Large-Scale Flow Generation due to Inhomogeneous Turbulent Helicity

Abstract:

Nobumitsu Yokoi$^1$ and Axel Brandenburg$^2$

$^1$ Institute of Industrial Science (IIS), Univ. of Tokyo, $^2$ Nordic Institute for Theoretical Physics (NORDITA)

In non-mirror-symmetric systems, such as rotating planets and stars, a finite turbulent helicity density $\langle {{\bf{u}}' \cdot \mbox{\boldmath$\omega$}'} \rangle$ is expected to be locally present in turbulence. Such helical turbulence exhibits interesting features: Turbulence works not only to enhance the effective transport leading to a decrease of coherent large-scale structure, but also to suppress the effective transport leading to the sustainment and generation of large-scale localized flows. Yokoi and Yoshizawa (1993) showed that the inhomogeneous turbulent helicity coupled with the large-scale vortical motion and/or rotation induces a large-scale flow. This suggests a possibility of a new way of the vortex dynamo even in incompressible turbulence without resorting to any baroclinicity effect. It is distinguished from the previously studied anisotropic kinetic alpha (AKA) effect. Following their suggestions, we examine the flow generation due to inhomogeneous helicity with the aid of a direct numerical simulation (DNS) of rotating helical turbulence. It is shown that a large-scale flow is induced by the inhomogeneous turbulent helicity effect whereas no flow is generated in the zero and/or homogeneous turbulent helicity cases. Physical origin of this helicity effect is discussed by considering the Reynolds stress and turbulent vortexmotive (pondermotive) force.

Yokoi, N. and Yoshizawa, A. Phys. Fluids A5, 464-477 (1993).