Impact of large-scale flow states on small-scale 3D turbulence

Turbulent flows often feature an intricate interplay between anisotropic, even quasi-two-dimensional, large-scale features and three-dimensional small-scale fluctuations. To systematically study the relation between large-scale flow structures and small-scale turbulence, we investigate a conceptually simple shear flow -- a generalized turbulent Kolmogorov flow. The flow is forced with a single shear mode and is subject to large-scale friction, which effectively allows to control transitions between different large-scale states. We present a detailed investigation of the energetics of the system, and we find that the excitation of three-dimensional small-scale turbulence provides a dissipation channel for the large scales in the sense of classical eddy viscosity. We show that the energy transfer rate depends on the large-scale flow state, which establishes a direct coupling between the large scales and smaller-scale flow features such as small-scale intermittency.