Research On Typical Coupling Problems And The Decoupling Conditions For Groundwater Flow Linked With Air Flow And Thermal Flow

The traditional theory of hydrogeology focuses on groundwater flow and its hydrogeochemical background; it usually simplifies or even ignores the dynamic coupling between groundwater flow and other factors in the geological environments. However, the coupling processes linked with groundwater flow exist everywhere, which play important roles in moulding the geological environments. Among these processes, the coupling between groundwater flow and air flow and the coupling between groundwater flow and heat flow have become to the hot research topics in the contemporary groundwater dynamics and in the geological environments fields. In comparison with traditional groundwater dynamics, coupling behaviors always refer to more complicated mathematical problems. As well, it is much more difficult to obtain the analytical solutions. Thus, numerical solution is generally adopted to survey the coupling behaviors for special cases, losing the advantage of the analytical method by which the relationships among different factors can be clearly illustrated. The physical mechanisms of the coupling processes are hard to be understood because they always emerged behind the complex and unclear scenes.As a matter of fact, groundwater flow is a dominate factor in many cases and the other dynamic items have little impacts on groundwater flow. It allows the application of decoupling method to obtain the approximate solution of groundwater flow in coupling circumstances. It is the basis of the availability for the classical groundwater hydraulics. Only when the situation satisfies the decoupling conditions, the classical groundwater hydraulics can be effectively applied. However, how can we ignore the coupling effect on groundwater flow is still a poorly understood problem. This study attempts to present a better answer to the problem.The thesis investigates two typical coupling processes:one is the coupling between groundwater flow and air flow in an unconfined aquifer; the other is the coupling between groundwater flow field and the temperature field in a basin. The characteristics of the coupling behaviors are observed and the decoupling conditions of these two problems are analyzed. The findings can be summarized as follow:(1) The controlling factors on the coupling problem of radial groundwater flow and air flow are identified and applied by building the mathematical model with several dimensionless parameters. It leads to a much more clear description of the problem in comparison with previous presentations. The air flow has little impact on groundwater flow as indicated by the small difference of estimated groundwater levels between the coupling and decoupling models. In other words, the decoupling condition for groundwater flow is generally satisfied. However, air flow is sensitive to groundwater flow so that the decoupling result of groundwater level could not be sufficiently used to obtain the change in air pressure in the vadose zone.(2) Based on the finite element model setup in the Comsol software, the coupling between groundwater flow and heat flow in a basin is successfully simulated. It is found that the analytic solutions of groundwater-heat flows in a unit basin (Domenico and Palciauskas,1973) may bring obvious errors.This study demonstrates that strong heat convection following groundwater flow could break the symmetry patterns in the spatial distribution of temperature.In most cases (average status of the terrestrial heat flow), groundwater-heat flows in a basin satisfy the decoupling condition and the decoupling result of groundwater flow can be sufficiently used to obtain the distributed temperature.These findings improve the understanding of the mechanisms in groundwater-heat flow and multiple phase flow problems in hydrogeology. It is an advantage for geological projects and environmental projects to use the classical groundwater dynamics reasonably. Nevertheless, the models in this thesis are presented on the basis of some simplifications and hypothesis, which bring limitations in the conclusions. Investigations on the problems need further efforts to be extended and improved.