Kinetic helicity effects in stellar angular-momentum transport

Abstract

In non-mirror-symmetric system, turbulent helicity enters the Reynolds-stress expression as the coupling coefficient of the mean absolute vorticity (anti-symmetric part of the mean velocity shear) (: velocity fluctuation, : vorticity fluctuation, and : ensemble average). This gives marked contrast to the turbulent energy, which plays an essential role in the eddy-viscosity representation of the Reynolds stress as the coupling coefficient of the mean velocity strain (symmetric part of the mean velocity shear). By considering the turbulent vortex-motive force in the mean vorticity equation, it is shown that a large-scale flow can be induced in the direction of the mean absolute vorticity (mean vorticity and system rotation) mediated by the inhomogeneous turbulent helicity. This inhomogeneous helicity effect is applied to the large-scale flow generation and sustainment in the stellar convective zone. The contribution of the inhomogeneous helicity effect to the angular-momentum transport in the stellar convection is discussed with the aid of some direct numerical simulations. Emphasis is also made on the turbulent helicity as a link between the large-scale flow structures, like differential azimuthal rotation and the meridional circulation, and the statistical properties of turbulence.