title: Large-scale flow generation by inhomogeneous helicity.
abstract: The effect of kinetic helicity (velocity-vorticity correlation) on turbulent momentum transport is investigated. The turbulent kinetic helicity (pseudoscalar) enters the Reynolds stress (mirror-symmetric 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 nonuniform 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.
authors.affiliation: Institute of Industrial Science, University of Tokyo, Tokyo, Japan.
authors.forename: N
authors.initials: N
authors.lastname: Yokoi
authors.affiliation: Nordita, KTH Royal Institute of Technology and Stockholm University, SE-10691 Stockholm, Sweden; Department of Astronomy, Stockholm University, SE-10691 Stockholm, Sweden; and JILA and Department of Astrophysical and Planetary Sciences, Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado 80303, USA.
authors.forename: A
authors.initials: A
authors.lastname: Brandenburg