Alpha effect and diffusivity in helical turbulence with shear

Abstract

We study the dependence of turbulent transport coefficients, such as the components of the $\alpha$ tensor (${\alpha }_{ij}$) and the turbulent magnetic diffusivity tensor (${\eta }_{ij}$), on shear and magnetic Reynolds number in the presence of helical forcing. We use three-dimensional direct numerical simulations with periodic boundary conditions and measure the turbulent transport coefficients using the kinematic test field method. In all cases the magnetic Prandtl number is taken as unity. We find that with increasing shear the diagonal components of ${\alpha }_{ij}$ quench, whereas those of ${\eta }_{ij}$ increase. The antisymmetric parts of both tensors increase with increasing shear. We also propose a simple expression for the turbulent pumping velocity (or $\gamma$ effect). This pumping velocity is proportional to the kinetic helicity of the turbulence and the vorticity of the mean flow. For negative helicity, i.e. for a positive trace of ${\alpha }_{ij}$, it points in the direction of the mean vorticity, i.e. perpendicular to the plane of the shear flow. Our simulations support this expression for low shear and magnetic Reynolds number. The transport coefficients depend on the wavenumber of the mean flow in a Lorentzian fashion, just as for non-shearing turbulence.

A&A, in press

References in corpus (16)

• The case for a distributed solar dynamo shaped by near-surface shear
• Simulations of nonhelical hydromagnetic turbulence
• Large-scale dynamos in turbulent convection with shear
• Generation of Magnetic Field by Combined Action of Turbulence and Shear
• Kinematic alpha effect in isotropic turbulence simulations
• Scale dependence of alpha effect and turbulent diffusivity
• Alpha effect and turbulent diffusion from convection
• Nonlinear current helicity fluxes in turbulent dynamos and alpha quenching
• Nonlinear magnetic diffusivity and alpha tensors in helical turbulence
• Large-scale Dynamo Action Driven by Velocity Shear and Rotating Convection
• Catastrophic alpha quenching alleviated by helicity flux and shear
• Alpha effect and diffusivity in helical turbulence with shear
• A growing dynamo from a saturated Roberts flow dynamo
• Numerical study of large-scale vorticity generation in shear-flow turbulence
• Dynamo quenching due to shear flow
• How do dynamos saturate?

Cited by in corpus (6)

• Alpha effect and turbulent diffusion from convection
• Alpha effect and diffusivity in helical turbulence with shear
• Advances in theory and simulations of large-scale dynamos
• Turbulent diffusion with rotation or magnetic fields
• The role of the Yoshizawa effect in the Archontis dynamo
• Dynamo efficiency with shear in helical turbulence