Characteristics of active and inactive motions in high-Reynolds-number turbulent boundary layers

TL;DR

This study investigates the roles of active and inactive eddy motions in high-Reynolds-number turbulent boundary layers, revealing their distinct contributions to drag and momentum transport through empirical analysis across a wide range of Reynolds numbers.

Contribution

It introduces an energy-decomposition scheme to empirically distinguish active and inactive eddy components and their specific roles in turbulent boundary layers.

Findings

Active motions generate Reynolds shear stresses.

Inactive motions transport streamwise momentum to the wall.

Distinct contributions of eddy components confirmed across Reynolds numbers.

Abstract

Wall-scaled (attached) eddies play a significant role in the overall drag experienced in high-Reynolds-number turbulent boundary layers (TBLs). This study aims to delve into the underlying mechanisms driving this phenomenon by dissecting the active and inactive components of these attached eddies, as initially proposed by Townsend (1976). Employing a recently introduced energy-decomposition scheme, we analyze TBL datasets covering a wide range of Reynolds numbers ($Re_{\tau}$ $\sim$ $\mathcal{O}$(10$^{3}$)--$\mathcal{O}$(10$^{6}$)). This analysis provides empirical evidence of the distinct contributions of these components to drag generation, and reveals that while active motions are responsible solely for generating Reynolds shear stresses, inactive motions are crucial for transporting streamwise momentum from the logarithmic region to the wall, thus corroborating earlier hypotheses in the literature.