Saturated-unsaturated Flow And Air-water Two-phase Flow In A Vertical Column Drained At Its Bottom At Constant Head

The unsteady-state drainage of water from a vertical sand column with and without a finer layer on the top was studied theoretically and experimentally to investigate the air flow generated by the finer layer. Both saturated-unsaturated flow drainage experiment without the fine sand layer on the top of the coarse sand and air-water two-phase flow drainage experiment with the fine sand layer were performed. The sand column, saturated at its lower portion and initially in the condition of hydrostatic equilibrium, is drained at its bottom at constant head. The results show that significant vacuum can be generated in the vadose zone of the column with a finer layer on the top. The vacuum increases quickly in the earlier stage of the drainage, reaches a maximum and gradually becomes zero. The maximum vacuum is 2.58 kPa when the initial hydraulic head difference is 30 cm. Because of the effect of the vacuum in the vadose zone, water is sucked and the cumulative outflow from the column with the finer layer is much smaller than without the layer during most of the drainage process.Based on the mixed-form of the Richards equation and a finite difference scheme, a VBA program is developed to perform the numerical simulation of the saturated-unsaturated flow drainage experiment. Satisfactory agreement is achieved. Theoretical analyses based on the VBA program show that there is a substantial amount of water in the vadose zone at earlier times of drainage. As time progresses, the water saturation profile becomes closer to the steady-state profile, which corresponds to the soil-water characteristic curve of the coarse sand. The total head in the upper portion of the column is higher than the head at the constant head boundary even at the end of drainage.Ordinary differential equations (ODE), which require only saturated hydraulic properties of the porous media, are derived to predict the location of the surface of saturation and vacuum in the vadose zone in air-water two-phase flow. The solutions of ODE match very satisfactorily with the experimental data and give better results than TOUGH2. Theoretical analyses based on solutions of ODE show that the surface of saturation in the column is much higher than the water table during most of the drainage process because of the effect of vacuum. The vacuum in the vadose zone is closely related to the falling rate of the surface of saturation. Theoretical analysis based on TOUGH2 show that the saturation profile is substantially different from the drainage experiment without the finer layer on the top.