Formation of spanwise vorticy in oblique turbulent bands of transitional plane Couette flow, part 1: Numerical experiments

TL;DR

This study uses direct numerical simulations to analyze how spanwise vorticity forms in oblique turbulent bands of plane Couette flow, revealing the mechanisms of vorticity roll-up and its advection by large-scale flows.

Contribution

It introduces a new criterion for detecting spanwise vorticity events and systematically measures their correlation lengths and advection velocities in turbulent bands.

Findings

Spanwise vorticity is generated by roll-up of shear layers in velocity streaks.

Vorticity is advected along the bands by large-scale flow.

The size of laminar regions may influence vorticity ejection.

Abstract

This article investigates the formation of spanwise vorticity in the velocity streaks of the oblique laminar- turbulent bands of plane Couette flow (PCF) by mean of Direct Numerical Simulations (DNS). The spanwise vorticity is created by a roll-up type development of the streamwise-wall normal shear layer of the velocity streaks. It is advected by the large scale flow along the bands. We propose a criterion on spanwise vorticity which detects these events in order to perform systematic measurements. Beside of the streamwise and spanwise correlation lengths of the rolls, their advection velocity is measured and shown to match the large scale flow along the band near the turbulent region. Eventually, we discuss the possible relation between ejection of vorticity away from the bands near the laminar region and the size of said laminar region.