Fluid mixing (stretching) by time periodic sequences for weak flows

Abstract
The stretching of material lines in time periodic sequences of ‘‘weak’’ flows (i.e., characterized by a linear stretching for long times) in which streamlines change at the end of each periodic unit is quantified in terms of a mixing efficiency. Two different flows and two different strain distributions give nearly identical results in terms of the mixing efficiency and indicate the existence of an optimal operating condition for maximum stretch. The results are relevant to fluid mixing in chaotic flows.