Random shearing by zonal flows and transport reduction

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 tau(ZF) decorrelates two nearby fluid elements less efficiently than a mean shear flow does. The decorrelation time is tau(D)=(tau(eta)/tau(ZF)Omega(rms)(2))(1/2) (tau(eta) is the turbulent scattering time, and Omega(rms) is the rms shear), leading to larger scalar field amplitude with a slightly different scaling (proportional totau(D)/Omega(rms)), as compared to the case of coherent shearing. In the strong shear limit, the flux scales as proportional toOmega(rms)(-1). (C) 2004 American Institute Physics.