  The transition to turbulence in the boundary flow of superfluid $^4$He is
investigated using a vortex--free vibrating wire. At high wire vibration
velocities, we found that stable alternating flow around the wire enters a
turbulent phase triggered by free vortex rings. Numerical simulations of vortex
dynamics demonstrate that vortex rings can attach to the surface of an
oscillating obstacle and expand unstably due to the boundary flow of the
superfluid, forming turbulence. Experimental investigations indicate that the
turbulent phase continues even after stopping the injection of vortex rings,
which is also confirmed by the simulations.
