Divergence of Laminar Flow With Eddy ???

Peter Kostka

Pardon my ignorance, I'm quite new to CFD, I'm modeling flow downstream of a moving surface (a 3in diameter speaker). Looking at reynolds numbers it seems that the flow should be laminar (u < 5m/s, l < 0.2m), so I have set up a 2D laminar transient model using CFDesign. However after some number of speaker cycles, the flow sets up a small eddy and the solution diverges. I've tried reducing the mesh size and time step. If I run a trubulent case, the solution converges. Am I making a fundamental error ? What is the reason for my laminar solution diverging ? and can it be fixed ? Thank you for your help.