  This paper is concerned with the validity of the Prandtl boundary layer
theory in the inviscid limit of the steady incompressible Navier-Stokes
equations, which is an extension of the pioneer paper (Y. Guo et al., 2017,
Ann. PDE) from a domain of $[0,L]\times\mathbb{R}_+$ to $[0,L]\times[0,2]$.
Under the symmetry assumption, we establish the validity of the Prandtl
boundary layer expansions and the error estimates. The convergence rate as
$\e\rightarrow 0$ is also given.
