Three-Dimensional Nature of 2D Hairpin Packet Signatures in a DNS of a Turbulent Boundary Layer

Abstract

The existence of the outer region “hairpins” and “hairpin packets” is visually assessed in a well-resolved DNS of a zero-pressure-gradient turbulent boundary layer at moderately high Reynolds number. For this purpose, 50 independent 2D streamwise–wall-normal slices at $Re_{\theta }=4300$ are extracted. The slices are then used to mimic the coarser resolution PIV velocity fields of Adrian et al. J. Fluid Mech, 422:1–54, 2000 [2] using the mimicking procedure of Rahgozar et al. J. Turbul, 14(10):37–66, 2013 [5] based on Gaussian filtering and linear interpolation. Afterwards, in the same manner as Adrian et al. J. Fluid Mech, 422:1–54, 2000 [2], the mimicked fields are inspected in order to discover the signatures of hairpin and hairpin packets. The vortices that are identified as hairpins are then isolated and visualized in three dimensions using the fully resolved DNS data. In agreement with Adrian et al. J. Fluid Mech, 422:1–54, 2000 [2], signatures associated by them to hairpin and hairpin packets are observed frequently in the mimicked planes. However, the 3D character of the 2D signatures is found to be more convoluted than the proposed hairpin packet model.