Effects of shear on the turbulent diffusivity tensor

Earlier measurements of some components of the turbulent diffusivity tensor D(ij) in a nearly homogeneous turbulent shear flow with constant mean temperature gradient parallel to the mean shear are now supplemented with measurements of additional components in the same flow but with a mean temperature gradient perpendicular to both mean velocity and mean shear. A quasi-Lagrangian analysis provides expressions for all components of D(ij) and confirms the expectation that D(ij) is nondiagonal and nonsymmetric. The analysis also suggests the possibility that D(11) and D(21) might become positive at sufficiently large values of mean shear. Independent estimates of D(ij) based on a numerical simulation of homogeneous shear flow as well as on a rapid distortion-type analysis are also reported.