EXPERIMENTAL-STUDY OF THE FINE-SCALE STRUCTURE OF CONSERVED SCALAR MIXING IN TURBULENT SHEAR FLOWS .1. SC-MUCH-GREATER-THAN-1

We present results from an experimental investigation into the fine-sc ale structure associated with the mixing of a dynamically passive cons erved scalar quantity on the inner scales of turbulent shear flows. Th e present study was based on highly resolved two- and three-dimensiona l spatio-temporal imaging measurements. For the conditions studied, th e Schmidt number (Sc = nu/D) was approximately 2000 and the local oute r-scale Reynolds number (Re-delta = u delta/nu,) ranged from 2000 to 1 0000. The resolution and signal quality allow direct differentiation o f the measured scalar field zeta(x, t) to give the instantaneous scala r energy dissipation rate field (ReSc)(-1)del zeta .del zeta(x, t). Th e results show that the fine-scale structure of the scalar dissipation field, when viewed on the inner-flow scales for Sc much greater than 1, consists entirely of thin strained laminar sheet-like diffusion lay ers. The internal structure of these scalar dissipation sheets agrees with the one-dimensional self-similar solution for the local strain-di ffusion competition in the presence of a spatially uniform but time-va rying strain rate field. This similarity solution also shows that line -like structures in the scalar dissipation field decay exponentially i n time, while in the vorticity field both line-like and sheet-like str uctures can be sustained. This sheet-like structure produces a high le vel of intermittency in the scalar dissipation field - at these condit ions approximately 4% of the flow volume accounts for nearly 25% of th e total mixing achieved. The scalar gradient vector field del zeta(x, t) for large Sc is found to be nearly isotropic, with a weak tendency for the dissipation sheets to align with the principal axes of the mea n flow strain rate tenser. Joint probability densities of the conserve d scalar and scalar dissipation rate have a shape consistent with this canonical layer-like fine-scale structure. Statistics of the conserve d scalar and scalar dissipation rate fields are found to demonstrate s imilarity on inner-scale variables even at the relatively low Reynolds numbers investigated.