  We have imagined a numerical experiment to explore the onset of turbulent
intermittency associated with a spatial perturbation of the correlation length.
We place two isotropic regions, with different integral scales, inside a volume
where the turbulent kinetic energy is initially uniform and leave them to
interact and evolve in time. The different length scales produce different
decay rates in the two regions. Since the smaller-scale region decays faster, a
transient turbulent energy gradient is generated at the interface between the
two regions. The transient is characterized by three phases in which the
kinetic energy gradient across the interface grows, peaks and then slowly
decays. The transient lifetime is almost proportional to the initial ratio of
the correlation lengths. The direct numerical simulations also show that the
interface width grows in time. The velocity moments inside this interaction
zone are seen to depart from their initial isotropic values and, with a certain
lag, the anisotropy is seen to spread to small scales. The longitudinal
derivative moments also become anisotropic after a few eddy turnover times.
This anisotropic behaviour is different from that observed in sheared
homogeneous turbulent flows, where high transverse derivative moments are
generated, but longitudinal moments almost maintain the isotropic turbulence
values. Apart from the behaviour of the energy gradient transients, the results
also show the timescaling of the interface diffusion width, and data on the
anisotropy of the large and small scales, observed through one-point statistics
determined inside the intermittency sublayer, which is associated with the
interaction zone.
