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' APPENDIX D <br /> DARCY FLOW MODEL <br />' This appendix includes a description of the Darcy flow model The steady-state form of Darcy's <br /> equation as applied to radial flow conditions is used to model air flow through the subsurface and <br /> to estimate the soil parameters. <br /> DARCY FLOW MODEL CONDITIONS <br /> Steady-state conditions prevail, after a transient period, for a well extractingdrama e <br /> a g area <br /> (cell) with a completely open outer boundary The steady-state conditions assume that air <br />' extracted from the soil will be exactly balanced by air entering the soil at the outer boundary <br /> In the Darcy model, the radius of influence is defined as the outer radius of the drainage area <br />' at which point the pressure is always equal to the initial (atmospheric) pressure once steady-state <br /> flow conditions prevail. <br />' GOVERNING EQUATIONS FOR THE DARCY FLOW MODEL <br /> The general form of the compressible-flow radial diffusivity equation is: <br /> I S (p- r ao -- I L(p) (1) <br />' r Sr µz Sr k St z <br />' where p = pressure <br /> r = radius <br /> = porosity <br /> k = permeability <br /> µ = viscosity <br /> z = gas compressibility factor (z=1 for ideal gas flow) <br />' For steady state, the following boundary conditions apply* <br /> F = F, = Pressure at r = r. (outer boundary) <br /> = 0 for all r and t (z = 1) <br /> t St <br /> The general form of Darcy's law for radial flow is: . <br /> q = k 27crh db (2) <br /> µ dr <br /> 1 D-1 <br />