COMPLEX EDDY-VISCOSITY - A 3-DIMENSIONAL EFFECT

It is shown that the response of spatially periodic three-dimensional flow to a 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/root 3)sin x).