Estimating Topological Entropy from the Motion of Stirring Rods

Stirring a two-dimensional viscous fluid with rods is often an effective way to mix. The topological features of periodic rod motions give a lower bound on the topological entropy of the induced flow map, since material lines must ‘catch’ on the rods. But how good is this lower bound? We present examples from numerical simulations and speculate on what affects the ‘gap’ between the lower bound and the measured topological entropy. The key is the sign of the rod motion's action on first homology of the orientation double cover of the punctured disk.