The accuracy of a fluid flow simulation depends not only on the numerical method used for discretizing the governing equations, but also on the distribution and topology of the mesh elements. Mesh adaptation is a technique for automatically modifying the mesh in order to improve the simulation accuracy in an attempt to reduce the manual work required for mesh generation. The conventional approach to mesh adaptation is based on a feature-based criterion that identifies the distinctive features in the flow field such as shock waves and boundary layers. Although this approach has proved to be simple and effective in many CFD applications, its implementation may require a lot of trial and error for determining the appropriate criterion in certain applications. An alternative approach to mesh adaptation is the residual-based approach in which the discretization error of the fluid flow quantities across the mesh faces is used to construct an adaptation criterion. Although this approach provides a general framework for developing robust mesh adaptation criteria, its incorporation leads to significant computational overhead. The main objective of the thesis is to present a methodology for developing an appropriate mesh adaptation criterion for fluid flow problems that offers the simplicity of a feature-based criterion and the robustness of a residual-based criterion. This methodology is demonstrated in the context of a second-order accurate cell-centred finite volume method for simulating laminar steady incompressible flows of constant property fluids. In this methodology, the error of mass and momentum flows across the faces of each control volume are estimated with a Taylor series analysis. Then these face flow errors are used to construct the desired adaptation criteria for triangular isotropic meshes and quadrilateral anisotropic meshes. The adaptation results for the lid-driven cavity flow show that the solution error on the resulting adapted meshes is 80 to 90 percent lower than that of a uniform mesh with the same number of control volumes. The advantage of the proposed mesh adaptation method is the capability to produce meshes that lead to more accurate solutions compared to those of the conventional methods with approximately the same amount of computational effort.

Computational fluid dynamics (CFD) has been widely applied to simulate turbulent flows in an urban environment. The two basic methodologies in CFD that have been applied here are a Reynolds-averaged Navier-Stokes (RANS) modeling and a large-eddy simulation (LES). The nature of the flow in a built-up urban area consisting of an arbitrary aggregation of buildings is dominated by unsteady large-scale turbulent structures. Recognizing that RANS is unable to correctly capture these turbulent structures while LES is associated with high computational costs, a hybrid RANS/LES methodology that combines the computational efficiency of RANS with the predictive accuracy of LES can be a promising simulation approach for the application to urban flows. In the non-zonal approach of hybrid RANS/LES methodology, a single generalized turbulence model is used in the entire computational domain. This model can function as a RANS turbulence closure model or as a LES subgrid scale model, depending on the local grid resolution or flow properties. A variant of non-zonal approaches, referred as partially resolved numerical simulation (PRNS) in this study, obtains the generalized turbulence model from the rescaling of a conventional RANS model through the incorporation of a resolution control function (F_R). The resolution control function F_R is used to characterize the degree of modeling required to represent the unresolved scales of motion. A new generalized functional form for F_R in PRNS is proposed in this thesis. The predictive performance of PRNS is compared with unsteady RANS (URANS) and LES computations, for a plane channel flow, and for fully-developed and developing flows over a matrix of cubes resembling a group of buildings. It is demonstrated that PRNS behaves similarly to LES, in terms of the predictions of the mean flow and turbulence, but outperforms URANS in general. This indicates PRNS is a promising approach for the simulation of complex turbulent flows in an urban environment.

This master’s thesis was provided by the Swedish Defence Research Agency, FOI. The task is to test several RANS (Reynolds-averaged Navier-Stokes) models on two different case geometries and compare the results with LES and experimental data. The first is two dimensional, constructed for flow separation at a sharp edge. The second is three dimensional and flow separation occurs at a smooth surface. The models tested are implemented in the open source CFD (Computational Fluid Dynamics) program, OpenFOAM. OpenFOAM uses the finite volume method and the SIMPLE algorithm as solution procedure. The main flow features evaluated is the shape, position and size of the flow separation. Most of the models tested have problems describing the complex dynamics of flow separation in these particular cases. In addition to the simulations, the RANS k-epsilon turbulence model is presented and the RANS equations and the equation for the turbulent kinetic energy are derived from the Navier-Stokes equations. The theory behind wall functions is described and these equations together with the equations in the k-epsilon model are compared with the equations implemented in OpenFOAM.

