摘要
地下水位的上升或下降会驱动包气带中的空气流动，并与水位升降运动发生相互作用。当潜水含水层被低渗透介质覆盖时，这种水位升降与空气流的耦合现象会更加的明显。本文利用2-7.5cm厚度的薄层细砂作为低渗透介质盖层与厚层的粗砂组合成双层结构砂柱，并进行了注水和排水实验的研究。
在排水实验中，当粗砂层被细砂层覆盖时，随着砂柱水位面下降，包气带会形成显著的真空，吸取外界大气进入砂柱。有细砂覆盖层的排水实验由于真空对水形成吸持作用，使得水不能顺利排出，其排水速率显著低于无细砂层覆盖的情况，证实了真空阻滞效应的存在。在注水实验中，砂柱水位面抬升，包气带气压增大，使得空气向外流动。无论是注水还是排水实验，包气带气压随时间的变化曲线呈单峰形式，先快速增大然后缓慢减小。峰值的大小受细砂层厚度的影响，细砂盖层越厚，包气带相对气压的峰值越大，达到峰值的时间也越长。
考虑砂柱饱水带的Darcy流和包气带可压缩空气的线性渗流，提出了一个描述砂柱水-气运动的简化动力学模型，通过Runge-Kutta法进行实验过程的数值模拟，包气带气压变化的模拟曲线重现了观测到的包气带气压变化特征，说明模型揭示了砂柱实验的主要动力学特征。模拟结果表明包气带气压的极值随着低渗透盖层厚度的增加而呈非线性增大的趋势。在简化动力学模型基础上，还得到了排水实验早期和晚期真空度变化的近似解析公式。说明本文提出的动力学模型只需少量介质参数便可以解释包气带相对气压和饱和带水位面的变化过程，包气带相对气压主要取决于强渗透介质的饱和渗透系数、弱渗透介质的透气性及其厚度。对实验结果进行的参数分析发现，盖层透气性在排水晚期会明显大于排水早期，反映了盖层含水量对透气性的影响。
使用基于有限元的多物理场模拟软件COMSOL改写水-气二相流的控制方程，考虑空气的压缩性，建立了适合于本文实验研究的COMSOL模型。对细砂覆盖层厚度分别为2cm和5cm的非均质砂柱排水实验进行了数值模拟。COMSOL的模拟结果印证了低渗透介质覆盖条件下真空阻滞效应的存在，但是与实测数据的拟合结果并不理想，COMSOL低估了砂柱中的真空，相应的高估了累计排水量，具体原因还有待进一步研究。
The change of groundwater level would drive air flow in the vadose zone and theair flow will interact with groundwater flow. This kind of coupling betweengroundwater level change and air flow will become more apparent when theunconfined aquifer is covered by a low-permeability layer. Intake and drainageexperiments were carried out in a double-layer sand column with fine sand (thicknessis2-7.5cm) over coarse sand, which used the thin find sand layer as thelow-permeability confining layer.
As the decline of the water level in the drainage experiment, significant vacuumcan be generated in the vadose zone of the column with a finer layer on the top and airflows from atmosphere into the column. Because of the effect of the vacuum in thevadose zone, water is sucked and the drainage speed is significantly smaller than thatwithout the fine sand layer, confirming the existence of vacuum decay effect. Incontrast to the drainage experiment, when the water level uplifts in the intakeexperiment, air pressure in the vadose zone increases and air flows outward. Thechange of vadose zone air pressure with time both shows a single peak in the drainageand intake experiments. The air pressure increases quickly in the earlier stage of theexperiments, reaches a maximum and gradually becomes zero. The maximum isaffected by the thickness of the fine sand layer, the more the thickness of the confininglayer, the value of the maximum is more essential and the time reach to the peak ismore longer.
Based on the Darcy flow of groundwater in the saturated zone and the linearseepage of compressible air in the vadose zone, a simplified kinetic model is proposedto explain the air-water movement in the sand column and Runge-Kutta algorithm wasused to solve the model, the observed vadose zone air pressure was reproduced and themain kinetic characteristics of the sand column experiments is revealed. Simulationresults show that the maximum air pressure in the vadose zone increases nonlinearlywith the increasing of the thickness of the low-permeability layer. Based on thesimplified kinetic model, the approximate analytical solutions are obtained for the early and late periods of the drainage. In this paper, the kinetic model only need asmall amount of medium parameters to explain the changes of the pressure in thevadose zone and the location of the surface of saturation. The pressure in the vadosezone depends on the saturated permeability of the coarse, the air-permeability of thefine sand and its thickness. It is also found through parameter identifications that theair-permeability in the late period is significantly higher than that in the early period,which inflects the influence of water-content in the confining layer. This study showsthe significance of air flow in groundwater drainage from sands if a low permeabilitysoil exits on the top.
Using the multi-physics simulation software COMSOL which based on the finiteelement method to simulate the drainage experiments of unconfined aquifer coveredby2cm or5cm fine sand layer on the top.Considering the compression of the air torewrite the air-water–two-phase flow control equation, the model suitable for theexperiment studied in this paper is established. The simulation results confirm thatvacuum can be generated in the vadose zone when the unconfined aquifer is coveredby a low-permeability layer. But the simulation results do not fit very well with themeasured data, the vacuum value is underestimated and the cumulative outflow isoverestimated so the specific reason need further studied.
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