  Self-organized large-scale flow structures occur in a wide range of turbulent
flows. Yet, their emergence, dynamics, and interplay with small-scale
turbulence are not well understood. Here, we investigate such self-organized
turbulent superstructures in three-dimensional turbulent Kolmogorov flow with
large-scale drag. Through extensive simulations, we uncover their
low-dimensional dynamics featuring transitions between several stable and
meta-stable large-scale structures as a function of the damping parameter. The
main dissipation mechanism for the turbulent superstructures is the generation
of small-scale turbulence, whose local structure depends strongly on the
large-scale flow. Our results elucidate the generic emergence and
low-dimensional dynamics of large-scale flow structures in fully developed
turbulence and reveal a strong coupling of large- and small-scale flow
features.
