Negative viscosity effect in large-scale flows

This study considers a large-scale flow maintained by a small-scale periodic force field. If the field is essentially anisotropic, the system of small-scale eddies thereby generated turns out to be unstable to long-wave disturbances, i.e., the effective viscosity of the corresponding large-scale flow is negative. If the applied field is sufficiently isotropic, the long-wave instability disappears so that the effective viscosity is positive.