  Edge shear flow and its effect on regulating turbulent transport have long
been suspected to play an important role in plasmas operating near the
Greenwald density limit $ n_G $. In this study, equilibrium profiles as well as
the turbulent particle flux and Reynolds stress across the separatrix in the
HL-2A tokamak are examined as $ n_G $ is approached in ohmic L-mode discharges.
As the normalized line-averaged density $ \bar{n}_e/n_G $ is raised, the
shearing rate of the mean poloidal flow $ \omega_{\rm sh} $ drops, and the
turbulent drive for the low-frequency zonal flow (the Reynolds power $
\mathcal{P}_{Re} $) collapses. Correspondingly, the turbulent particle
transport increases drastically with increasing collision rates. The geodesic
acoustic modes (GAMs) gain more energy from the ambient turbulence at higher
densities, but have smaller shearing rate than low-frequency zonal flows. The
increased density also introduces decreased adiabaticity which not only
enhances the particle transport but is also related to a reduction in the
eddy-tilting and the Reynolds power. Both effects may lead to the cooling of
edge plasmas and therefore the onset of MHD instabilities that limit the plasma
density.
