  We investigate by numerical simulations the pattern formation after an
oscillating attractive-repulsive obstacle inserted into the flow of a
Bose-Einstein condensate. For slow oscillations we observe a complex emission
of vortex dipoles. For moderate oscillations organized lined up vortex dipoles
are emitted. For high frequencies no dipoles are observed but only lined up
dark fragments. The results shows that the drag force turns negative for
sufficiently high frequency. We also successfully model the {\it ship waves} in
front of the obstacle. In the limit of very fast oscillations all the
excitations of the system tend to vanish.
