We extend the vortex-surface field (VSF), a Lagrangian-based structure-identification method, to study the structural evolution in Klebanoff-type boundary-layer transition. The VSF is constructed from the instantaneous vorticity fields in a series of sliding windows at different times and streamwise locations using the boundary-constraint method. The visualization of VSF isosurfaces displays the Lagrangian-like evolution of vortex surfaces consisting of vortex lines. The near-wall VSF isosurfaces evolve from wall-parallel planar vortex surfaces through a train of hairpin-like structures into turbulent spots. In the inception of turbulent spots, the train of hairpin-like bulges of vortex surfaces induces sinuous spanwise perturbations on the adjacent flank of lower wavy vortex surfaces, and then triggers the Kelvin-Helmholtz instability in the spanwise direction. The resultant sinuous distortion of vortex surfaces propagates in the spanwise direction, which can lead to the mass production of small-scale, skewed hairpin-like structures and the rapid lateral growth of turbulent spots. The streaky and hairpin-like structures have strong interactions in the present Klebanoff-type transition. On the other hand, these signature structures can be interpreted as different evolutionary geometries of the same set of VSF isosurfaces at different times and spatial regions, rather than considering them as two different elementary structures visualized by different structure-identification methods. Furthermore, the quantified distortion of near-wall vortex surfaces in the lateral growth of young turbulent spots is highly correlated to the surge of the skin-friction coefficient in the transition.

• Received 9 January 2018

DOI:https://doi.org/10.1103/PhysRevFluids.3.074701