Spanwise structure and scale growth in turbulent boundary layers

TLDR

The study experimentally investigates spanwise structure and growth mechanisms in turbulent boundary layers and proposes that vortex packets merge via vortex reconnection to drive additional scale growth. PIV measurements in the streamwise–spanwise plane are used to analyze vortex packet dynamics, revealing that vortex reconnection underlies the observed merging of packets. Statistical analysis shows vortex organization along the streamwise direction, a hairpin signature linked to local minima in streamwise velocity, spanwise lengthscales that grow linearly with wall distance indicating average self‑similarity, yet individual structures do not grow strictly self‑similarly, and the proposed reconnection mechanism connects vortex pairing to streak coalescence in the inner layer.

Abstract

Spanwise structure and growth mechanisms in a turbulent boundary layer are investigated experimentally. PIV measurements are obtained in the streamwise–spanwise (. Statistical evidence is also presented for two important aspects of the vortex packet paradigm: vortex organization in the streamwise direction, and the clear association of the hairpin signature with local minima in streamwise velocity. Several spanwise lengthscales are shown to vary linearly with distance from the wall, revealing self-similar growth of spanwise structure in an average sense. Inspection of the data, however, suggests that individual structures do not grow strictly self-similarly in time. It is proposed that additional scale growth occurs by the merging of vortex packets on an eddy-by-eddy basis via a vortex re-connection mechanism similar to that suggested by Wark & Nagib (1989). The proposed mechanism provides a link between vortex-pairing concepts and the observed coalescence of streaky low-speed regions in the inner layer.