Contribution of Vortices to Laminar-Turbulent Transition in a Boundary Layer Disturbed by a Roughness Element

This paper presents analyses of the flow topology around and behind a roughness element in a laminar flat plate boundary layer. Hydrogen-bubble and dye visualization performed in a laminar water channel give insights into primary structures, such as the horseshoe vortex, hairpin vortex and recirculation zone. Moreover, two secondary structures are presented: the \(\varLambda _1\) vortex, known from classical laminar-turbulent transition, and a novel vortex, here named \(\varLambda _2\) vortex. The \(\varLambda _2\) vortex is elongated and orientated like the \(\varLambda _1\) vortex parallel to the wall. However, it lies near to the wall and is believed to be initiated by the \(\varLambda _1\) vortex. Hot-film measurements reveal that the fluctuation amplitude of the \(\varLambda _1\) vortex increases up to the highest value downstream of the roughness element. This indicates that the \(\varLambda _1\) vortex contributes most to laminar-turbulent breakdown.