I've modeled vortex shedding using a time step that is 1/30 of the period of shedding(Fluent manual suggests at least 1/20) with good results. For the Re #'s and Str #'s I've been looking at this corresponds to a step of approx. 0.03 s. Yet, I've read several postings where someone is modeling a similar phenomenon, shedding from a cir. cylinder for example, and they're using a delta t aound 0.00001. Why so small?

The period of shedding and time step required for flow around cylinder in narrow channels is much less. May be these postings are for these cases.

Sounds reasonable. I've actually modeled a thin plate in a channel for different blockage ratios and found that the Str# does increase significantly, but I didn't have to decrease my time step size quite so small to capture 1/30th of the period. Thanks