Dual channels of helicity cascade in turbulent flows

TL;DR

This paper reveals two distinct channels of helicity cascade in turbulent flows, linked to vortex twisting and stretching, with different behaviors across turbulence types and implications for energy transfer mechanisms.

Contribution

It introduces the concept of dual helicity cascade channels and analyzes their different behaviors in various turbulent flow conditions.

Findings

Two helicity cascade channels originate from vortex twisting and stretching.

Different behaviors of helicity flux in homogeneous versus other turbulent flows.

Identification of a mechanism hindering inverse energy cascade related to the second channel.

Abstract

Helicity, as one of only two inviscid invariants in three-dimensional turbulence, plays an important role in the generation and evolution of turbulence. From the traditional viewpoint, there exists only one channel of helicity cascade similar to that of kinetic energy cascade. Through theoretical analysis, we find that there are two channels in helicity cascade process. The first channel mainly originates from vortex twisting process, and the second channel mainly originates from vortex stretching process. By analysing the data of direct numerical simulations of typical turbulent flows, we find that these two channels behave differently. The ensemble averages of helicity flux in different channels are equal in homogeneous and isotropic turbulence, while they are different in other type of turbulent flows. The second channel is more intermittent and acts more like a scalar, especially on small scales. Besides, we find a novel mechanism of hindered even inverse energy cascade, which could be attributed to the second-channel helicity flux with large amplitude.