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 (36)

• Astrophysical magnetic fields and nonlinear dynamo theory
• The inverse cascade and nonlinear alpha-effect in simulations of isotropic helical hydromagnetic turbulence
• The case for a distributed solar dynamo shaped by near-surface shear
• Simulations of nonhelical hydromagnetic turbulence
• Magnetic Helicity Conservation and Astrophysical Dynamos
• Mean-field concept and direct numerical simulations of rotating magnetoconvection and the geodynamo
• Constraints on the magnitude of alpha in dynamo theory
• Dynamic nonlinearity in large scale dynamos with shear
• Magnetic diffusivity tensor and dynamo effects in rotating and shearing turbulence
• Electromotive Force and Large-Scale Magnetic Dynamo in a Turbulent Flow with a Mean Shear
• Nonlinear theory of a "shear-current" effect and mean-field magnetic dynamos
• Large-scale dynamos in turbulent convection with shear
• Generation of Magnetic Field by Combined Action of Turbulence and Shear
• Doubly Helical Coronal Ejections from Dynamos and their Role in Sustaining the Solar Cycle
• Dynamical Quenching of the ${\alpha }^2$ Dynamo
• Kinematic alpha effect in isotropic turbulence simulations
• Magnetoconvection and dynamo coefficients: II. Field-direction dependent pumping of magnetic field
• 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
• The mean electromotive force due to turbulence of a conducting fluid in the presence of mean flow
• Nonlinear magnetic diffusivity and alpha tensors in helical turbulence
• Effects of fluctuation on alpha-omega dynamo models
• Large-scale Dynamo Action Driven by Velocity Shear and Rotating Convection
• Large-scale dynamos at low magnetic Prandtl numbers
• Magnetoconvection and dynamo coefficients III: alpha-effect and magnetic pumping in the rapid rotation regime
• Catastrophic alpha quenching alleviated by helicity flux and shear
• Alpha effect and diffusivity in helical turbulence with shear
• The helicity constraint in turbulent dynamos with shear
• Do mean-field dynamos in nonrotating turbulent shear-flows exist?
• A growing dynamo from a saturated Roberts flow dynamo
• Generation of Large-Scale Vorticity in a Homogeneous Turbulence with a Mean Velocity Shear
• Numerical study of large-scale vorticity generation in shear-flow turbulence
• Mean-field effects in the Galloway-Proctor flow
• Dynamo quenching due to shear flow
• How do dynamos saturate?

Cited by in corpus (32)

• Astrophysical hydromagnetic turbulence
• Alpha effect and turbulent diffusion from convection
• Astrophysical turbulence modeling
• Characterizing the mean-field dynamo in turbulent accretion disks
• Large-scale dynamos in rigidly rotating turbulent convection
• Turbulent dynamos with shear and fractional helicity
• Turbulent transport coefficients in spherical wedge dynamo simulations of solar-like stars
• Test-field method for mean-field coefficients with MHD background
• Alpha effect and diffusivity in helical turbulence with shear
• Quenching and anisotropy of hydromagnetic turbulent transport
• Advances in theory and simulations of large-scale dynamos
• Turbulent diffusion with rotation or magnetic fields
• Dynamo action and magnetic buoyancy in convection simulations with vertical shear
• Mean-field transport in stratified and/or rotating turbulence
• The alpha effect with imposed and dynamo-generated magnetic fields
• The alpha effect in rotating convection with sinusoidal shear
• Pumping velocity in homogeneous helical turbulence with shear
• Reynolds stresses from hydrodynamic turbulence with shear and rotation
• On the problem of large-scale magnetic field generation in rotating compressible convection
• Calibrating passive scalar transport in shear-flow turbulence
• The role of the Yoshizawa effect in the Archontis dynamo
• Turbulent viscosity and effective magnetic Prandtl number from simulations of isotropically forced turbulence
• Turbulent transport in hydromagnetic flows
• Influence of Ohmic diffusion on the excitation and dynamics of MRI
• On the spatial and temporal non-locality of dynamo mean-field effects in supersonic interstellar turbulence
• Strong nonlocality variations in a spherical mean-field dynamo
• Dynamo efficiency with shear in helical turbulence
• Mean field dynamo action in shear flows. I: fixed kinetic helicity
• Turbulent radiative diffusion and turbulent Newtonian cooling
• Helicity-vorticity turbulent pumping of magnetic fields in solar convection zone
• Helicity effect on turbulent passive and active scalar diffusivities
• Helicity--vorticity turbulent pumping of magnetic fields in the solar dynamo