Dynamo effect in decaying helical turbulence

Axel Brandenburg, Tina Kahniashvili, Sayan Mandal, Alberto Roper Pol, Alexander G. Tevzadze, Tanmay Vachaspati

Research output: Contribution to journalArticle

2 Scopus citations

Abstract

We show that in decaying hydromagnetic turbulence with initial kinetic helicity, a weak magnetic field eventually becomes fully helical. The sign of magnetic helicity is opposite to that of the kinetic helicity - regardless of whether the initial magnetic field was helical. The magnetic field undergoes inverse cascading with the magnetic energy decaying approximately like t-1/2. This is even slower than in the fully helical case, where it decays like t-2/3. In this parameter range, the product of magnetic energy and correlation length raised to a certain power slightly larger than unity is approximately constant. This scaling of magnetic energy persists over long timescales. At very late times and for domain sizes large enough to accommodate the growing spatial scales, we expect a crossover to the t-2/3 decay law that is commonly observed for fully helical magnetic fields. Regardless of the presence or absence of initial kinetic helicity, the magnetic field experiences exponential growth during the first few turnover times, which is suggestive of small-scale dynamo action. Our results have applications to a wide range of experimental dynamos and astrophysical time-dependent plasmas, including primordial turbulence in the early universe.

Original languageEnglish (US)
JournalPhysical Review Fluids
Volume4
Issue number2
DOIs
StatePublished - Feb 1 2019

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ASJC Scopus subject areas

  • Computational Mechanics
  • Modeling and Simulation
  • Fluid Flow and Transfer Processes

Cite this

Brandenburg, A., Kahniashvili, T., Mandal, S., Pol, A. R., Tevzadze, A. G., & Vachaspati, T. (2019). Dynamo effect in decaying helical turbulence. Physical Review Fluids, 4(2). https://doi.org/10.1103/PhysRevFluids.4.024608