STRUCTURE OF THE STRETCHING FIELD IN CHAOTIC CAVITY FLOWS

Stretching of material elements in time-periodic cavity flows is inves tigated numerically. The spatial structure of the stretching field is determined not only by nonchaotic islands and by unstable manifolds of hyperbolic periodic points, but also by singularities of the flow fie ld at the cavity corners. For the short time scales interesting to mos t mixing applications, regions of very high stretching (good local mix ing) are determined by unstable manifolds that pass close to the corne rs of the cavity. Low stretching (poor local mixing) regions are usual ly found both inside and near islands. In some cases, however, the uns table manifolds wrap themselves around the islands, preventing the for mation of segregated low stretching subregions within the chaotic regi on.