INTRODUCTION TO MODAIR
MODAIR and its companion code P3DAIR answer questions critical to SVE system design: What will be the airflow rate to an extraction well for a given applied pressure? What will be the resultant pressure distribution in the soil? What area will the capture zone of the extraction system encompass?
MODAIR is a software package for modeling airflow in the unsaturated zone, including airflow to an extraction well. MODAIR is based on MODFLOW, the popular ground-water modeling code developed by the U.S. Geological Survey (McDonald and Harbaugh, 1988). The MODAIR software package consists of: MODFLOW; AIRGEN, a preprocessor which prepares the input files so that airflow simulations can be made using MODFLOW; and PMAIR, a postprocessor which enables users to plot two-dimensional air-pressure distributions. MODFLOW packages supported are BAS, BCF, WEL, CHD, SIP, and OPC. The three types of boundary conditions at the ground surface are: constant pressure, no-flow, and variable permeability. MODAIR provides maximum flexibility for data input and for the types of conditions that can be simulated. It allows data to be entered in units preferred by the user, even mixed types for length, time, mass, pressure and air permeability. It also allows simulation of either steady-state or transient conditions. The user can prepare output files of the pressure distributions across a layer or a vertical cross section along a row or column. MODAIR is recommended for use with P3DAIR, a software package for advective transport in the unsaturated zone.
MODAIR KEY FEATURES
MODAIR FREQUENTLY-ASKED QUESTIONS
Steady-state or transient simulations
Discharge-controlled or pressure-controlled wells
Time-varying boundary conditions
Capture zone analyses of SVE wells
Advective vapor transport simulations
How is MODFLOW adapted for airflow simulations?
One of the fundamental differences between ground-water flow and airflow is that the partial-differential equation describing airflow is nonlinear, if the flow is transient. To apply MODFLOW for airflow, the airflow equation is linearized with respect to pressure-squared based on the assumption that the pressure deviation from its ambient pressure is relatively small. Guo (1996) indicates that the errors induced by linearization are acceptable if the linearization is made with respect to pressure - squared in the range of pressure changes commonly encountered in the field.
How does MODAIR differ from Air3D?
Air3D is a computer code which also adapts MODFLOW to simulate airflow in the unsaturated zone (Joss and Baehr, 1995). Air3D and MODAIR differ primarily in their treatment of vapor extraction wells. Vapor extraction wells are represented as constant pressure cells in Air3D. MODAIR's treatment of extraction wells allows for more flexibility: wells can be specified as either volumetric-flow-rate-controlled or pressure-controlled. Volumetric-flow-rate-controlled extraction wells are simulated using the WEL (well) package. Extraction rates may change with time from one stress period to another stress period. A pressure-controlled extraction well or a horizontal well can be represented using the CPB (time-varying constant pressure boundary) package in which the pressure at wells is specified.
How does MODAIR handle different units in airflow simulations?
Keeping consistent units in airflow simulations can sometimes be frustrating. Unlike the units in ground-water simulations (time and length), airflow simulations involve several more parameters whose units have to be specified: viscosity, ambient pressure, etc. Their units can be very different, for example, atmosphere atm mmHg and in H2O for air pressure. MODAIR has a panel specially designed to help users deal with units. As long as users select one unit for one type of input parameter, MODAIR will automatically convert different types of units into an internally consistent centimeter-gram-second system.
Is the MODFLOW in MODAIR different from the USGS MODFLOW?
The executable code MODFLOW.EXE accompanies MODAIR and is almost identical to the USGS version of MODFLOW except that the MODFLOW.EXE in MODAIR supports the CPB (time-varying constant pressure boundary) package, which is similar to the CHD (time-varying constant head boundary) package in ground-water simulations.
What MODFLOW packages are included in MODAIR?
MODAIR includes the following packages of MODFLOW:
BAS (Basic Input Parameters)
BCF (Block-Centered Flow Package)
WEL (Well Package)
CPB (Time-Varying Constant Pressure Boundary Package)
OPC (Output Control Package)
SIP (Strongly-Implicit Procedure)
Is MODAIR a Windows program or a DOS program?
MODAIR contains both DOS and Windows programs. When MODAIR is executed, a Windows program called AIRWIN is executed first. AIRWIN provides a series of input Windows and dialog boxes for data entry. Users can also execute MODFLOW or other programs within AIRWIN. AIRWIN was developed using Borland C++ 4.5 and OWL (Object Windows Language) 2.5, and compiled as 16-bit codes on the platforms of MS Windows 3.11. It is also compatible with Windows 95 and Windows NT.
MODAIR Requirements: Pentium with 2 MB RAM. Requires SURFER or other graphics package for graphical output.