The Research Of Water Migration In Soil Using The Pore-scale Modelling Method

Water migration in soil controls almost all physical and biogeochemical processes in terrestrial ecosystems,and so there is a significant importance to study the water migration in soil.All physical and biological processes occur in the soil pores,and research of soil water migration based on macroscopic numerical models often ignore microscopic characteristics.The simulation of pore scale process is hence essential to improve the macro model and understand the micro mechanism of water migration in soil.Pore-scale simulation method can quickly,simply and effectively understand the law of water migration and grasp the situation of water migration.However,the current methods have disadvantages in the application of high-density-ratio immiscible fluid,and are rarely used in the study of contact angle in real soil structure and the impact of artificial farming activities on soil water.This thesis has the following work to overcome these challenges.1.An alternative model is proposed to simulate unsaturated water flow at pore scale.After determine water distribution in the pore geometry using a morphological model,the water–air interface is fixed in space.The water flow numerically is simulated by solving the Stokes equations in which the water–air interface is treated as a boundary.We test the water flow model against analytical solution of thin water film flow over a non-slip solid wall,and then combine it with the morphological model to simulate water flow in 3D soil structures.As a validation,we compare the permeability calculated by the model under di erent saturations with those estimated by the van Genuchten formula using the saturated permeability and water retention parameters obtained from the morphological model.2.The pore water migration and redistribution in two soil samples with different contact angles are simulated by the Lattice Boltzmann method combined with X-ray Computed tomography,and the influence of contact angle on the distribution of pore water is analyzed.The results show that in the soil with small contact angle,the liquid water will fill the pores with smaller diameter preferentially,expel the air and make the air gather in the pores to form bubbles;the area of the liquid-solid interface in soil increases as the contact angle decreases,but the area of the gas-solid interface is just the opposite;the diameter of the liquid transport channel becomes smaller as the contact angle decreases,while the diameter of the gas transport channel increases first and then decreases as the contact angle decreases;the density of water on the surface of the solid phase is the largest in the soil;the density of liquid water changes little with the contact angle,but the density of water vapor decreases significantly with the decrease of the contact angle;when the contact angle is large,the saturation has no obvious effect on the water vapor density;when the contact angle is small,the increase of the saturation will significantly increase the water vapor density.3.Under the background of three soil texture,three long-term field tillage system were studied.The nature of the pore systems was visualized and quantified in 3D by X-ray Computed Tomography at the micro scale.Then the lattice Boltzmann model of saturated soil water migration was used to simulate the permeability of soils with different structures.The results show that,on the micrometer scale,grass and arable land cause clay porosity to increase significantly,connectivity is higher than that of no-tillage soil,and plant coverage is a major factor in this effect.However,that didn’t significant affect the structure of sandy soil at the micro scale.Fertilizer can promote the effect of tillage on soil structure,and the type of fertilizer determines the degree of promotion.Permeability calculation results show that,on the micrometer scale,tillage and the plant cover has a significant impact on the permeability of three texture soils,but not all kinds of fertilizers promote the effects of tillage on soil permeability.According to the change of soil characteristics,the effect of tillage methods on soil characteristics is related to soil texture.Through the research in this paper,the laws and influencing factors of soil water migration are revealed from the microscopic mechanism,which provides a basis for further simulating fluid movement and material migration in soil,and provides theoretical guidance for the research of soil water resources and optimization of agricultural production.It is also very important to understand and study the fluid conditions in other porous media.