# 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 α -effect in Fourier space. Concerning the small-scale perturbations, we predict theoretically and confirm numerically that the largest instability is achived for the magnetic component with the same helicity of the flow, in agreement with the Stretch–Twist–Fold mechanism. Vice versa, in the presence of Lorentz feedback on the velocity, we find that the inverse cascade of magnetic helicity is mostly local if magnetic and kinetic helicities have opposite signs, while it is more nonlocal and more intense ifmore »

- Authors:

- Department of Physics and INFN, University of Rome Tor Vergata, Via della Ricerca Scientifica 1, I-00133 Rome (Italy)
- School of Physics and Astronomy, University of Edinburgh, Peter Guthrie Tait Road, EH9 3FD, Edinburgh (United Kingdom)

- Publication Date:

- OSTI Identifier:
- 22663883

- Resource Type:
- Journal Article

- Resource Relation:
- Journal Name: Astrophysical Journal; Journal Volume: 836; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)

- Country of Publication:
- United States

- Language:
- English

- Subject:
- 79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; COMPARATIVE EVALUATIONS; COMPUTERIZED SIMULATION; DECOMPOSITION; DISTRIBUTION; DISTURBANCES; EQUATIONS; FEEDBACK; FLUIDS; HELICITY; INSTABILITY; LAMINAR FLOW; MAGNETIC FIELDS; MAGNETOHYDRODYNAMICS; PERTURBATION THEORY; QUANTUM ENTANGLEMENT; SPACE; TURBULENCE; TURBULENT FLOW; VELOCITY

### Citation Formats

```
Linkmann, Moritz, Sahoo, Ganapati, Biferale, Luca, McKay, Mairi, and Berera, Arjun.
```*Effects of Magnetic and Kinetic Helicities on the Growth of Magnetic Fields in Laminar and Turbulent Flows by Helical Fourier Decomposition*. United States: N. p., 2017.
Web. doi:10.3847/1538-4357/836/1/26.

```
Linkmann, Moritz, Sahoo, Ganapati, Biferale, Luca, McKay, Mairi, & Berera, Arjun.
```*Effects of Magnetic and Kinetic Helicities on the Growth of Magnetic Fields in Laminar and Turbulent Flows by Helical Fourier Decomposition*. United States. doi:10.3847/1538-4357/836/1/26.

```
Linkmann, Moritz, Sahoo, Ganapati, Biferale, Luca, McKay, Mairi, and Berera, Arjun. Fri .
"Effects of Magnetic and Kinetic Helicities on the Growth of Magnetic Fields in Laminar and Turbulent Flows by Helical Fourier Decomposition". United States.
doi:10.3847/1538-4357/836/1/26.
```

```
@article{osti_22663883,
```

title = {Effects of Magnetic and Kinetic Helicities on the Growth of Magnetic Fields in Laminar and Turbulent Flows by Helical Fourier Decomposition},

author = {Linkmann, Moritz and Sahoo, Ganapati and Biferale, Luca and McKay, Mairi and Berera, Arjun},

abstractNote = {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 α -effect in Fourier space. Concerning the small-scale perturbations, we predict theoretically and confirm numerically that the largest instability is achived for the magnetic component with the same helicity of the flow, in agreement with the Stretch–Twist–Fold mechanism. Vice versa, in the presence of Lorentz feedback on the velocity, we find that the inverse cascade of magnetic helicity is mostly local if magnetic and kinetic helicities have opposite signs, 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.},

doi = {10.3847/1538-4357/836/1/26},

journal = {Astrophysical Journal},

number = 1,

volume = 836,

place = {United States},

year = {Fri Feb 10 00:00:00 EST 2017},

month = {Fri Feb 10 00:00:00 EST 2017}

}