Abstract
The physics of random shearing by zonal flows and the consequent reduction of scalar field transport are studied. In contrast to mean shear flows, zonal flows have a finite autocorrelation time and can exhibit complex spatial structure. A random zonal flow with a finite correlation time τZF decorrelates two nearby fluid elements less efficiently than a mean shear flow does. The decorrelation time is τD=(τη/τZFΩrms2)1/2 (τη is the turbulent scattering time, and Ωrms is the rms shear), leading to larger scalar field amplitude with a slightly different scaling (∝τD/Ωrms), as compared to the case of coherent shearing. In the strong shear limit, the flux scales as ∝Ωrms−1.
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