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3DFEMFAT

Detailed Description

3DFEMFAT

3DFEMFAT





Introduction

3DFEMFAT is a 3-Dimensional Finite-Element Model of Flow And Transport through Saturated-Unsaturated Media. Typical applications are infiltration, wellhead protection, agriculture pesticides, sanitary landfill, radionuclide disposal sites, hazardous waste disposal sites, density-induced flow and transport, saltwater intrusion, etc. 3DFEMFAT can do simulations of flow only, transport only, combined sequential flow and transport, or coupled density-dependent flow and transport. In comparison to conventional finite-element or finite-difference models, the transport module of 3DFEMFAT offers several advantages: (1) it completely eliminates numerical oscillation due to advection terms, (2) it can be applied to mesh Peclet number ranging from 0 to infinity, (3) it can use a very large time step size to greatly reduce numerical diffusion, and (4) the hybrid Lagrangian-Eulerian finite-element approach is always superior to and will never be worse than its corresponding upstream finite-element or finite-difference method. Because of these advantages, 3DFEMFAT is ideal for applications to large field problems. The special features of 3DFEMFAT are its flexibility and versatility in modeling a wide range of real-world problems.

  • Heterogeneous and anisotropic media.
  • Spatially and temporally-dependent element and point sources/sinks.
  • Prescribed initial condition or the simulated steady-state solution as the initial condition.
  • Five types of boundary conditions for the flow module: (1) prescribed heads, (2) prescribed gradient fluxes, (3) prescribed total fluxes, (4) iteratively determined infiltration, seepage, and/or evaporation boundaries, and (5) river boundaries. All boundary values are allowed to vary with space and time.
  • Four types of boundary conditions for the transport module: (1) prescribed concentrations, (2) prescribed gradient fluxes, (3) prescribed total fluxes, and (4) flow direction-dependent inflow and outflow boundaries. All boundary values are allowed to vary with time and space.
  • Three options (exact, under- or overrelaxation) for estimating the nonlinear matrix.
  • Six options (subregion block iterations, basic point iterations, and four PCG methods) for solving the linearized matrix equations.
  • Two options (consistent and lumping) of treating the mass matrix.
  • Two options (nodal quadrature and Gaussian quadrature) for surface and element integrations.
  • Three adsorption models (linear isotherm, nonlinear Langmuir and Freundlich isotherms) in the transport module.
  • Employ hexahedral elements, triangular prism, tetrahedral elements, or the mixtures of these three types of elements to facilitate the discretization of the region of interest.
  • Automatically reset time step size when boundary conditions or sources/sinks change abruptly.
  • Check the mass balance computation over the entire region for every time step.
Picture

3DFEMFAT Method: The generalized Richards' equation and Darcy's law governing pressure distribution and water flow in saturated-unsaturated media are simulated with the Galerkin finite-element method subject to appropriate initial and four types of boundary conditions. The transport equation is derived based on the principle of conversation of mass. The contaminant transport equation is simulated with either the conventional finite-element methods or the hybrid Lagrangian-Eulerian finite-element method with the adaptive local grid refinement and peak capturing scheme subject to appropriate initial and four types of boundary conditions. Mixed hexahedral elements, triangular prisms, and tetrahedral elements are used to facilitate the discretization of the region of interest.

3DFEMFAT Input: (1) Geometry in terms of nodes and elements, and boundaries in terms of nodes and segments; (2) soil properties including (a) saturated hydraulic conductivities or permeabilities; (b) compressibility of water and the media, respectively; (c) bulk density; (d) three soil characteristic curves for each type of soil or geologic unit which are the retention curve, relative conductivity vs. head curve, and water capacity curve; (e) effective porosity; and (f) dispersivities and effective molecular diffusion coefficient for each soil type or geologic unit; (3) initial distribution of pressure head over the region of interest; (4) net precipitation, allowed ponding depth, potential evaporation, and allowed minimum pressure head in the soil; (5) prescribed pressure head on Dirichlet boundaries; (6) prescribed fluxes of contaminants on Cauchy and/or Neumann boundaries; (7) artificial withdrawals or injections of water; (8) adsorption constants; (9) artificial source/sink of water and contaminants; (10) prescribed concentrations of contaminants on Dirichlet boundaries; (11) prescribed fluxes of contaminants on variable boundaries; and (12) initial distribution of contaminants. All inputs in items 4 through 11 can be time-dependent or constant with time.

3DFEMFAT Output: (1) pressure head, total head, moisture content, and flow velocity over the two-dimensional grid at any desired time; (2) water fluxes through all types of boundaries and amount of water accumulated in the media at any desired time; (3) distribution of contaminants over a three-dimensional grid at any desired time; and (4) amount of contaminants through all boundary segments.

3DFEMFAT Requirements: 486/Pentium (Pentium recommended) with 16 MB RAM and FORTRAN Compiler. Any Workstation, e.g., IBM RS6000, DEC Alpha, Silicon Graphics, Sun SparcStation, and HP 9000 Series.

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