  The effect of kinetic helicity (velocity--vorticity correlation) on turbulent
momentum transport is investigated. The turbulent kinetic helicity
(pseudoscalar) enters the Reynolds stress (mirrorsymmetric tensor) expression
in the form of a helicity gradient as the coupling coefficient for the mean
vorticity and/or the angular velocity (axial vector), which suggests the
possibility of mean-flow generation in the presence of inhomogeneous helicity.
This inhomogeneous helicity effect, which was previously confirmed at the level
of a turbulence- or closure-model simulation, is examined with the aid of
direct numerical simulations of rotating turbulence with non-uniform helicity
sustained by an external forcing. The numerical simulations show that the
spatial distribution of the Reynolds stress is in agreement with the
helicity-related term coupled with the angular velocity, and that a large-scale
flow is generated in the direction of angular velocity. Such a large-scale flow
is not induced in the case of homogeneous turbulent helicity. This result
confirms the validity of the inhomogeneous helicity effect in large-scale flow
generation and suggests that a vortex dynamo is possible even in incompressible
turbulence where there is no baroclinicity effect.
