How to cause flow around a bluff body to go unsteady

Hi all,

I'm simulating turbulent flow in a pipe around a bluff body with time dependent solvers so I can look at the vortex shedding that occurs. I'm solving the RANS equations with the standard k-epsilon turbulence model.

For most bluff body geometries and configurations, I can get the flow to transition to unsteady flow (vortex shedding!), however sometimes the solution converges to a steady state solution (no vortex shedding).

I've been running successful simulations for months now and have been avoiding the ones that won't go unsteady, however I can no longer avoid them. Does anyone have suggestions for simple ways to cause the flow to go unsteady over a bluff body?

Thanks!