RÉSUMÉ

We apply the vortex-surface field (VSF), a Lagrangian-based structure-identification method, to the DNS database of the K-type transitional boundary layer (Sayadi et al., 2013). The VSFs are constructed from the vorticity fields within a sliding window at different times and streamwise locations using a recently developed boundary-constraint method. The isosurfaces of the VSF, representing vortex surfaces consisting of vortex lines with different wall distances in the laminar stage, show different evolutionary geometries in the transition. We observe that the vortex surfaces evolve from wall-parallel planar sheets through hairpin-like structures into a turbulent spot with offspring hairpins. From quantitative analysis, we show that a small number of representative vortex surfaces can contribute significantly to the increase of the skin-friction coefficient in the transition, which implies a reduced-order model based on the VSF.