Transport reduction by shear flows in dynamical models

The reduction in the transport of particles and heat by a strong mean shear flow is studied, in the context of interchange and ion-temperature gradient turbulence models. Compared to passive scalar transport, a stronger reduction in the transport (scaling with the shearing rate $\Omega$ as ${\mathrm{\propto \Omega }}^{\text{−3}}\hspace{0.5em}\ln \mathrm{\hspace{0.5em}\Omega }$ ) results from a severe reduction in the amplitude of turbulent velocity in both models. However, the cross phase is only modestly reduced, as in the scalar field case. These results are in qualitative agreement with the results from both gyrokinetic and gyrofluid simulations of toroidal ion-temperature gradient turbulence [Lin et al., Phys. Rev. Lett. 83, 3645 (1999); Falchetto and Ottaviani, Phys. Rev. Lett. 92, 025002 (2004)], but contradict recent claims in some literature, highlighting the importance of the detailed properties of the flow in determining the overall transport level.