FlowDict Computations
The flow properties and the permeability of porous media can be calculated by FlowDict on 3D material models obtained from 3D image data (µCT, FIB-SEM) or from generation with GeoDict’s generator modules.
FlowDict computes permeability, flow resistivity, and average flow velocity (or pressure drop) across porous media. Flow permeability is an intrinsic property of the structure that strongly depends on its geometry.
Thus, the choice of the physics equations to solve depends not only on how to describe the fluid flow (linear or non-linear, as seen in the Partial Differential Equations chapter) but also on the particular material. When the material is simply made up of empty and solid voxels, as in a porous material, the Stokes equations or the Navier-Stokes equation are appropriate.
When the particles are much smaller than the scale of the structure, for example, in a pleat formed by a metal wire mesh (solid voxels) in empty (voxel) space supporting a filter media (porous voxels with definable permeabilities), the equations Stokes-Brinkman or Navier-Stokes-Brinkman are needed.
If additionally no through-path exists, e.g., in a rock on the macro scale and the permeabilities are low, Darcy Flow must be used.
The flow equations underlying the FlowDict solvers computations apply only when the flow is stationary, not transient (time-dependent flow). To ensure stationary flow conditions at the boundary of the media, for Stokes (-Brinkman) and Navier-Stokes (-Brinkman) the presence of inlet (inflow region) and outlet (outflow region) is fundamental.
The inlet and outlet of a structure can be set directly during the generation of the structure in many GeoDict generator modules.
For structures modeled from imported 3D image data, these regions can be added by embedding empty voxels in the positive and negative flow direction (e.g., Z-direction) using ProcessGeo (Model → ProcessGeo, Process-Embed).
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Important! Note that the length (L) over which the pressure drop is taking place increases with the addition of inflow and outflow regions. For the Stokes flow, when inlet and outlet are present, the permeability tensor is computed over the total length, not only for the structure, and it is not the actual permeability of the material. |
Another possibility would be to add implicit regions for inflow and outflow as Boundary Conditions in the Solver Options.
For structures used to Simulate a Flow Experiment inflow and outflow regions also are needed for accurate results. They must already be defined explicitly in the structure, usually in form of tubes.
For Darcy Flow, no inlet or outlet are needed.
