The minimal flow unit in near-wall turbulence

TLDR

The study aims to isolate and analyze the basic flow unit’s morphology and dynamics to assess its role in fully developed channel turbulence. Direct numerical simulations of low‑to‑moderate Reynolds number channel flow were conducted in small computational boxes that enforce a doubly periodic array of identical structures. For boxes wider than about 100 wall units, turbulence reproduces experimental near‑wall statistics; narrower boxes exhibit one‑wall turbulence or laminar flow, yet the elementary box consistently contains a single low‑velocity streak with lifted spanwise vorticity, and a fundamental intermittency period is observed in which wall‑layer vorticity wraps around an inclined vortex.

Abstract

Direct numerical simulations of unsteady channel flow were performed at low to moderate Reynolds numbers on computational boxes chosen small enough so that the flow consists of a doubly periodic (in x and z ) array of identical structures. The goal is to isolate the basic flow unit, to study its morphology and dynamics, and to evaluate its contribution to turbulence in fully developed channels. For boxes wider than approximately 100 wall units in the spanwise direction, the flow is turbulent and the low-order turbulence statistics are in good agreement with experiments in the near-wall region. For a narrow range of widths below that threshold, the flow near only one wall remains turbulent, but its statistics are still in fairly good agreement with experimental data when scaled with the local wall stress. For narrower boxes only laminar solutions are found. In all cases, the elementary box contains a single low-velocity streak, consisting of a longitudinal strip on which a thin layer of spanwise vorticity is lifted away from the wall. A fundamental period of intermittency for the regeneration of turbulence is identified, and that process is observed to consist of the wrapping of the wall-layer vorticity around a single inclined longitudinal vortex.