Simulation Study Of Two-Phases(Water And Air) Flow In Unsaturated Zone Under Infiltration Condition

The study of two phases flow(water and air)is a more concerned problem in unsaturated zone under infiltration condition. Most studies are only about water phase flow,but the air phase flow studies are less, or neglect the influential action of air phase directly. In fact, the water phase flow of unsaturated zone is a mutual displacement and complex process in soil pore, especially under high intensity precipitation and pressure underground infiltration in arid and semi-arid area, because the air phase can't dissipate fully under the influential action of infiltration, the air pressure sharp increases, which can't be neglected for water flow.This paper summarized and analyzed previous achievements, mainly studied the water and air flow in unsaturated zone, according to physical simulation experiment,numerical simulation and theoretical analysis,the author revealed the change of water and air pressure,dynamic fields characteristics and influencing factors, and employed measured data to identify and correct two phases coupled model of water and air and used this coupled model further revealed the transport mechanism of water and air Gaining the following results and conclusions:(1) Using measured data to identify and correct two phases coupled model of water and air, from the fitting of gradient fields and deceleration fields we could see that the macro-effects of whole morphology were better, which basically reflected the migration rule of two phases flow.(2)According to the change of water and air saturation and air pressure,we further verified the relationship between air phase and water phase:there was a negative correlation between air pressure and water content(water saturation),a positive correlation between air phase and negative head (air saturation).(3)According to the analysis of model's sensitivity, the sensitivities of intrinsic permeability,relative permeability and capillary pressure were higher for calculating value of air pressure, the sensitivities of density and porosity were lower.(4)Water flow was main vertical movement under ifiltration, but the vertical flow infiltration rate was higher than horizontal; Air phase would breakthrough overlying saturated zone and escaped from soil surface when air was compressed by infiltration, air was main about vertical movement.However, air phase would transport to water level by the pushing of wetting front, because of the blocking effect of water level, air was main horizontal movement, besides, different lateral boundary conditions had a very important effect on draining of air(5)Air pressure values was main related to precipitation intensity in unsaturated zone. Different precipitation intensities had a different effect on air pressure, the precipitation intensity was higher, the water flow infiltration rate was faster, so the variation of air pressure was obvious. The variation of precipitation intensity resulted in the changes of water flow infiltration rate and it was the most direct factor affected the air pressure in unsaturated zone.(6)According to comparative analysis between the simulation results of profile air pressure and physical simulation experiment results, the variation characteristics were divided into there areas:air pressure stable area, it was near the soil surface, air pressure was basically steady; Air pressure variation area:it distributed the depth between 20cm and 60cm, air was compressed and air pressure sharp increased in a state of fluctuation, this was the main area which mutual displacement of two phases occurred; Air pressure low-growth area:it distributed above the water level, the air pressure was slowly increased.(7)According to the results of water pressure between water and air coupled model and single water flow, the results of coupled model were close to measured values and the fitting errors were less than 5%, but the results of single flow model were too large. According to the computation of water level recharge flux, when the precipitation reached 120cm per hour, the results deviation of two kinds of models was almost 240cm3 per minute; When the precipitation was 0.24cm per hour, the results deviation was about 20 cm3 per minute.The higher precipitation intensity, the larger deviation of both computation results. It showed that the influential action couldn't be neglected under high intensity precipitation. When the precipitation was less than 0.24cm per hour,the coupled model results of bottom flux were nearly equal, and the results deviation of bottom flux between coupled model and single model were too small, and the relative error was about 10%. So it could be neglected for the influential action of air phase when the precipitation intensity was less than 0.24cm per hour.