Hydrodynamic Irreversibility in Creeping Flow.

Using experiments as well as lattice Boltzmann and finite difference simulations we study creeping flow in a Hele-Shaw cell at Reynolds numbers below unity. We describe an ``echo'' technique that is very sensitive to effects of hydrodynamic irreversibility. By combining experimental and numerical studies we show that the irreversibility observed in the experiments is due to inertial forces. As a by-product we validate the numerical techniques used---in particular the recently introduced lattice Boltzmann model.