Description |
AIR is a numerical model for three-dimensional simulation of compressible or incompressible gas flow in the unsaturated zone. The pressure based governing flow equation is solved using a node-centered finite difference method using rectangular prismatic cells. The resulting set of algebraic equations are solved with a preconditioned conjugate-gradient method (PCG). The model assumes that: the soil moisture distribution is constant in time; the flow is single-phase gas flow only; the free ground-water surface (water table) is an impervious boundary for gas and its location does not change with the air pressure field; and flow is laminar and can be described by Darcy's law. Pathlines (3D) are computed by particle tracking using Euler integration. The model can handle heterogeneous and anisotropic soil permeability conditions, injection/pumping wells with constant flow rates, constant flux boundaries, and fixed pressure boundaries. The model has the option to take compressibility into consideration for pressures greater than 40,000 Pa. The model supports a grid of up to 25 x 25 x 10 cells and includes a detailed air flow balance. |