|Alternative Title||A Zoning Vertical Integratal Theory and Numerical Model for Turbidity Currents|
|Thesis Advisor||刘青泉 ; 安翼|
|Place of Conferral||北京|
|Keyword||浊流 垂向积分模型 床面变形方程 黎曼问题 自加速|
Turbidity currents are turbulent underflows with low sediment concentration. It can involve water from the environment, roll up sediment from and deposit sediment to the bed. In these mechanics of dynamic equilibrium, a large amount of sediment can be transported at a very large distance. In the process of sediment transport, turbidity currents become the controlling factor of the seabed topography and landform. The high-speed movement of turbidity currents is also very destructive and can pose a great threat to submarine facilities. Sediment deposited from turbidity currents is an important source and storage site for offshore oil and gas resources. Therefore, the study for the movement of turbidity currents has very important scientific significance and engineering value. So far, studies of turbidity currents can be inversions based on sediment deposits, laboratory experiments, and mechanical models. The first two methods are limited by the uniqueness and scaling problem so that their research reliability is limited.
There are still many conceptual problems in mechanical models for turbidity current, a more general theory is developed and a new vertical-averaged model is established through strict derivation. Starting from the general dispersed two-phase turbulence theory, the compressibility of turbidity currents is deduced theoretically. The region of turbidity currents is divided into several zones along the vertical direction. Thus, the model can distinguish flows of environmental water, turbidity currents and flows near the sea bed. A more rigorous bed deformation model coupled with turbidity currents is proposed, the four important factors affecting turbidity current are considered in the model, including water entrainment, erosion and deposition, the static pressure gradient along the upper attached water body and reduced gravity.
After the hyperbolic property of the model is confirmed, a numerical scheme for the new model is constructed with the Well-Balanced property and integrity being proved for the first time. The exact solution of the Riemann Problem for turbidity current is gotten for the first time, and a formula in double-rarefactions form for turbidity current is obtained, a special structure of turbidity current problem different from the open channel flow is obtained and a new structure of solution to the Riemann Problem for turbidity current is suggested when the density of the environmental water varies.
Then, numerical experiments are carried out for the important law of a steady equilibrium of turbidity current. The influence of density and flow of steady water on the steady-state equilibrium of turbidity current is explored by using the new model. The results show that in a few special cases considered, the variation of density or only flows of environmental water will not affect the remotely asymptotic property of the steady equilibrium state of the new model, but do affect the upstream solution and will change the self acceleration criterion of turbidity currents.
Finally, a real-scale turbidity current event (Agadir Canyon turbidity current) is numerically simulated using a widely used coupled model (FCM). The results show that a limited-scaled event may happen, otherwise it will not meet the limited erosional area near the canyon mouth shown by the marine geologist. A dimensionless parameter scheme for the global erosion volume is also suggested by the results.
Besides, as part of the research results for my Ph.D. program during the master stage, an extra chaper that summarizes the numerical simulation results of droplets impacting a thin liquid layer at high speed is attached at the end of the thesis. Focusing on the control law of the movement of the splashing crown spray formed by droplet impact, and further, the role of viscosity in it, the CLSVOF method was used to simulate this process. The simulation results and analysis show that the dissipation level of viscous material varies during the three stages after impact. Viscous dissipation occurs mainly in the neck of the droplet and the liquid layer at the initial stage of impact, and the magnitude can be neglected; after the crown spray formation, viscous dissipation is mainly on the wall; and in the second stage, the main dissipation zone moves from the neck of the droplet to the wall, and the dissipation magnitude is the main part of the whole impact process.
|杨世豪. 沿深度分区的浊流垂向积分理论和数值模型[D]. 北京. 中国科学院大学,2019.|
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