Fluctuation covariance-based study of roll-streak dynamics in Poiseuille flow turbulence

TL;DR

This study investigates the physical mechanisms behind the formation and maintenance of roll-streak structures in turbulent Poiseuille flow, revealing continuous reinforcement by specific turbulent fluctuations.

Contribution

It demonstrates how sinuous and varicose fluctuations reinforce streaks through Reynolds stresses, using second-order statistical analysis and the SSD framework.

Findings

Reynolds stresses reinforce streaks continuously, not intermittently.

Sinuous and varicose fluctuations selectively reinforce low- and high-speed streaks.

Dominant POD modes of fluctuations drive the maintenance of the streamwise-mean roll.

Abstract

Although the roll-streak (R-S) is fundamentally involved in the dynamics of wall-turbulence, the physical mechanism responsible for its formation and maintenance remains controversial. In this work we investigate the dynamics maintaining the R-S in turbulent Poiseuille flow at R=1650. Spanwise collocation is used to remove spanwise displacement of the streaks and associated flow components, which isolates the streamwise-mean flow R-S component and the second-order statistics of the streamwise-varying fluctuations that are collocated with the R-S. This streamwise-mean/fluctuation partition of the dynamics facilitates exploiting insights gained from the analytic characterization of turbulence in the second-order statistical state dynamics (SSD), referred to as S3T, and its closely associated restricted nonlinear dynamics (RNL) approximation. Symmetry of the statistics about the streak centerline permits separation of the fluctuations into sinuous and varicose components. The Reynolds stress forcing induced by the sinuous and varicose fluctuations acting on the R-S is shown to reinforce low- and high-speed streaks respectively. This targeted reinforcement of streaks by the Reynolds stresses occurs continuously as the fluctuation field is strained by the streamwise-mean streak and not intermittently as would be associated with streak-breakdown events. The Reynolds stresses maintaining the streamwise-mean roll arise primarily from the dominant POD modes of the fluctuations, which can be identified with the time average structure of optimal perturbations growing on the streak. These results are consistent with a universal process of R-S growth and maintenance in turbulent shear flow arising from roll forcing generated by straining turbulent fluctuations, which was identified using the S3T SSD.