INTRODUCTION TO MOFAT

MOFAT for Windows includes a graphical pre-processor, Mesh Editor and post-processor with on-line help.

Simulate multiphase (water, oil and gas) flow and transport of up to five non-inert chemical species in MOFAT. Model flow of light or dense organic liquids in three fluid phase systems. Simulate dynamic or passive gas as a full three-phase flow problem. Model water flow only, oil-water flow, or water-oil-gas flow in variably-saturated porous media. MOFAT achieves a high degree of computational efficiency by solving flow equations at each node (on the finite-element mesh) only for phases that are undergoing changes in pressures and saturations above specified tolerances using a new adaptive solution domain method. Therefore, if NAPL is absent or exists at a residual saturation, MOFAT will locally eliminate those flow equations. MOFAT analyzes convective-dispersive transport in water, NAPL, and gas phases by assuming local equilibrium or nonequilibrium partitioning among the fluid and solid phases. MOFAT considers interphase mass transfer and compositional dependence of phase densities. A concise but accurate description of soil capillary pressure relations is used which assures natural continuity between single-phase, two-phase and three-phase conditions. The user can estimate soil properties using SOILPARA.

MOFAT FEATURES

• Simulate multiphase transport of up to five non-inert chemical species.
• Model flow of light or dense organic liquids in three fluid phase systems.
• Solve flow equations for phases exhibiting transient behavior using the ASD method.
• Simulate dynamic or passive gas as a full three-phase flow problem.
• Use a three-phase van Genuchten model for saturation-pressure-permeability relations.
• Handle flux type, specified head, specified concentration or mixed type boundary conditions.
• Consider hysteresis in oil permeability due to fluid entrapment.
• Model water flow only, coupled oil-water flow, or water-oil-gas flow.

MOFAT TECHNICAL INFORMATION

This section is extracted from the EPA Document EPA/600/2-91/020 May 1991:

MOFAT: A TWO-DIMENSIONAL FINITE ELEMENT PROGRAM FOR MULTIPHASE FLOW AND MULTICOMPONENT TRANSPORT

ABSTRACT

This report describes a two-dimensional finite element-model for coupled multiphase flow and multicomponent transport in planar or radially-symmetric vertical sections. Flow and transport of three fluid phases - water, nonaqueous phase liquid (NAPL) and gas - is considered by the program which also handles cases in which gas and/or NAPL phases are absent in part or all of the domain at any given time. The program will simulate flow only or coupled flow and transport. The flow module can be used to analyze two-phase flow of water and NAPL in a system with gas present but at constant pressure or explicit three-phase flow of water, NAPL and gas at variable pressure. The transport module can handle up to five components which partition among water, NAPL, gas and solid phases assuming either local equilibrium interphase mass transfer or first-order kinetically controlled mass transfer. The governing equations are solved using an efficient upstream-weighted finite element scheme. Required input for flow analyses consists of initial conditions, soil hydraulic properties, fluid properties, time integration parameters, boundary condition data and mesh geometry. Three-phase permeability-saturation-capillary pressure relations are defined by an extension of the van Genuchten model which considers effects of oil entrapment during periods of water imbibition. For transport analyses, additional input data are porous media dispersivities, initial water phase concentrations, equilibrium partition coefficients, component densities, diffusion coefficients, first-order decay coefficients, mass transfer coefficients (for nonequilibrium analyses) and boundary condition data. Time-dependent boundary conditions for the flow analysis may involve user-specified phase heads at nodes or phase fluxes along a boundary segment with zero flux as the default condition. For transport analyses, initial conditions are specified in terms of equilibrium water phase concentrations of each partitionable component. Time-dependent boundary conditions may be stipulated as equilibrium water phase concentrations in the porous medium as prescribed fluxes defined in terms of a specified concentration in the influent liquid or with zero dispersive flux specified. Program output consists of basic information on input parameters, mesh details and initial conditions plus pressure heads, saturations and velocities for each phase at every node for specified output intervals. For transport analyses, the phase concentrations at each node are output at each printout interval.

MOFAT WINDOWS INTERFACE

What is the MOFAT Pre-processor?

The MOFAT pre-processor was designed to write data files and store data for MOFAT numerical model runs. The pre-processor works in concert with the Mesh Editor and with the post-processor to make a complete graphical interface to the US EPA's Multiphase Organic Flow and Transport simulator. There are four distinct programs in MOFAT: the MOFAT numerical model, the MOFAT Pre-processor, the Mesh Editor, and the Post-processor. Each rely on one another for data input and output. For instance, the Mesh Editor is a dumb program that doesn't care whether it is working with MOFAT, or with BIOF&T, MOVER or MARS. It simply reads in a template file that lets it know what associations (for example, soil types, boundary conditions, etc.) are legal for MOFAT and what associations have been entered in the pre-processor for that mesh file. This arrangement allows for development of the Mesh Editor to include more features in the future without disturbing the main MOFAT program.

