= Effective permeability
{title2=$k_{\rm eff}$}
<Effective permeability> is a bulk <permeability of a porous medium> that reproduces the total <Darcy flux> through a heterogeneous sample under specified boundary conditions. For a horizontal sample with length $L$, <dynamic viscosity> $\mu$, and positive inlet-to-outlet <fluid pressure> drop $\Delta p$, define $k_{\rm eff}$ by $Q/A=k_{\rm eff}\Delta p/(\mu L)$. The description is useful when individual layers or passages are smaller than the scale of the calculation. It depends on direction, geometry and boundary conditions; it is not generally the <arithmetic mean> of local <intrinsic permeabilities>.
<Layered porous permeability> illustrates the mechanism: when every path must cross a resistant layer, successive <hydraulic resistances> add; when more permeable paths span the whole sample alongside less permeable ones, their discharges add. This is the same organizing idea as series and parallel <resistors>. For fixed <fluid> properties the corresponding <hydraulic conductivities> average in the same way.
Back to article page
