time dependant patching??

Glenn

hi.. Im modelling oscillating flow over a 3D circular cylinder and I would like to investigate an instability in the axial-direction. I want to 'kick' the solution into to doing this. Is it possible to induce a axial velocity for a set amount of time??. Ive tried patching but this only sets a axial velocity for an instant... And I dont think a udf using a momentum source would be good because this could cause inaccuracies in the continuity (am I correct saying that??). So what Im really looking for is a sort of patching that will be time dependant so that for example I could induce a axial velocity for ..say..1/2 cycle of the oscillating flow. Any ideas??