Hairpins et al. in Turbulent Boundary Layers

TL;DR

This study visualizes turbulent boundary layers at various Reynolds numbers, revealing the dominance of hairpin vortices at low Re and their loss of coherence at higher Re, while identifying large-scale streaks in the flow.

Contribution

It provides new 3D visualizations of turbulent boundary layers across a wide Reynolds number range, highlighting the evolution of coherent structures.

Findings

Hairpin vortices dominate at low Re

Coherence of vortical structures diminishes at high Re

Large-scale streaks are observed in high Re boundary layers

Abstract

A new set of three-dimensional visualisations of a large-scale direct numerical simulations (DNS) of a turbulent boundary layer is presented. The Reynolds number ranges from $Re_\theta=180$ to 4300, based on the momentum-loss thickness $\theta$ and the free-stream velocity $U_\infty$. The focus of the present fluid dynamics video is on analysing the coherent vortical structures in the boundary layer: It is clearly shown that the initial phases are dominated by coherent so-called hairpin vortices which are characteristic remainders of the laminar-turbulent transition at lower Reynolds numbers. At higher $Re$ (say $Re_\theta>2000$), these structures are no longer seen as being dominant; the coherence is clearly lost, both in the near-wall region as well as in the outer layer of the boundary layer. Note, however, that large-scale streaks in the streamwise velocity, which have their peak energy at about half the boundary-layer thickness, are unambiguously observed. In addition to visualisation with classical three-dimensional isosurfaces, the video is also rendered using stereoscopic views using red-cyan anaglyphs.