Diffusive cut-off scale of fractal surfaces in chaotic mixing

Results concerning the fractal properties of isovalue surfaces of a passive , diffusive scalar field being advected by a three-dimensional, steady how exhibiting chaotic streamlines are presented. For all the Peclet numbers an d the flow fields studied, the mixing efficiency is shown to be related to the extent of the so-called advective zone for which the similarity dimensi on D-s is optimal in the sense of mixing (D-s = 3). For global chaos, the e ffective mixing time can be related to the diffusive cut-off scale which bo unds this zone. For partial chaos, this cut-off scale corresponds to the si ze of the regular region which limits asymptotically the mixing.