Mixing of weakly and strongly diffusive passive scalars in isotropic turbulence

Scalar mixing in isotropic turbulence with Schmidt numbers ranging from 0.04 up to 144 is studied by numerical simulation. The scalar spectra show a k(-l) range for Sc > 1. Also for Sc < 1 the spectra appear to have a short K-l range. The scalar dissipation spectra are universally self-similar under Batchelor scaling. The form of the dissipation spectra agrees well with the Kraichnan model. The scalar gradient aligns with the direction of the compressive strain both for Sc < 1 and for Sc > 1. The local fine-scale structure of the velocity field appears to have an effect on the scalar mixing process for all Schmidt numbers.