Numerical simulation of water-gas two-phase displacement process in unsaturated granite residual soil

Infiltration of current in unsaturated soil is essentially a two-phase flow problem of water displacing air in the process of infiltration. The accuracy of traditional two-phase flow research methods cannot meet the needs of engineering, and it is not conducive to repetitive research. In contrast, the numerical simulation method of multiphase flow at the meso-scale can better simulate the whole dynamic process of water flooding. Mao Huan, Qren and others have achieved great results in the field of pore meso-scale research, but on the one hand, most of them focus on the study of rock multiphase flow, and there are some differences between the research object and the actual pore structure. On the other hand, the widely used pore network model method cannot directly show the change of particle velocity at any time and cannot present the phase interface movement state. In view of this, in order to reveal the dynamic percolation mechanism of water-gas two-phase displacement of unsaturated granite residual soil, this paper selected undisturbed granite residual soil in Fuzhou as the research object and studied the dynamic characteristics of two-phase displacement of undisturbed soil samples by using industrial CT scanning images and Level Set method. The results show that the Level Set method can properly capture the interface position between two immiscible fluids for meso-scale water-gas two-phase displacement simulation. The water-gas two-phase displacement process has the characteristics of large pore preferential flow, and the ‘low around’ phenomenon can easily appear in the higher ground of porosity roundness in general. The displacement rate is mainly controlled by the tortuosity of the channel, displacement speed is relatively high in the straight and wide channel. There is an obvious phenomenon of ‘preferential passage’, and its seepage time is positively correlated with the first rapid and then slow characteristics, and the maximum and minimum growth rate are 10.77 % and 1.90 %, respectively. The velocity distribution of the pore cross-section is related to the pore structure, and the phenomena of ‘reflux’ and ‘flow around’ cause the displacement velocity to drop sharply, whose decreasing degree can reach 21.62 %. The maximum displacement resistance appears at the hole wall, and the narrower the hole, the greater the resistance. The displacement efficiency is directly proportional to the displacement pressure difference, and the initial pressure growth effect is significant (up to 25.49 %, and only 1.47 % later). The research results can provide a theoretical basis for the study of the water migration mechanism of porous slopes, and also can enrich the theoretical basis of rainfall-induced landslides and are helpful in preventing natural disasters.