Effects of a fluctuating sheared flow on cross phase in passive-scalar turbulent diffusion

Transport barriers are key elements concerning energy and particle confinement in fusion devices. They play a fundamental role in the L→H transition observed in most tokamaks' edges. It has been shown that a shear in the E×B velocity could trigger and sustain such a barrier. The E×B velocity shear model has proven to be of great interest in the study of the formation and characteristics of transport barriers. Here we address a particular case of flow shear stabilization, namely the effect of a shear flow on the diffusion of a passive scalar. A shear flow reduces the radial flux (radial transport) Γ of a passive scalar field (we consider the pressure field) via the reduction of the turbulence energy ⟨p2⟩ and/or via the reduction of the cross phase cosδ between the fluctuations of the pressure and velocity fields. We compare our results with those of different analytical models for passive-scalar advection or diffusion [Terry et al., Phys. Rev. Lett. 87, 185001 (2001); Kim and Diamond, Phys. Rev. Lett. 91, 075001 (2003)]. However, these studies yielded contradictory results. The purpose of this study is to shed light on this particular issue using numerical simulations to clarify the role of the reduction of the amplitude of turbulence and cross phase in regulating the radial transport.