Non-Kolmogorov cascade of helicity driven turbulence

TL;DR

This study investigates how helicity injection affects turbulence by solving Navier-Stokes equations with dual forcings, revealing non-Kolmogorov energy spectra and mode asymmetries, challenging traditional turbulence models.

Contribution

It introduces a novel approach of simultaneous energy and helicity forcing, demonstrating non-Kolmogorov spectra and mode dominance in helicity-driven turbulence.

Findings

Energy flux remains unchanged with increased helicity injection.

Energy spectrum transitions from $k^{-5/3}$ to $k^{-7/3}$ as helicity increases.

A characteristic time based on helicity injection replaces the turnover time.

Abstract

We solve the Navier-Stokes equations with two simultaneous forcings. One forcing is applied at a given large-scale and it injects energy. The other forcing is applied at all scales belonging to the inertial range and it injects helicity. In this way we can vary the degree of turbulence helicity from non helical to maximally helical. We find that increasing the rate of helicity injection does not change the energy flux. On the other hand the level of total energy is strongly increased and the energy spectrum gets steeper. The energy spectrum spans from a Kolmogorov scaling law $k^{-5/3}$ for a non-helical turbulence, to a non-Kolmogorov scaling law $k^{-7/3}$ for a maximally helical turbulence. In the later case we find that the characteristic time of the turbulence is not the turnover time but a time based on the helicity injection rate. We also analyse the results in terms of helical modes decomposition. For a maximally helical turbulence one type of helical mode is found to be much more energetic than the other one, by several orders of magnitude. The energy cascade of the most energetic type of helical mode results from the sum of two fluxes. One flux is negative and can be understood in terms of a decimated model. This negative flux is however not sufficient to lead an inverse energy cascade. Indeed the other flux involving the least energetic type of helical mode is positive and the largest. The least energetic type of helical mode is then essential and cannot be neglected.