Cancellation exponents in helical and non-helical flows

TL;DR

This paper investigates the scaling laws of helicity fluctuations in turbulent flows using the cancellation exponent, revealing filamentary structures and independence from global helicity.

Contribution

It introduces the use of the cancellation exponent to analyze helicity fluctuations in both helical and non-helical turbulence, providing new insights into their geometric and statistical properties.

Findings

Helicity fluctuations exhibit filamentary structures.

Statistical properties of helicity are independent of global helicity.

The cancellation exponent relates to the fractal dimension of structures.

Abstract

Helicity is a quadratic invariant of the Euler equation in three dimensions. As the energy, when present helicity cascades to smaller scales where it dissipates. However, the role played by helicity in the energy cascade is still unclear. In non-helical flows, the velocity and the vorticity tend to align locally creating patches with opposite signs of helicity. Also in helical flows helicity changes sign rapidly in space. Not being a positive definite quantity, global studies considering its spectral scaling in the inertial range are inconclusive, except for cases where one sign of helicity is dominant. We use the cancellation exponent to characterize the scaling laws followed by helicity fluctuations in numerical simulations of helical and non-helical turbulent flows, with different forcing functions and spanning a range of Reynolds numbers from approximately 670 to 6200. The exponent is a measure of sign-singularity and can be related to the fractal dimension as well as to the first order helicity scaling exponent. The results are consistent with the geometry of helical structures being filamentary. Further analysis indicates that statistical properties of helicity fluctuations do not depend on the global helicity of the flow.