A numerical study of local isotropy of turbulence

The degree to which local isotropy applies is examined using databases obtained from numerical simulations of a variety of turbulent flows at low Reynolds numbers. Simulations of a turbulent channel flow were used to obtain most of the results but simulations of other flows, e.g. homogeneous isotropic turbulence, a homogeneous shear flow, and a turbulent mixing layer, have also been considered. Attention is mainly given to the high wave number part of the vorticity and temperature derivative spectra. The ratio of these spectra and their isotropic values depends on the particular quantity considered, the departure from isotropy being more pronounced for the temperature derivative than for the vorticity. When the Kolmogorov-normalized wave number is sufficiently large, isotropy is approximately satisfied provided the (Kolmogorov-normalized) mean strain rate is sufficiently small. This result appears to be independent of the quantity considered and of the Reynolds number.