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ρf is the fluid density.
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εp is the porosity.
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θs is the solid matrix volume fraction.
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ρs is the solid matrix density.
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keff is the effective thermal conductivity (a scalar or a tensor if the thermal conductivity is anisotropic).
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q is the conductive heat flux.
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u is the velocity field, either an analytic expression or computed from a Fluid Flow interface. It should be interpreted as the Darcy velocity, that is, the volume flow rate per unit cross sectional area. The average linear velocity (the velocity within the pores) can be calculated as uf = u/εp, where εp is the fluid’s volume fraction, or equivalently the porosity.
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Q is the heat source (or sink). Add one or several heat sources as separate physics features.
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A good estimate is given by the weighted geometric mean of kf and ks, as long as kf and ks are not too different from each other
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