Characteristics of vortex packets in turbulent boundary layers

TLDR

Stereoscopic PIV measured all three velocity components in streamwise–spanwise planes of a turbulent boundary layer at Reτ = 1060, with datasets from the logarithmic layer and beyond, and a feature‑extraction algorithm was developed to automate identification and characterization of hairpin vortex packets. Vortex packets were observed in the log layer, with coherent hairpin legs generating Reynolds shear stress up to 40 × (–uw), contributing 28–25 % of –uw while occupying only 4–4.5 % of the area, but this organization breaks down beyond the log layer.

Abstract

Stereoscopic particle image velocimetry (PIV) was used to measure all three instantaneous components of the velocity field in streamwise–spanwise planes of a turbulent boundary layer at Re τ =1060 ( Re θ =2500). Datasets were obtained in the logarithmic layer and beyond. The vector fields in the log layer ( z + =92 and 150) revealed signatures of vortex packets similar to those proposed by Adrian and co-workers in their PIV experiments. Groups of legs of hairpin vortices appeared to be coherently arranged in the streamwise direction. These regions also generated substantial Reynolds shear stress, sometimes as high as 40 times − uw . A feature extraction algorithm was developed to automate the identification and characterization of these packets of hairpin vortices. Identified patches contributed 28% to − uw while occupying only 4% of the total area at z + =92. At z + =150, these patches occupied 4.5% of the total area while contributing 25% to − uw . Beyond the log layer ( z + =198 and 530), the spatial organization into packets is seen to break down.