We consider the non-stationary incompressible Euler equations in a 2D porou
s medium. We suppose a periodic porous medium, with the period proportional
to the characteristic pore size epsilon and with connected fluid part. The
ow is subject to an external force, corresponding to an in ow. We start fr
om an initial irrotational velocity and prove that the effective filtration
velocity satisfies a transient filtration law. It has similarities with Da
rcy's law, but it now connects the time derivative of the filtration veloci
ty with the pressure gradient. The viscosity does not appear in the filtrat
ion law any more and the permeability tensor is determined through auxiliar
y problems of decomposition type. Using the limit problem, we construct the
correction for the fluid velocity and prove that C-1([0, T]; L-2(Ohm)(2))-
norm of the error is of order epsilon. Similarly, we estimate the differenc
e between the fluid pressure and its correction in C([0, T]; L-1(Ohm)) as C
epsilon.