create short pulse to cause instabilities
 

I am trying to model vortex shedding of a 2D cylinder at re 1000. my understanding is that after a while, errors will cause instailities which will intialise the vortex shedding. I have had my model running all night at timestep of 0.05 with 40 iterations per step, and it seems to still be pretty stable. Is there a way to create a pulse to initiate vortex shedding? Many thanks