  Recent experiments and simulations have shown that unsteady turbulent flows,
before reaching a dynamic equilibrium state, display a universal behaviour. We
show that the observed universal non-equilibrium scaling can be explained using
a non-equilibrium correction of Kolmogorov's energy spectrum. Given the
universality of the experimental and numerical observations, the ideas
presented here lay the foundation for the modeling of a wide class of unsteady
turbulent flows.
