted first before applying the 2D dispersion model in this study because the effect of secondary flows, etc., which play an important role in mixing contaminants at the confluence cannot be identified through the 2D model. In addition, since it is inefficient to use a 3D model when simulating the mixture of contaminants in the confluence, it is intended to quickly and accurately simulate by using a 2D dispersion coefficient reflecting the confluence characteristics. In practice, three-dimensional simulations are not often used to determine the mixing characteristics of contaminants in the long section of the confluence. When simulating the mixed behavior of pollutants introduced through tributaries, it is reasonable to use a two-dimensional advection-dispersion model to satisfy both the efficiency of the calculation and the accuracy of the simulation. But if the dispersion coefficient, the most important parameter in the two-dimensional advection-dispersion model, does not reflect the complex flow and mixing behavior at the confluence, the transverse dispersion cannot be simulated appropriately immediately after the confluence. In addition, if the same dispersion coefficient is applied throughout the simulated section at the confluence where the flow characteristics change from three dimensions to two dimensions, the lateral dispersion of the contaminant is overestimated or underestimated.
Therefore, in this work, a new 2D dispersion model was proposed. First, the characteristic sections are divided by analyzing the flow characteristics identified through the 3D numerical model, and a new 2D dispersion coefficient using the discharge and discordance ratio as the main factor is applied to each section. The newly proposed empirical equation showed the remarkable effect of the discordance and discharge ratios on the transverse dispersion coefficient. The dispersion coefficient equation, which reflected the influence of the discharge and discordance ratios, yielded more accurate results than using other hydraulic parameters.Abstract of dissertation Ⅰ
List of figures Ⅸ
List of tables ⅩⅢ
List of symbols ⅩⅤ
1. Introduction 1
1.1 Background and Necessities 1
1.2 Objectives and Methodology 7
2. Theoretical Background 10
2.1 Flow Characteristics at River Confluence 10
2.1.1 Flow Dynamics at River Confluence 10
2.1.2 Division of Flow Zones of River Confluence 14
2.1.3 Secondary Flow at Channel Confluence 19
2.1.4 Flow Controlling Factors at Confluence 23
2.1.5 Discordance of channel bed 25
2.2 Modeling Pollutant Transport at River Confluences 28
2.2.1 Mixing Characteristics at River Confluences 28
2.2.2 Numerical Modeling of Contaminant Transport at River Confluences 31
2.2.3 Dispersion Coefficients 39
2.2.3.1 Caluculation of Velocity-based Dispersion Coefficiens 39
2.2.3.2 Caluculation of Concentration-based Dispersion Coefficiens 44
3. Numerical Models 49
3.1 Numerical Model Strategy 49
3.2 3D Numerical Model 51
3.2.1 Governing Equations 51
3.2.2 Turbulence Models 55
3.3 2D Numerical Model 59
3.3.1 2D Hydrodynamic Model 59
3.3.2 2D Advection Dispersion Model 62
4. Three-dimensional Simulation of Flow and Mixing at Channel Confluence 66
4.1 Objectices of 3D Numerical Simulation 66
4.2 Experimental Data for Verification 68
4.2.1 Laboratory Confluence Channel and Experimental Equipment 68
4.2.2 Analysis of Experimental Data 73
4.3 Three-dimensional Numerical Simulation Setup 78
4.3.1 Computational Domain and Grid Sensitivity Analysis 78
4.3.2 Turbulence Model and Input Parameters 83
4.3.3 Simulation Cases 87
4.3.4 Validation of 3D Numerical Simulation 91
4.4 Analysis of Flow Dynamics 101
4.4.1 Analysis of Flow-field 101
4.4.1.1 Velocity Field 101
4.4.1.2 Flow Recirculation Zone 111
4.4.1.3 Turbulent Characteristics 117
4.4.2 Analysis of Secondary Flow 122
4.4.2.1 Characteristics of Secondary Flow 122
4.4.2.2 Analysis of Intensity of Secondary Flow 128
4.5 Analysis of Contaminant Mixing 133
4.5.1 Concentration Field 133
4.5.2 Transverse Mixing 139
4.5.2.1 Concentration Contour on y-z plane 139
4.5.2.2 Analysis of Transverse Concentration Gradient 144
5. Estimation of Dispersion Coefficients 149
5.1 Division of Mixing Zone 151
5.2 Velocity-based Transverse Dispersion Coefficients 154
5.2.1 Calculation Methods 154
5.2.2 Results of Velocity-based Dispersion Coefficients 155
5.2.2.1 Dispersion Coefficients in Zone 1 (FDZ) 155
5.2.2.2 Dispersion Coefficients in Zone 2 (FSZ) 160
5.2.2.3 Dispersion Coefficients in Zone 3 (FRZ) 164
5.3 Concentration-based Transverse Dispersion Coefficients 169
5.3.1 Calculation of Dispersion Coefficients from Concentration Data 169
5.3.2 Comparison of Velocity-based and Concentration-based Dispersion
Coefficients 176
6. Two-dimensional Model Application 180
6.1 Field Experiment Data 180
6.1.1 Study Site 180
6.1.2 Data Acquisition 183
6.1.3 Experimental Setup 186
6.1.4 Analysis of Velocity Data 188
6.1.5 Analysis of EC Data 193
6.2 Application of 2D Flow Model 196
6.2.1 Reflection of Study Site Characteristics 196
6.2.2 Sensitivity Analysis and Model Application 198
6.3 Application of 2D Mixing Model 204
7. Conclusion and Future study 211
7.1 Conclusions 211
7.2 Future Study 214
References 215
국문초록 223박
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