= Resident-weighted transit time in a layered flow
{title2=$\langle\tau\rangle_{\rm resident}=\langle L/v\rangle$}
Without transverse exchange, a particle chosen uniformly from pore volume samples each streamline's transit time with resident-volume weight. This differs from the <flux-weighted residence time in a porous layer>, which samples at the inlet proportionally to $v$. For constant <porosity> and a linear positive <velocity> $v=v_0+(v_1-v_0)y/H$, the two means are $L\log(v_1/v_0)/(v_1-v_0)$ and $2L/(v_0+v_1)$, respectively. Their limits agree for uniform <velocity>.
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