Even though the first theoretical example of chaotic advection was a three-dimensional
flow (Hénon 1966), the number of theoretical studies addressing chaos and mixing
in three-dimensional flows is small. One problem is that an experimentally tractable
three-dimensional system that allows detailed experimental and computational investigation
had not been available. A prototypical, bounded, three-dimensional, moderate-Reynolds-number flow is presented; this system lends itself to detailed experimental
observation and allows high-precision computational inspection of geometrical and
dynamical effects. The flow structure, captured by means of cuts with a laser sheet
(experimental Poincaré section), is visualized via continuously injected fluorescent dye
streams, and reveals detailed chaotic structures and chains of high-period islands.
Numerical experiments are performed and compared with particle image velocimetry (PIV)
and flow visualization results. Predictions of existing theories for chaotic
advection in three-dimensional volume-preserving flows are tested. The ratio of two
frequencies of particle motion – the frequency of motion around the vertical axis and
the frequency of recirculation in the plane containing the axis – is identified as the
crucial parameter. Using this parameter, the number of islands in the chain can be
predicted. The same parameter – using as a base-case the integrable motion – allows
the identification of operating conditions where small perturbations lead to nearly
complete mixing.
