  This paper is concerned with Darcy's law for an incompressible viscous fluid
flowing in a porous medium. We establish the sharp $O(\sqrt{\e})$ convergence
rate in a periodically perforated and bounded domain in $R^d$ for $d\ge 2$,
where $\e$ represents the size of solid obstacles. This is achieved by
constructing two boundary correctors to control the boundary layers created by
the incompressibility condition and the discrepancy of boundary values between
the solution and the leading term in its asymptotic expansion. One of the
correctors deals with the tangential boundary data, while the other handles the
normal boundary data.
