Complex eddy-viscosity: a three-dimensional effect
Résumé
It is shown that the response of spatially periodic three-dimensional flow to weak large-scale momentum perturbation may involve a complex eddy-viscosity. This is in contrast to the eddy-diffusivity which is always positive and to the eddy-viscosity of two-dimensional flow which is real (but not necessarily positive). A simple ''two-and-a-half dimensional'' flow with a complex eddy-viscosity is (sin(x + y - z), 0, sin(x + y - z) + (1/ √3) sin x).