Developments and Evolutions of ThreeDimensional Perturbations inTransitional Boundary Layer

Formations and developments of typical 3-D perturbations, typically shown as Λ structures, hairpin vortices and ring-like vortices, were observed by means of hydrogen-bubble method in a transitional flat-plate boundary-layer excited by a modulated 2-D artificial perturbation. Visual results show the relationships between a hairpin vortex and a spike structure, which convince the forming mechanisms of spike structures. Dark spots, often detected in turbulent boundary layer, are also observed during flow breakdown, and results show that the instability of local high-shear layer caused by dark spots is probably responsible for flow breakdown.