  We study the viscous dissipation in pipe flows in long channels with porous
or semipermeable walls, taking into account both the dissipation in the bulk of
the channel and in the pores. We give simple closed form expressions for the
dissipation in terms of the axially varying flow rate $Q(x)$ and the pressure
$p(x)$, generalizing the well known expression $\dot W=Q\,\Delta p$ for the
case of impenetrable walls with constant $Q$ and a pressure difference $\Delta
p$ between the ends of the pipe. When the pressure $p_0$ outside the pipe is
constant, the result is the straightforward generalization $\dot W=\Delta
\left[(p-p_0) \,Q\right]$. Finally, applications to osmotic flows are
considered.