Ozonation is currently considered as one of the most effective microbial disinfection technologies due to its powerful disinfection capacity and reduction in levels of chlorinated disinfection by-products (DBP). However, ozonation of waters containing bromide can produce bromate ion above regulated levels, leading to tradeoffs between microbial and chemical risks. In efforts to meet increasingly stringent drinking water regulations and to be cost-effective, water suppliers are required to optimize ozone dosage. Therefore, there is a need to develop a robust and flexible tool to accurately describe ozone disinfection processes and contribute to their design and operation. Computational fluid dynamics (CFD) has come into use recently for evaluating disinfection systems. However, the focus of its application has been largely on modelling the hydraulic behaviour of contactors, which is only one component of system design. The significance of this dissertation is that a fully comprehensive three dimensional (3D) multiphase CFD model has been developed to address all the major components of ozone disinfection processes: contactor hydraulics, ozone mass transfer, ozone decay, and microbial inactivation. The model was validated using full-scale experimental data, including tracer test results and ozone profiles from full-scale ozone contactors in two Canadian drinking water treatment plants (WTPs): the DesBaillets WTP in Montréal, Quebec and the Mannheim WTP in Kitchener, Ontario. Good agreement was observed between the numerical simulation and experimental data. The CFD model was applied to investigate ozone contactor performance at the DesBaillets WTP. The CFD-predicted flow fields showed that recirculation zones and short circuiting existed in the DesBaillets contactors. The simulation results suggested that additional baffles could be installed to increase the residence time and improve disinfection efficiency. The CFD model was also used to simulate ozone contactor performance at the Mannheim Water Treatment Plant before and after installing new liquid oxygen (LOX) ozone generators and removing some diffusers from the system. The modelling results indicated that such changes led to an increase in effective residence time, and therefore an adjustment to operational parameters was required after system modification. Another significant contribution is that, for the first time, the Eulerian and Lagrangian (or particle tracking) approaches, two commonly utilized methods for predicting microbial inactivation efficiency have been compared for the study of ozone disinfection processes. The modelling results of two hypothetical ozone reactors and a full scale contactor suggested that the effective CT values predicted by the Lagriangian approach were slightly lower than those obtained from the Eulerian approach but their differences were within 10%. Therefore, both approaches can be used to predict ozone disinfection efficiency. For the full scale contactor investigated, the tracer residence time distribution predicted by the Euerlian approach provided a better fit to the experimental results, which indicated that the Eulerian approach might be more suitable for the simulation of chemical tracer performance. The results of this part of work provided important insight in understanding the complex performance of multiphase ozonation systems and played an important role in further improving CFD modelling approaches for full-scale ozone disinfection systems. The third significant contribution of this work is that a CFD model was applied to illustrate the importance of ozone residual monitoring locations and suggest an improved strategy for ozone residual monitoring. For the DesBaillets ozone contactors, the CFD modelling results showed that ozone residuals in the cross section of the outlets of some contactor chambers differed by an order of magnitude. The “optimal” area of monitoring locations however varied at different operational conditions. Therefore, it was suggested that multiple ozone residual sampling points should be installed based on CFD analysis and experimental studies, to provide more accurate indicators to system operators. The CFD model was also used to study the factors affecting the residence time distribution (RTD). The results suggested that the selection of the tracer injection locations as well as tracer sampling locations might affect the RTD prediction or measurement. The CFD-predicted T10 values at different outlet locations varied by more than 10% variation. It is therefore recommended that CFD modelling be used to determine tracer test strategy before conducting a full-scale tracer test, and multiple sampling points should be employed during tracer tests, if possible. In addition, a research based on full-scale investigation has also been done to compare the three different CT prediction approaches: CT10, integrated disinfection design framework (IDDF), and CFD, to determine the most appropriate method for design and operation of ozone systems. The CFD approach yielded more accurate predictions of inactivation efficacy than the other two approaches. The current results also suggested that the differences in the three approaches in CT predictions became smaller at higher contactor T10/T ratios conditions as the contactors performed more closely to ideal plug flow reactors. This study has demonstrated that the computational fluid dynamics (CFD) approach is an efficient tool for improving ozone disinfection performance of existing water treatment plants and designing new ozonation systems. The model developed in this study can be used for ozone contactor design, evaluation, and troubleshooting. It can also be used as a virtual experimental tool to optimize ozone contactor behaviour under varying water quality and operational conditions.

