  We study numerically the directed motion of vortices (antivortices) under an
applied ac bias in two-dimensional Josephson junction arrays (JJA) with an
asymmetrically modulated periodic vortex pinning potential. We find that the
ratchet effect in large 2D JJA can be obtained using the RSJ model for the
overdamped vortex dynamics. The rectification effect shows a strong dependence
on vortex density as well as an inversion of the vortex flow direction with the
ac amplitude, for a wide range of high magnetic field around f=1/2 (f being the
vortex density). Our results are in good agreement with very recent experiments
by D.E. Shalom and H. Pastoriza [Phys. Rev. Lett. 94, 177001, (2005)].
