Ascending-descending and direct-inverse cascades of Reynolds stresses in turbulent Couette flow

TL;DR

This study uses DNS and AGKE to analyze the complex interactions of Reynolds stresses in turbulent Couette flow, revealing simultaneous bottom-up and top-down energy cascades across scales and wall-normal directions.

Contribution

It provides a detailed, scale- and space-resolved analysis of Reynolds stress cascades, highlighting the coexistence of direct and inverse cascades and top-down versus bottom-up dynamics in turbulent Couette flow.

Findings

Large-scale motions influence small-scale turbulence production.

Both bottom-up and top-down interactions occur in the flow.

Inverse cascade is prominent in wall-normal Reynolds stresses.

Abstract

The interaction between small- and large-scale structures, and the coexisting bottom-up and top-down processes are studied in a turbulent plane Couette flow, where space-filling longitudinal rolls appear at relatively low values of the Reynolds number $Re$. A DNS database at $Re_\tau=101$ is built to replicate the highest $Re$ considered in a recent experimental work by Kawata and Alfredsson (Phys. Rev. Lett., vol.120, 2018, 244501). Our study is based on the exact budget equations for the second-order structure function tensor $\langle \delta u_i \delta u_j \rangle$, i.e. the Anisotropic Generalized Kolmogorov Equations (AGKE). The AGKE study production, redistribution, transport and dissipation of every Reynolds stress tensor component, considering simultaneously the physical space and the space of scales, and properly define the concept of scale in the inhomogeneous wall-normal direction. We show how the large-scale energy-containing motions are involved in the production and redistribution of the turbulent fluctuations. Both bottom-up and top-down interactions occur, and the same is true for direct and inverse cascading. The wall-parallel components $\langle \delta u \delta u \rangle$ and $\langle \delta w \delta w \rangle$ show that the both small and large near-wall scales feed the large scales away from the wall. The wall-normal component $\langle \delta v \delta v \rangle$ is different, and shows a dominant top-down dynamics, being produced via pressure-strain redistribution away from the wall and transferred towards near-wall larger scales via an inverse cascade. The off-diagonal component shows a top-down interaction, with both direct and inverse cascade, albeit the latter takes place within a limited range of scales.