Intense structures of different momentum fluxes in turbulent channels

TL;DR

This study compares different definitions of momentum flux in turbulent channels and finds that, despite statistical differences, the intense structures are similar and represent fundamental coherent structures like sweeps, ejections, and rollers.

Contribution

It demonstrates the robustness of coherent structure properties across different momentum flux definitions in turbulent channel flows.

Findings

Intense structures are similar across flux definitions.

Structures include both high- and low-momentum regions.

Properties are robust despite statistical differences.

Abstract

The effect of different definitions of the momentum flux on the properties of the coherent structures of the logarithmic region of wall-bounded turbulence is investigated by comparing the structures of intense tangential Reynolds stress with those of the alternative flux proposed in [J. Jimenez, J. Fluid Mech. 809, 585 (2016)]. Despite the fairly different statistical properties of the two flux definitions, it is found that their intense structures show many similarities, such as the dominance of 'wall-attached' objects, and geometric self-similarity. The new structures are wider, but not taller, than the classical ones, and include both high- and low-momentum regions within the same object. It is concluded that they represent the same phenomenon as the classical groups of a sweep, an ejection, and a roller, suggesting that these groups should be considered the fundamental coherent structures of the momentum flux. The present results show that the properties of these composite momentum structures are robust with respect to the definition of the fluxes.