  In this work, we introduce an a posteriori error indicator for the reduced
basis modelling of turbulent flows. It is based upon the $k^{-5/3}$ Kolmogorov
turbulence theory, thus it may be applied to any numerical discretisation of
LES turbulence models. The main idea of this indicator is that if the
full-order solution and the Reduced Order solution are close enough, then their
flow energy spectrum within the inertial range should also be close. We present
some numerical tests which supports that the use of this indicator is helpful,
obtaining large computational speed-ups. We use as full-order model a Finite
Element discretisation of the unsteady LES Smagorinsky turbulence model.
