地下水位升降与空气流耦合的砂柱实验及其模拟

Sand Column Experiments and Modeling Study on Coupling between Groundwater Level Change and Air Flow

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更加明显.利用细砂盖层作为低渗透介质与厚层粗砂组合成双层结构砂柱,进行了注水和排水实验研究.在排水实验中砂柱水位面下降,包气带会形成显著真空,吸取外界大气.在注水实验中砂柱水位面抬升,包气带气压增大,空气向外流动.包气带气压随时间的变化曲线呈单峰形式,且受到细砂盖层厚度的影响.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,重现了观测到的包气带气压变化特征.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趋势.

The change of groundwater level drives air flow in the vadose zone,and the air flow in turn interacts with groundwater flow.This kind of coupling between groundwater level change and air flow becomes more apparent when the unconfined aquifer is covered by a low-permeability layer.Intake and drainage experiments were carried out in a double-layer sand column with fine sand over coarse sand,using the thin find sand layer as the low-permeability confining layer in this study.As the the water level declines in the drainage experiment,significant vacuum can be generated in the vadose zone and air flows from atmosphere into the column.In contrast to the drainage experiment,when the water level uplifts in the intake experiment,air pressure in the vadose zone increases and air flows outward.The change of vadose zone air pressure with time shows a single peak and is affected by the thickness of the fine sand layer.Based on the Darcy flow of groundwater in the saturated zone and the linear seepage of compressible air in the vadose zone,a simplified kinetic model is proposed to explain the air-water movement in the sand column and Runge-Kutta algorithm was used to solve the model,the observed vadose zone air pressure was reproduced.Simulation results show that the maximum air pressure in the vodose zone increases nonlinearly with the increasing of the thickness of the low-permeability layer.