Ozonation is currently considered as one of the most effective microbial disinfection technologies due to its powerful disinfection capacity and reduction in levels of chlorinated disinfection by-products (DBP). However, ozonation of waters containing bromide can produce bromate ion above regulated levels, leading to tradeoffs between microbial and chemical risks. In efforts to meet increasingly stringent drinking water regulations and to be cost-effective, water suppliers are required to optimize ozone dosage. Therefore, there is a need to develop a robust and flexible tool to accurately describe ozone disinfection processes and contribute to their design and operation. Computational fluid dynamics (CFD) has come into use recently for evaluating disinfection systems. However, the focus of its application has been largely on modelling the hydraulic behaviour of contactors, which is only one component of system design. The significance of this dissertation is that a fully comprehensive three dimensional (3D) multiphase CFD model has been developed to address all the major components of ozone disinfection processes: contactor hydraulics, ozone mass transfer, ozone decay, and microbial inactivation. The model was validated using full-scale experimental data, including tracer test results and ozone profiles from full-scale ozone contactors in two Canadian drinking water treatment plants (WTPs): the DesBaillets WTP in Montréal, Quebec and the Mannheim WTP in Kitchener, Ontario. Good agreement was observed between the numerical simulation and experimental data. The CFD model was applied to investigate ozone contactor performance at the DesBaillets WTP. The CFD-predicted flow fields showed that recirculation zones and short circuiting existed in the DesBaillets contactors. The simulation results suggested that additional baffles could be installed to increase the residence time and improve disinfection efficiency. The CFD model was also used to simulate ozone contactor performance at the Mannheim Water Treatment Plant before and after installing new liquid oxygen (LOX) ozone generators and removing some diffusers from the system. The modelling results indicated that such changes led to an increase in effective residence time, and therefore an adjustment to operational parameters was required after system modification. Another significant contribution is that, for the first time, the Eulerian and Lagrangian (or particle tracking) approaches, two commonly utilized methods for predicting microbial inactivation efficiency have been compared for the study of ozone disinfection processes. The modelling results of two hypothetical ozone reactors and a full scale contactor suggested that the effective CT values predicted by the Lagriangian approach were slightly lower than those obtained from the Eulerian approach but their differences were within 10%. Therefore, both approaches can be used to predict ozone disinfection efficiency. For the full scale contactor investigated, the tracer residence time distribution predicted by the Euerlian approach provided a better fit to the experimental results, which indicated that the Eulerian approach might be more suitable for the simulation of chemical tracer performance. The results of this part of work provided important insight in understanding the complex performance of multiphase ozonation systems and played an important role in further improving CFD modelling approaches for full-scale ozone disinfection systems. The third significant contribution of this work is that a CFD model was applied to illustrate the importance of ozone residual monitoring locations and suggest an improved strategy for ozone residual monitoring. For the DesBaillets ozone contactors, the CFD modelling results showed that ozone residuals in the cross section of the outlets of some contactor chambers differed by an order of magnitude. The “optimal” area of monitoring locations however varied at different operational conditions. Therefore, it was suggested that multiple ozone residual sampling points should be installed based on CFD analysis and experimental studies, to provide more accurate indicators to system operators. The CFD model was also used to study the factors affecting the residence time distribution (RTD). The results suggested that the selection of the tracer injection locations as well as tracer sampling locations might affect the RTD prediction or measurement. The CFD-predicted T10 values at different outlet locations varied by more than 10% variation. It is therefore recommended that CFD modelling be used to determine tracer test strategy before conducting a full-scale tracer test, and multiple sampling points should be employed during tracer tests, if possible. In addition, a research based on full-scale investigation has also been done to compare the three different CT prediction approaches: CT10, integrated disinfection design framework (IDDF), and CFD, to determine the most appropriate method for design and operation of ozone systems. The CFD approach yielded more accurate predictions of inactivation efficacy than the other two approaches. The current results also suggested that the differences in the three approaches in CT predictions became smaller at higher contactor T10/T ratios conditions as the contactors performed more closely to ideal plug flow reactors. This study has demonstrated that the computational fluid dynamics (CFD) approach is an efficient tool for improving ozone disinfection performance of existing water treatment plants and designing new ozonation systems. The model developed in this study can be used for ozone contactor design, evaluation, and troubleshooting. It can also be used as a virtual experimental tool to optimize ozone contactor behaviour under varying water quality and operational conditions.

