  We study, by means of extensive direct numerical simulations, the turbulent
flow produced by a two-dimensional cellular forcing in a cubic box with
periodic boundary conditions. In spite of the strong anisotropy of the forcing,
we find that turbulence recovers almost complete isotropy at small scales.
Nonetheless, the signature of the forcing remains in the mean flow (averaged
over time and over the homogeneous direction) and this allows to introduce a
friction factor, whose dependence on the Reynolds number is investigated. We
further find that the flow is characterized by large temporal fluctuations of
the total energy, as a consequence of the exchange between the forced mean flow
at large scales and turbulent fluctuations at small scales. Such temporal
fluctuations produce a correction to the energy spectrum that can be explained
by a simple dimensional argument.
