  We study the motion of an ideal incompressible fluid in a perforated domain.
The porous medium is composed of inclusions of size $a$ separated by distances
$\tilde d$ and the fluid fills the exterior. We analyse the asymptotic behavior
of the fluid when $(a,\tilde d) \to (0,0)$.
  If the inclusions are distributed on the unit square, this issue is studied
recently when $\frac{\tilde d}a$ tends to zero or infinity, leaving aside the
critical case where the volume fraction of the porous medium is below its
possible maximal value but non-zero. In this paper, we provide the first result
in this regime. In contrast with former results, we obtain an Euler type
equation where a homogenized term appears in the elliptic problem relating the
velocity and the vorticity.
  Our analysis is based on the so-called method of reflections whose
convergence provides novel estimates on the solutions to the div-curl problem
which is involved in the 2D-Euler equations.