The accuracy of a fluid flow simulation depends not only on the numerical method used for discretizing the governing equations, but also on the distribution and topology of the mesh elements. Mesh adaptation is a technique for automatically modifying the mesh in order to improve the simulation accuracy in an attempt to reduce the manual work required for mesh generation. The conventional approach to mesh adaptation is based on a feature-based criterion that identifies the distinctive features in the flow field such as shock waves and boundary layers. Although this approach has proved to be simple and effective in many CFD applications, its implementation may require a lot of trial and error for determining the appropriate criterion in certain applications. An alternative approach to mesh adaptation is the residual-based approach in which the discretization error of the fluid flow quantities across the mesh faces is used to construct an adaptation criterion. Although this approach provides a general framework for developing robust mesh adaptation criteria, its incorporation leads to significant computational overhead. The main objective of the thesis is to present a methodology for developing an appropriate mesh adaptation criterion for fluid flow problems that offers the simplicity of a feature-based criterion and the robustness of a residual-based criterion. This methodology is demonstrated in the context of a second-order accurate cell-centred finite volume method for simulating laminar steady incompressible flows of constant property fluids. In this methodology, the error of mass and momentum flows across the faces of each control volume are estimated with a Taylor series analysis. Then these face flow errors are used to construct the desired adaptation criteria for triangular isotropic meshes and quadrilateral anisotropic meshes. The adaptation results for the lid-driven cavity flow show that the solution error on the resulting adapted meshes is 80 to 90 percent lower than that of a uniform mesh with the same number of control volumes. The advantage of the proposed mesh adaptation method is the capability to produce meshes that lead to more accurate solutions compared to those of the conventional methods with approximately the same amount of computational effort.

Computational fluid dynamics (CFD) has been widely applied to simulate turbulent flows in an urban environment. The two basic methodologies in CFD that have been applied here are a Reynolds-averaged Navier-Stokes (RANS) modeling and a large-eddy simulation (LES). The nature of the flow in a built-up urban area consisting of an arbitrary aggregation of buildings is dominated by unsteady large-scale turbulent structures. Recognizing that RANS is unable to correctly capture these turbulent structures while LES is associated with high computational costs, a hybrid RANS/LES methodology that combines the computational efficiency of RANS with the predictive accuracy of LES can be a promising simulation approach for the application to urban flows. In the non-zonal approach of hybrid RANS/LES methodology, a single generalized turbulence model is used in the entire computational domain. This model can function as a RANS turbulence closure model or as a LES subgrid scale model, depending on the local grid resolution or flow properties. A variant of non-zonal approaches, referred as partially resolved numerical simulation (PRNS) in this study, obtains the generalized turbulence model from the rescaling of a conventional RANS model through the incorporation of a resolution control function (F_R). The resolution control function F_R is used to characterize the degree of modeling required to represent the unresolved scales of motion. A new generalized functional form for F_R in PRNS is proposed in this thesis. The predictive performance of PRNS is compared with unsteady RANS (URANS) and LES computations, for a plane channel flow, and for fully-developed and developing flows over a matrix of cubes resembling a group of buildings. It is demonstrated that PRNS behaves similarly to LES, in terms of the predictions of the mean flow and turbulence, but outperforms URANS in general. This indicates PRNS is a promising approach for the simulation of complex turbulent flows in an urban environment.

