| Flood is one of the major environmental hazards, and closely related to the safety of people’s life and property. Study on the flooding process is beneficial to the further understanding of the occurrence of floods and its response mechanism. Dynamic simulation of floods is able to provide detailed information of flooding process, such as the position of inundation, water depth, velocity and so on, thus plays an important role in flood risk assessment; in order to obtain reliable flood risk assessment results, one needs to have the flooding simulation results as the basis for accuracy. At present, most of the hydrodynamic flooding models are one-dimensional, two-dimensional and 1D/2D coupled models, which have successfully solved many engineering problems. However, the1D/2D shallow water equations cannot realistically describe the motion of floods. The reason is that, take 2D numerical flooding models for example, the variables are treated as uniformly distributed along the vertical direction, the vertical velocity and acceleration are ignored, and the pressure are assumed as hydrostatic. However, in reality the flood water movement is various in three directions(longitudinal, lateral and vertical),and presents obvious three dimensional characteristics in the process of movement, and with a variety of secondary flows. Therefore, for the flood flows in actual conditions,2D models can not fully reflect its real dynamic characteristics. This has inspired the interest in the study of 3D flooding models. Despite the fact that the 3D flood models in literature is still relatively rare currently, people have already begun to try to study the three-dimensional hydrodynamic models in recent years, along with the development of the super computer and parallel computing technology. International interest on this topic has greatly increased. Under this background, this thesis carried out the research of threedimensional hydrodynamic flooding model. The main research contents and innovations are as follows:(1) Solved the numerical problem that it is hard to solve the matrix equations when the aspect ratio is high in wetting and drying process, by introducing suitable vertical absorption terms, improved the wetting and drying algorithm. Wetting and drying process is one of the major di?culties in numerical flooding models. One of the reasons is that in the wetting and drying process, the aspect ratio of the finite element is usually very large, which leads to increase the condition number of the resulting pressure matrix equation, making the matrix equation very ill-conditioned and di?cult to solve. To solve this problem, suitable vertical absorption terms are introduced in this work to control the conditioning of the discretized equations(the Laplacian for the combined pressure/free-surface variable). This leads to an improvement in stability and speed of convergence of the iterative solvers, thus ensures the matrix equations to be well-conditioned and stable.(2) Based on the improved wetting and drying algorithm, developed a 3D numerical dynamic flooding model, improved the computational e?ciency and stability. Based on the improved wetting and drying algorithm, developed a 3D numerical dynamic flooding model. In this model, The 3D Navier-Stokes equations are solved and a finite element method with arbitrary unstructured tetrahedral meshes is adopted. This proposed method allows the free surface height to be treated with a high level of implicitness and stability allowing relatively large time steps to be used. It also ensures numerical consistency when using a fully unstructured mesh in the horizontal and vertical as the free surface height, and in particular its derivatives, do not need to be interpolated from the free surface to the internal volume of the domain. On the free surface the no normal flow boundary condition is weakly enforced as an area becomes dry. This provides an elegant way of relaxing the free surface height to a particular value if the water depth becomes small, less than 1 cm, for example. The e?ciency, robustness and sensitivity to topography of the proposed model have been tested with a series of flooding cases.(3) Compared the 3D model results with 2D model results to analyse the 3D hydrodynamic characteristics in flooding process, and verified the reliability of the newly developed 3D numerical hydrodynamic flooding model by comparing the results with the experimental data. By comparing the model results with the experimental data, analysed the reliability of the 3D hydrodynamic flooding model; By compared with2 D model results, analysed the 3D hydrodynamic characteristics in flooding process. The study found that the 3D model results presents larger di?erences with 2D models when the vertical inertia is large; with the water becoming shallower and the vertical inertia becoming smaller, the di?erences gradually turns smaller. Besides, the 3D model has captured very detailed and complex information of the flooding flows, and is able to generate smooth wetting front.(4) Established the three-dimensional hydrodynamic model of urban surface runo?, analysed the 3D hydrodynamic characteristics of urban flooding. Three dimensional hydrodynamic characteristics of urban flood have been further studied. Research on the impact of di?erent mesh resolution on modelling results and CPU time has been carried out. A flooding event in a densely urbanized area within the city of Glasgow has been simulated using the newly developed 3D flooding model. By compared with the other 2D models, this work found that the 3D model provides similar results as 2D models in ponding areas, but in those ares with larger slopes(> 3%), the results are much more di?erent, and the 3D model gives larger vertical velocity component(> 0.2 m/s)than poinding areas. Besides, considering that the urban flooding consists of surface and sub-surface flooding processes, this work has developed an interface between SWMM and Fluidity, in order to achieve 1-D sub-surface and 3D surface coupling modelling of urban floods.This work systematically studied the 3D numerical hydrodynamic flooding model for the first time and realised the 3D modelling of real flooding events, and helps illustrate the case that 3D modelling techniques are promising to improve accuracy and obtain more detailed information related to urban flooding dynamics. This helps to obtain better assessments of flood damage and vulnerability of urban areas. |
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