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土壤水分布的孔隙尺度格子玻尔兹曼模拟研究

PORE-SCALE LATTICE BOLTZMANN MODELING OF SOIL WATER DISTRIBUTION

  • 摘要: 土壤水控制着陆地生态系统几乎所有的物理和生物化学过程,准确描述土壤中水的分布与运动状态对人类发展和生态环境保护均有重要意义.土壤水分布不仅与孔隙结构有关, 而且受土壤固相表面润湿性的影响.一般对土壤水分布都是从宏观尺度进行描述,但土壤中的物理及生化过程都发生在孔隙中, 从孔隙尺度分析土壤水的分布规律,有助于更准确地理解土壤中的各种宏观现象.本文通过X射线扫描成像技术获取两个土样的三维孔隙结构,并采用改进的Shan-Chen格子玻尔兹曼模型,模拟了在不同润湿性条件下两个土样中孔隙水分布,分析了接触角对孔隙水分布的影响, 结果表明:接触角较大时,孔隙直径对液态水和水蒸气分布影响较小, 随着接触角减小,孔隙直径对液态水和水蒸气的分布影响增大;接触角对液、气、固之间的界面面积以及流体输运通道直径也有较大影响,液体输运通道直径随接触角减小而变小, 气体输运通道直径则随接触角的减小先增大,后减小; 土壤中液态水密度随接触角变化很小,但水蒸气密度随接触角减小而显著降低; 接触角较大时,饱和度对水蒸气密度无明显影响, 接触角较小时,饱和度的增大会显著提高水蒸气的密度.

     

    Abstract: Water in soil controls almost all physical and biogeochemical processes in terrestrial ecosystems and correctly describing its distribution and flow is critical in human development and ecological environment protection. Water distribution at pore scale is modulated by a multitude of abiotic and biotic factors such as the exudates secreted by plant roots and microbes, which could alter soil wettability and water surface tension. The combined impact of all these factors can be described by a single parameter, the contact angle. Practical studies on soil water distribution normally focus on large scale using continuum approaches by volumetrically averaging the microscopic processes out, but it is the physical and biochemical processes occurring in the pores that underpin the emerging phenomena at large scales. Studying the microscopic mechanisms underlying the microscopic water distribution is hence essential to improving the understanding of macroscopic phenomena. Since it is difficult to observe the water distribution at pore-scale due to the complexity of pores structure and the opaque nature of the soils, pore-scale modelling in combination with tomography has been increasingly used to bridge this gap. In this paper, we numerically investigated how a change in the contact angle reshaped water distribution using the Lattice Boltzmann model and X-ray computed tomography. Two soils with contrasting structures were acquired using X-ray computed tomography and they were then segmented to binary images consisting of pore and solid voxels. Water distribution in pore spaces of the soils was assumed to be controlled by capillary force and was simulated using a modified two-phase lattice Boltzmann model. The results show that with the contact angle increasing, the impact of the pore diameter on water distribution in both soils waned, and that a change in the contact angle also led to a change in the channel diameter for fluid flow and interfacial areas between liquid, solid and gas. It was found that as the contact angle decreased, the channel diameter for the liquid decreased while that for the gas increased first followed by a decline. The density of the liquid water was independent of the contact angle, but the density of the vapor decreased significantly as the contact angle increased. The effects of saturation on density of the vapor also increased as the contact angle decreased.

     

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