The generation and regeneration of single hairpin vortices

The generation and growth of single hairpin vortices created by controlled surface fluid injection were examined experimentally within a laminar boundary layer over a range of Reynolds numbers. Flow visualization, using both dye and hydrogen bubbles, was employed in conjunction with hot-film anemometry to investigate the growth characteristics and evolution of these single hairpin vortices. Hydrogen-bubble visualization results reveal that the passage of a hairpin vortex can give rise to a low-speed streak pattern adjacent to the surface, and a turbulent pocket-like pattern farther removed from the surface. When the displacement and injection Reynolds numbers exceed critical levels, regeneration processes occur, which result in the development of new hairpin-like vortices by both (i) lateral deformation of the vortex lines comprising the initial hairpin vortex and (ii) a process of vortex-surface interaction, which causes the ejection of surface fluid and subsequent hairpin formation by viscous-inviscid interactions. The combination of these processes results in both lateral and streamwise growth of the initial hairpin-vortex structure, yielding a symmetric turbulent-spot type of behaviour.