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We have imagined a numerical experiment to explore the onset of turbulentintermittency associated with a spatial perturbation of the correlation length.We place two isotropic regions, with different integral scales, inside a volumewhere the turbulent kinetic energy is initially uniform and leave them tointeract and evolve in time. The different length scales produce differentdecay rates in the two regions. Since the smaller-scale region decays faster, atransient turbulent energy gradient is generated at the interface between thetwo regions. The transient is characterized by three phases in which thekinetic energy gradient across the interface grows, peaks and then slowlydecays. The transient lifetime is almost proportional to the initial ratio ofthe correlation lengths. The direct numerical simulations also show that theinterface width grows in time. The velocity moments inside this interactionzone are seen to depart from their initial isotropic values and, with a certainlag, the anisotropy is seen to spread to small scales. The longitudinalderivative moments also become anisotropic after a few eddy turnover times.This anisotropic behaviour is different from that observed in shearedhomogeneous turbulent flows, where high transverse derivative moments aregenerated, but longitudinal moments almost maintain the isotropic turbulencevalues. Apart from the behaviour of the energy gradient transients, the resultsalso show the timescaling of the interface diffusion width, and data on theanisotropy of the large and small scales, observed through one-point statisticsdetermined inside the intermittency sublayer, which is associated with theinteraction zone.

Decaying turbulence: what happens when the correlation length varies spatially in two adjacent zones