Effects of magnetic and kinetic helicities on the growth of magnetic fields in laminar and turbulent flows by helical-Fourier decomposition

Abstract

We present a numerical and analytical study of incompressible homogeneous conducting fluids using a helical Fourier representation. We analytically study both small- and large-scale dynamo properties, as well as the inverse cascade of magnetic helicity, in the most general minimal subset of interacting velocity and magnetic fields on a closed Fourier triad. We mainly focus on the dependency of magnetic field growth as a function of the distribution of kinetic and magnetic helicities among the three interacting wavenumbers. By combining direct numerical simulations of the full magnetohydrodynamics equations with the helical Fourier decomposition we numerically confirm that in the kinematic dynamo regime the system develops a large-scale magnetic helicity with opposite sign compared to the small-scale kinetic helicity, a sort of triad-by-triad $\alpha$-effect in Fourier space. Concerning the small-scale perturbations, we predict theoretically and confirm numerically that the largest instability is achieved for the magnetic component with the same helicity of the flow, in agreement with the Stretch-Twist-Fold mechanism. Vice versa, in presence of a Lorentz feedback on the velocity, we find that the inverse cascade of magnetic helicity is mostly local if magnetic and kinetic helicities have opposite sign, while it is more nonlocal and more intense if they have the same sign, as predicted by the analytical approach. Our analytical and numerical results further demonstrate the potential of the helical Fourier decomposition to elucidate the entangled dynamics of magnetic and kinetic helicities both in fully developed turbulence and in laminar flows.

References in corpus (4)

• Scale interactions and scaling laws in rotating flows at moderate Rossby numbers and large Reynolds numbers
• Excess wing in glass-forming glycerol and LiCl-glycerol mixtures detected by neutron scattering
• Magnetic Helicity Conservation and Inverse Energy Cascade in Electron Magnetohydrodynamic Wave Packets
• Joint inverse cascade of magnetic energy and magnetic helicity in MHD turbulence

Cited by in corpus (5)

• A closure theory for the split energy-helicity cascades in homogeneous isotropic homochiral turbulence
• Effects of forcing mechanisms on the multiscale properties of magnetohydrodynamics
• New Exact Betchov-like Relation for the Helicity Flux in Homogeneous Turbulence
• Inverse transfer of magnetic helicity in direct numerical simulations of compressible isothermal turbulence: scaling laws
• Inverse transfer of magnetic helicity in direct numerical simulations of compressible isothermal turbulence: helical transfers