  It has been generally assumed, since the work of von Karman and Howarth in
1938, that free decay of fully-developed turbulence is self-similar. We present
here a simple phenomenological model of the decay of 3D incompressible
turbulence, which predicts breakdown of self-similarity for low-wavenumber
spectral exponents $n$ in the range $n_c<n<4$, where $n_c$ is some threshold
wavenumber. Calculations with the eddy-damped quasi-normal Markovian
approximation give the value as $n_c\approx 3.45$. The energy spectrum for this
range of exponents develops two length-scales, separating three distinct
wavenumber ranges.