The MOFAT Pre-processor allows for entry of Control Parameters (for example, whether or not transport will be solved for in the run), Initial Conditions (initial heads of water and oil), Species Properties for up to five species, Fluid Properties, Transport Properties, Boundary Conditions, and Material Properties for up to ten soils. Many values entered in the pre-processor are used in the Mesh Editor. Values like non-uniform water heads are defined in the pre-processor, then later assigned to nodes in the Mesh Editor. Material Properties, Boundary Conditions, non-uniform oil heads, and individual node printouts are all assigned to nodes or edges in the Mesh Editor.

The MOFAT Pre-processor also has a run module for writing input data files and executing the MOFAT numerical model. It is important to understand that the pre-processor does not read MOFAT numerical model data files, it only writes them. The pre-processor and Mesh Editor use text files to save project data to disk and to open projects. After a data file for the numerical models has been written, you can change individual control parameters but these changes will not be reflected in the pre-processor.

The MOFAT Pre-processor Interface

After clicking on the MOFAT pre-processor for the first time, a setup notebook will appear on the screen. This notebook acts as a binder for all MOFAT data files, for links to the Mesh Editor and a project mesh, for writing input files and running the numerical model, and for cue cards and additional help. This notebook is available at any time by selecting Tools|Setup from the main menu or Tools|Runner. To prevent this notebook from automatically displaying every time the pre-processor is run, click off the Show Again check box on the Cue Cards page.

Note: all variable names like ITRN are in capital letters and match exactly the names used in the MOFAT FORTRAN source code.

The MOFAT pre-processor runs under Windows 3.X, Windows 95 and Windows NT. Currently all MOFAT programs are 16-bit, so there is no speed or multitasking advantage to running the programs under a 32-bit operating system. For most problems, the pre-processor will run well under 4 MB of RAM. When dealing with large finite element meshes, you will find that reading and writing files, and working with the Mesh Editor will greatly slow down under 4 MB of RAM. Also, the numerical model will not run under 4 MB of RAM for larger problems.

The MOFAT pre-processor uses a commonly-used tabbed notebook interface to allow quick editing of input files. The main program has two sets of tabs, one along the bottom which separates major sections of the interface, and, on some of the large notebook pages, tabs along the top that separate subsections to make the most use of available screen space. For example, clicking on the bottom tab "Boundary Schedules" takes you to the boundary schedule notebook. Here there is a tabbed notebook for editing type 1 and type 2 boundary schedules.

MOFAT for Windows does not have a pre-processor checklist or linear approach for a user to step through each necessary data input value. MOFAT starts up with default values selected for logic switches (for example, ITRN - solve for transport is default set to off), but data values are set to zero (for example, TH - time weighting factor). The MOFAT numerical model will not run if these data values are not input by the user. While the pre-processor will check to be sure that at least one soil type has been entered before running the numerical model, it will not check control parameter data values. While this method may lead to some misunderstanding for the beginning modeler, most who use the preprocessor more than a few times will appreciate not having dialog boxes pop up every time they don't want to enter a value for a variable.

Opening and Saving Projects in MOFAT

A MOFAT project is similar to a multi-table database. Data for the pre-processor is stored in ASCII text files as is data for the Mesh Editor and Post-processor. When a new project is created and saved to file, the pre-processor prompts you for a project name (an .mfp file extension is the default type). The pre-processor will then create sub-data files for species data and materials data.

What is the MOFAT Mesh Editor?

The Mesh Editor in MOFAT was designed to work with these numerical models to create and edit finite element meshes. The Mesh Editor allows designing irregular quadrilateral meshes in two dimensions and hexahedral meshes in three dimensions. MOFAT version 2.2 only uses 2D rectangular elements. Working with a numerical model pre-processor, the Mesh Editor provides a graphical interface for assigning properties to a mesh such as initial concentrations of contaminants, soil properties, boundary conditions, etc.

Speed Buttons

Pan

Panning allows the mesh to be moved in the Mesh Editor window. This puts the Mesh Editor into its pan state. You can then hold down the left mouse button and move the mesh on the screen. Letting go of the left mouse button "drops" the mesh in place. Nodes cannot be selected individually in this state.

Rotate

The rotate button is the leftmost red button on the tool bar. It puts the Mesh Editor into a rotate state where the mouse can be used to rotate the mesh in three dimensions. The mouse will rotate the mesh on the XYZ axis intersection. Rotating the Mesh Editor takes a little getting used to, but if you ever get lost the handy X-Y, Y-Z, and Z-X buttons will snap the mesh back into place.

Editing

The rightmost red button called Node Control puts the Mesh Editor in its editing state. Now a node or group of nodes can be selected to have values assigned to them or be moved in the X, Y and/or Z direction. To edit associations (i.e., soil type, recharge zones, type-1 boundary conditions, etc.) for a node, select the node, then right-click the mouse. This will bring up a list of available associations. Clicking on an association will bring up a secondary list of associations that can be assigned to the node.