Due to advances in technology and sophistication of many efficient algorithms, accurate numerical results can be achieved by using highly efficient computational software for research in wave action on coastal and harbor structures. These advances have benefitted the research in the physical phenomena of internal wave generation, propagation and breaking, which are some of the important topics in oceanography. In this study, the Flow-3D CFD (Computational Fluid Dynamics) software is used to simulate internal solitary wave motion in a density stratified fluid, in which the upper and lower layers are fresh and brine water, respectively. An internal solitary wave (ISW) is produced numerically by gravitational collapse mechanism in a numerical wave flume of 0.7 x 0.5 x 12.0 m (height x width x length ). The ISW in depression is then allow to propagate and across four different bottom obstacles (long uniform slope, trapezoidal section with short platform and isosceles triangle), in order to explore its waveform evolution and flow field distribution. This study also describes the setting and operation of the Flow-3D software, its application to the internal wave experiment, as well as verification of the numerically simulated results using previous laboratory experimental data. In this study, the lifting speed for the sluice gate was vital for not only the amplitude of an ISW, but also the speed of wave propagation in the flume. The result showed that the faster the gate opening, the faster propagation speeds and larger amplitude for the ISW so generated. Conversely, a slower gate opening led to weak wave speed and small amplitude to an ISW. Upon analyzing the results, we have found that the relationship between the speed of the gate opening and the wave propagation speed are not linear. Moreover, preliminary analysis and discussion are given for the ISW propagation over an obstacle (uniform long slope, trapezoidal section with short platform, and isosceles triangle), particularly on waveform evolution, vortex motions and flow field variations. It is believed that we can gain a better and thorough understanding of the internal wave characteristics, compared to physical laboratory experiments, if the numerical tool is applied with very fine grids and detailed analysis on the numerical outputs.

The dissertation presents a CFD approach for 3D simulation of long risers. Long riser VIV simulation is at the frontier of the CFD research area due to its high demand on computational resources and techniques. It also has broad practical application potentials, especially in the oil and gas industry. In this dissertation, I used a time domain simulation program - Finite-Analytic Navier-Stokes (FANS) code to achieve the 3D simulations of riser VIV. First, I developed a riser modal motion solver and a direct integration solver to calculate riser dynamic motions when subject to external forces. The direct integration solver provides good flexibility on inclusion of riser bending stiffness and structural damping coefficients. I also developed a static catenary riser solver based on trial and error iteration technique, which allowed the motion solvers to handle catenary risers and jumpers with arbitrary mass distribution. I then integrated the riser motion solvers to the existing FANS code, and applied the CFD approach to a series of riser VIV problems including a 2D fixed/vibrating riser, a 3D vertical riser in uniform and shear currents, a 3D horizontal riser in uniform and shear current, a hypothetic 3,000 ft marine top tensioned riser in uniform current, a practical 1,100m flexible catenary riser in uniform current, and a hypothetic 265m flexible jumper partially submerged in uniform current. I developed a VIV induced fatigue calculation module based on rain flow counting technique and S-N curve method. I also developed a modal extraction module based on the least squares method. The VIV details, including flow field vorticities, rms a/D, riser motion trajectories, PSDs, modal components, VIV induced stress characteristics, and VIV induced fatigue damages were studied and compared to the published experimental data and results calculated using other commercial software tools. I concluded that the CFD approach is valid for VIV simulations in 3D. I found that the long riser VIV response shows complex behaviors, which suggests further investigation on the lock-in phenomenon, high harmonics response, and sensitivity to the lateral deflections.

Includes bibliographical references (p. 207-212).